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1. 初始Java

1.1 Java 概述

1.1.1什么是 Java

Java是一种优秀的程序设计语言，它具有令人赏心悦目的语法和易于理解的语义。

不仅如此，Java还是一个有一系列计算机软件和规范形成的技术体系，这个技术体系提供了完整的用于软件开发和跨平台部署的支持环境，并广泛应用于嵌入式系统、移动终端、企业服务器、大型机等各种场合。

1.1.2 Java 背景

• Java 是美国sun公司在1995年推出的一门计算机高级编程语言。
• Java 早期称为Oak（橡树），后期改名Java。
• Java 之父：詹姆斯·高斯林（祖师爷）。
• 2009年 sun公司被 Oracle（甲骨文）公司收购。

1.1.3 为什么使用 Java

• 世界上最流行的编程语言之一，在国内使用最为广泛的编程语言
• 可移植性、安全可靠、性能较好
• 开发社区最完善、功能最丰富

1.1.4 Java能做什么

• 桌面应用开发
• 企业级应用开发
• 移动应用开发
• 服务器系统
• 大数据开发
• 游戏开发

1.1.5 Java 技术体系

1.1.6 Java 发展史

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1.1.7 Java的主要特征

以下 Java 特性来自 Java 白皮书

• 简单性：

Java 语言的语法与C语言和C++语言很接近，使得大多数程序员很容易学习和使用，另一方面，Java 丢弃了C++中很少使用的、很难理解的、令人迷惑的那些特性，如操作符重载、多继承、自动的强制类型转换。特别地，Java语言不使用指针，而是引用。并提供了自动的废料收集，使得程序员不必为内存管理而担忧。

• 面向对象：

Java语言提供类、接口和继承等面向对象的特性，为了简单起见，只支持类之间的单继承，但支持接口之间的实现机制（关键字为implements）。Java语言全面支持动态绑定，而C++语言只对虚函数使用动态绑定。总之，Java语言是一个纯的面向对象程序设计语言

• 分布式：

Java有一个丰富的例程库，用于处理像 HTTP 和FTP之类的TCP/IP协议。
Java应用程序能够通过URL打开和访问网络上的对象，其便捷程序就好像访问本地文件一样。

• 健壮性：

Java 的设计目标之一在于使得 Java 编写的程序具有多方面的可靠性。Java 非常强调进行早期的问题检测、后期动态的（运行时）检测，以及消除容易出错的情况… Java 与C/C++ 最大的不同在于Java 采用的指针模型可以消除重写内存和损坏数据的可能性。

• 安全性：

Java 要适用于网络/分布式环境。为了实现这个目标，安全性颇受重视。使用Java 可以构建防病毒、防篡改的系统。

Java 设计能够防范各种攻击，其中包括：

• 运行时堆栈溢出，这是蠕虫和病毒常用的攻击手段。
• 破坏自己的进程空间之外的内存，
• 未经授权读写文件

• 体系结构中立：

Java程序（后缀为java的文件）在Java平台上被编译为体系结构中立的字节码格式（后缀为class的文件），然后可以在实现这个Java平台的任何系统中运行。这种途径适合于异构的网络环境和软件的分发。

• 可移植性：

Java程序（后缀为java的文件）在Java平台上被编译为体系结构中立的字节码格式（后缀为class的文件），然后可以在实现这个Java平台的任何系统中运行。这种途径适合于异构的网络环境和软件的分发

• 解释型：

如前所述，Java程序在Java平台上被编译为字节码格式，然后可以在实现这个Java平台的任何系统中运行。在运行时，Java平台中的Java解释器对这些字节码进行解释执行，执行过程中需要的类在联接阶段被载入到运行环境中。

• 高性能：

与那些解释型的高级脚本语言相比，Java的确是高性能的。事实上，Java的运行速度随着JIT(Just-In-Time）编译器技术的发展越来越接近于C++

• 多线程：

在Java语言中，线程是一种特殊的对象，它必须由Thread类或其子（孙）类来创建。通常有两种方法来创建线程：其一，使用型构为Thread(Runnable)的构造子类将一个实现了Runnable接口的对象包装成一个线程，其二，从Thread类派生出子类并重写run方法，使用该子类创建的对象即为线程。值得注意的是Thread类已经实现了Runnable接口，因此，任何一个线程均有它的run方法，而run方法中包含了线程所要运行的代码。线程的活动由一组方法来控制。Java语言支持多个线程的同时执行，并提供多线程之间的同步机制（关键字为synchronized）

• 动态性：

Java语言的设计目标之一是适应于动态变化的环境。Java程序需要的类能够动态地被载入到运行环境，也可以通过网络来载入所需要的类。这也有利于软件的升级。另外，Java中的类有一个运行时刻的表示，能进行运行时刻的类型检查

因此：Java不仅仅是一门编程语言，也是一个由一些列计算机软件和规范组成的技术体系。

1.2JDK

sum 公司提供了一套 Java 开发环境，简称 JDK（Java Java Development Kit）。JDK 包括 Java编译器、Java运行工具、Java 文档生成工具、Java 打包工具等。

sum 公司除了提供 JDK外，还提供了一种 JRE（Java Runtime Environment）工具，它是提供给普通用户使用的 Java 运行环境。与 JDK 相比，JRE 工具只包含 Java运行工具，不包含 Jav 编译工具。需要说明的是，为了方便使用，sum 公司在 JDK 工具总封装了一个 JRE 工具，即开发环境中包含了运行环境。

1.2.1 安装 JDK和配置环境变量

1.2.2 JDK 目录介绍

JDK 安装完毕后，会在磁盘上生成一个目录，该目录被称为 JDK 目录。

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PS：可能有些小伙伴会疑问自己为什么没有src这个文件夹，这里是因为博主解压了src这个压缩包。

1. bin目录：该目录用于存放一些可执行程序，如：javac.exe（Java 编译器）、java.exe（Java 运行工具）、jar（打包工具）和javadoc.exe（文档生成工具）等。其中，最重要就是javac.exe和java.exe，下面我们对这两个程序进行讲解：
   • javac.exe是 Java 编译器，它可以将编写的 Java 文件编译成 Java 字节码文件（可执行的 Java 程序）。
   • java.exe是 Java 运行工具，它会启动一个 Java 虚拟机（JVM）进程，Java 虚拟机相当于一个虚拟的系统，专门运行由 Java 编译器生成的字节码文件（.class文件）
2. include目录：由于 JDK 是使用 C/C++ 开发的，因此在启动时需要引入一些C语言的头文件，该目录就是用于存放这些头文件的。
3. jre目录：jre 是 Java Runtime Environment 的缩写，意味 Java 程序运行时的环境。该目录是 Java 运行时环境的根目录，它包含 Java 虚拟机、运行时的类包、Java 应用启动器和一个bin 目录，但不包含开发环境中的开发工具。
4. lib目录：lib 是 library 的缩写，意为 Java 类库或库文件，是开发工具使用的归档包文件。
5. src.zip和javafx-src.zip文件：这两个文件中放置的是 JDK 核心类的源代码和 JavaFX 源代码，通过这两个文件可以查看 Java 基础类的源代码。

1.2.3 【面试题】：JDK、JRE、JVM之间的关系？*

• JDK(Java Development Kit):Java开发工具包，提供给Java程序员使用，包含了JRE，同时还包含了编译
  器javac与自带的调试工具Jconsole、jstack等。
• JRE(Java Runtime Environment):Java运行时环境，包含了JVM，Java基础类库。是使用Java语言编写程
  序运行的所需环境。
• JVM：Java虚拟机，运行Java代码

​

1.2.4 Java 跨平台、工作原理

• 一次编译，处处可用
• Java文件编译生成跟平台无关的字节码文件（class文件）
• JVM：Java虚拟机
• 由对应平台的JVM解析字节码为机器指令。

​

1.3 Java 程序开发

Java 程序，需要三个步骤：编写程序，编译程序，运行程序

​

1. 编写 Java 源文件

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"Hello world!"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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通过上述代码，我们可以看到一个完整的Java程序的结构，Java程序的结构由如下三个部分组成：

1. 源文件（扩展名为*.java)：源文件带有类的定义。类用来表示程序的一个组件，小程序或许只会有一个
   类。类的内容必须包含在花括号里面。
2. 类：类中带有一个或多个方法。方法必须在类的内部声明。
3. 方法：在方法的花括号中编写方法应该执行的语句。

总结：类存在于源文件里面；方法存在于类中；语句存在于方法中。

注意：在一个源文件中只能有一个public修饰的类，而且源文件名字必须与public修饰的类名字相同。

​

2. 运行 Java 程序

Java是一门半编译型、半解释型语言。先通过javac编译程序把源文件进行编译，编译后生成的.class文件是由字节码组成的平台无关、面向JVM的文件。最后启动java虚拟机来运行.class文件，此时JVM会将字节码转换成平台能够理解的形式来运行。

1. 编译程序：javac 文件名.java

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-cmd">javac HelloWorld.java </code></span></span>

   运行项目并下载源码cmd
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   ​

2. 运行程序：java 文件名

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-cmd">java HelloWorld </code></span></span>

   运行项目并下载源码cmd
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   ​

注意事项：

1. 第一个Java程序尽量用记事本写
2. 建议代码文件名全英文，首字母大写，满足驼峰模式，源代码文件后缀为.java

1.4 注释

1.4.1 什么注释

注释是写程序中对代码进行解释说明的文字，方便自己和其他人查看，以便理解程序的。

1.4.2 注释有哪些

• 单行注释：//
• 多行注释：/* */
• 文档注释：/** */

1.4.3 注释的特点

注释不影响程序的执行，编译后的class文件里面没有注释

​

1.5 标识符

标识符：在程序中由用户给类名、方法名或者变量所取的名字。

【硬性规则】

标识符中可以包含：字母、数字以及下划线和 $ 符号等等

注意：标识符不能以数字开头，不能是关键字，且严格区分大小写。

【软性建议】

• 类名：每个单词的首字母大写（大驼峰）
• 方法名：首字母小写，后面每个单词的首字母大写（小驼峰）
• 变量名：与方法名规则相同。

1.6 关键字

关键字是由Java语言提前定义好的，有特殊含义的标识符，或者保留字

注意：用户不能使用关键字定义标识符

​

2.数据类型和变量

2.1 字面常量

常量即程序运行时期，固定不变的量称为常量。

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"Hello world!"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#986801">100</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#986801">3.14</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">'A'</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#0184bb">true</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#0184bb">false</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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其中，“Hello world!”，100，3.14，‘A’，true/false都是常量，将其称为字面常量。

字面量的分类：

1. 字符串常量：由"“括起来的，比如"12345”、“hello”、“你好”。
2. 整型常量：程序中直接写的数字（注意没有小数点），比如：100、1000
3. 浮点型常量：程序中直接写的小数，比如：3.14、0.49
4. 字符常量：由’'括起来的单个字符，如：‘A’，‘1’
5. 布尔常量：只有两种true和false
6. 空常量：null

注意：字符串、整型、浮点型、字符型以及不而行，在Java中都称为数据类型。

2.2 数据类型

Java 是一种强类型语言。这意味着必须每一个变量声明一个类型。

Java 中数据类型主要分为两类：基本数据类型和引用数据类型。

​

注意：

• 无论是在32为系统还是64为系统，int都占用4个字节，long都占8个字节
• 整型和浮点型都是带有符号的
• 整型默认是int型，浮点型默认是double
• 字符串属于引用类型，后序介绍。

什么是字节?

字节是计算机中表示空间大小的基本单位.

计算机使用二进制表示数据. 我们认为 8 个二进制位(bit) 为一个字节(Byte).

我们平时的计算机为16GB 内存, 意思是16G 个字节.

其中 1KB = 1024 Byte, 1MB = 1024 KB, 1GB = 1024 MB.

所以 16GB 相当于 160 多亿个字节.

篇幅限制下面就只能给大家展示小册部分内容了。整理了一份Java零基础学习笔记包括了：Java基础、Javaweb、Java框架、后端中间件、Java微服务、计算机基础、算法、springcloud、Java虚拟机、spring源码、redis、MySQL、并发编程、微服务、

需要全套Java零基础进阶路线笔记【点击此处即可/免费获取】https://docs.qq.com/doc/DQUFvdURDeVF3UUpz?no_promotion=1&is_blank_or_template=blank

2.3 变量

2.3.1 变量的概念

在程序中，除了有始终不变的常量外，有些内容可以回经常改变，比如：人的年龄、身高、成绩分数、数学函数的计算结果等，对于这些经常改变的内容，在 Java 中称为变量。而数据类型就是用来定义不同种类变量

2.3.2 语法格式

定义变量的语法格式为：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java">数据类型 变量名 <span style="color:#a67f59">=</span> 初始值<span style="color:#999999">;</span> </code></span></span>

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比如：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#708090">// 定义整形变量a，a是变量名也称为标识符，该变量中放置的值为10</span> <span style="color:#0077aa">double</span> d <span style="color:#a67f59">=</span> <span style="color:#986801">3.14</span><span style="color:#999999">;</span> <span style="color:#0077aa">char</span> c <span style="color:#a67f59">=</span> <span style="color:#50a14f">'A'</span><span style="color:#999999">;</span> Boolean b <span style="color:#a67f59">=</span> <span style="color:#0184bb">true</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>d<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>c<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>b<span style="color:#999999">)</span><span style="color:#999999">;</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">100</span><span style="color:#999999">;</span> <span style="color:#708090">// a是变量，a中的值是可以修改的，</span> <span style="color:#708090">// 注意：= 在java中表示赋值，即将100交给a，a中保存的值就是100</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 注意：在一行可以定义多个相同类型的变量</span> <span style="color:#0077aa">int</span> a1 <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">,</span>a2 <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">,</span> a3 <span style="color:#a67f59">=</span> <span style="color:#986801">30</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a1<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a2<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a3<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

2.3.3 整型变量

2.3.3.1 整型变量

1. 方法一：定义时给出初始值

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#708090">// 方法一:定义时给出初始值</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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2. 方法二：定义时没有给初始值，但使用前必须设置初值

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#708090">// 方法二：定义时没有给初始值，但使用前必须设置初值</span> <span style="color:#0077aa">int</span> b<span style="color:#999999">;</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 使用方式二定义后，在使用前如果没有赋值，则编译期间会报错</span> <span style="color:#0077aa">int</span> c<span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>c<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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   ​

3. int型变量所能表示的范围：

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-Java">// int型变量所能表示的范围： System.out.println(Integer.MIN_VALUE);// -2147483648 System.out.println(Integer.MAX_VALUE);// 2147483647 // 注意：在定义int性变量时，所赋值不能超过int的范围 int d = 12345678901234; // 编译时报错，初值超过了int的范围 System.out.println(d); </code></span></span>

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   ​

注意事项：

1. int不论在何种系统下都是4个字节
2. 推荐使用方式一定义，如果没有合适的初始值，可以设置为0
3. 在给变量设置初始值时，值不能超过int的表示范围，否则会导致溢出
4. 变量在使用之前必须要赋初值，否则编译报错
5. int的包装类型为 Integer

2.3.3.2 长整型变量

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">long</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#708090">// long定义的长整型变量</span> <span style="color:#0077aa">long</span> c <span style="color:#a67f59">=</span> <span style="color:#986801">10L</span><span style="color:#999999">;</span> <span style="color:#708090">// 为了区分int和long类型，一般建议：long类型变量的初始值之后加L或者l</span> <span style="color:#0077aa">long</span> d <span style="color:#a67f59">=</span> <span style="color:#986801">10l</span><span style="color:#999999">;</span> <span style="color:#708090">// 一般更加以加大写L，因为小写l与1不好区分</span> <span style="color:#708090">// long型变量所能表示的范围：这个数据范围远超过 int 的表示范围. 足够绝大部分的工程场景使用.</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>Long<span style="color:#999999">.</span><span style="color:#986801">MIN_VALUE</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// -9223372036854775808</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>Long<span style="color:#999999">.</span><span style="color:#986801">MAX_VALUE</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// 9223372036854775807</span> </code></span></span>

注意事项：

1. 长整型变量的初始值后加L或者l，推荐加L
2. 长整型不论在那个系统下都占8个字节
3. 长整型的表示范围为：-263~263-1
4. long的包装类型为Long

2.3.3.3 短整型变量

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">short</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// short型变量所能表示的范围：</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>Short<span style="color:#999999">.</span><span style="color:#986801">MIN_VALUE</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// -32768</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>Short<span style="color:#999999">.</span><span style="color:#986801">MAX_VALUE</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// 32767</span> </code></span></span>

注意事项：

1. short在任何系统下都占2个字节
2. short的表示范围为：-32768 ~ 32767
3. 使用时注意不要超过范围(一般使用比较少)
4. short的包装类型为Short

2.3.3.4 字节型变量

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">byte</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// byte型变量所能表示的范围：</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>Byte<span style="color:#999999">.</span><span style="color:#986801">MIN_VALUE</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// -128</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>Byte<span style="color:#999999">.</span><span style="color:#986801">MAX_VALUE</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// 127</span> </code></span></span>

注意事项：

1. byte在任何系统下都占1个字节
2. byte的范围是：-128 ~ 127 3. 字节的包装类型为Byte

2.3.4 浮点型变量

2.3.4.1 双精度浮点型

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">double</span> d <span style="color:#a67f59">=</span> <span style="color:#986801">3.14</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>d<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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代码一：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">2</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">/</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 0</span> <span style="color:#708090">// 为什么不输出0.5?</span> </code></span></span>

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在 Java 中，int除以int的值仍然是int（会直接舍弃小数部分）。如果想要得到0.5，需要使用double类型计算。

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">double</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">1.0</span><span style="color:#999999">;</span> <span style="color:#0077aa">double</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">2.0</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">/</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// 0.5</span> </code></span></span>

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代码二：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">double</span> num <span style="color:#a67f59">=</span> <span style="color:#986801">1.1</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>num <span style="color:#a67f59">*</span> num<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 输出1.21?</span> <span style="color:#708090">// 1.2100000000000002</span> </code></span></span>

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注意事项：

1. double在任何系统下都占8个字节
2. 浮点数与整数在内存中的存储方式不同，不能单纯使用的形式来计算
3. double的包装类型为Double
4. double 类型的内存布局遵守 IEEE 754 标准(和C语言一样), 尝试使用有限的内存空间表示可能无限的小数, 势 必会存在一定的精度误差，因此浮点数是个近似值，并不是精确值

2.3.4.2 单精度浮点型

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java">loat num <span style="color:#a67f59">=</span> <span style="color:#986801">1.0f</span><span style="color:#999999">;</span> <span style="color:#708090">// 写作 1.0F 也可以</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>num<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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注意事项：

1. float 类型在 Java 中占四个字节, 同样遵守 IEEE 754 标准.
2. 由于表示的数据精度范围较小, 一般在工程上用到浮点数 都优先考虑 double, 不太推荐使用 float.
3. float的包装类型为Float。

2.3.5 字符型变量

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">char</span> c1 <span style="color:#a67f59">=</span> <span style="color:#50a14f">'A'</span><span style="color:#999999">;</span> <span style="color:#708090">// 大写字母</span> <span style="color:#0077aa">char</span> c2 <span style="color:#a67f59">=</span> <span style="color:#50a14f">'1'</span><span style="color:#999999">;</span> <span style="color:#708090">// 数字字符</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>c1<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>c2<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 注意：java中的字符可以存放整形</span> <span style="color:#0077aa">char</span> c3 <span style="color:#a67f59">=</span> <span style="color:#50a14f">'帅'</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>c3<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

注意事项：

1. Java 中使用 单引号 + 单个字母 的形式表示字符字面值.

2. 计算机中的字符本质上是一个整数. 在 C 语言中使用 ASCII 表示字符, 而 Java 中使用 Unicode 表示字符. 因此 一个字符占用两个字节, 表示的字符种类更多, 包括中文.

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">char</span> c3 <span style="color:#a67f59">=</span> <span style="color:#50a14f">'帅'</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>c3<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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3. char的包装类型为Character

2.3.6 布尔类型变量

布尔类型常用来表示真假，在现实生活中也是经常出现的，比如：听说xxx同学买彩票中了一个亿…，听到后估计 大部分人第一反应就是：我x，真的假的？

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">boolean</span> b <span style="color:#a67f59">=</span> <span style="color:#0184bb">true</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>b<span style="color:#999999">)</span><span style="color:#999999">;</span> b <span style="color:#a67f59">=</span> <span style="color:#0184bb">false</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>b<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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注意事项：

1. boolean 类型的变量只有两种取值,true表示真,false表示假.

2. Java 的 boolean 类型和 int 不能相互转换, 不存在1表示true,0表示false这样的用法.

​

1. Java虚拟机规范中，并没有明确规定boolean占几个字节，也没有专门用来处理boolean的字节码指令，在 Oracle公司的虚拟机实现中，boolean占1个字节.

2. boolean的包装类型为Boolean。

2.3.7 类型转换

我们经常需要将一种数据类型转换为另一种数据类型。

​

图中6个实线箭头，表示无信息丢失的转换；另外有3个虚线剪头，表示可能有精度损失的转换。

2.3.7.1 自动类型转换（隐式）

自动类型转换即：代码不需要经过任何处理，在代码编译时，编译器会自动进行处理。特点：数据范围小的转为数据范围大的时会自动进行。

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#986801">1024</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 整型默认情况下是int</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#986801">3.14</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 浮点型默认情况下是double</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">100</span><span style="color:#999999">;</span> <span style="color:#0077aa">long</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">10L</span><span style="color:#999999">;</span> b <span style="color:#a67f59">=</span> a<span style="color:#999999">;</span> <span style="color:#708090">// a和b都是整形，a的范围小，b的范围大，当将a赋值给b时，编译器会自动将a提升为long类型，然后赋值</span> a <span style="color:#a67f59">=</span> b<span style="color:#999999">;</span> <span style="color:#708090">// 编译报错，long的范围比int范围大，会有数据丢失，不安全</span> <span style="color:#0077aa">float</span> f <span style="color:#a67f59">=</span> <span style="color:#986801">3.14F</span><span style="color:#999999">;</span> <span style="color:#0077aa">double</span> d <span style="color:#a67f59">=</span> <span style="color:#986801">5.12</span><span style="color:#999999">;</span> d <span style="color:#a67f59">=</span> f<span style="color:#999999">;</span> <span style="color:#708090">// 编译器会将f转换为double，然后进行赋值</span> f <span style="color:#a67f59">=</span> d<span style="color:#999999">;</span> <span style="color:#708090">// double表示数据范围大，直接将float交给double会有数据丢失，不安全</span> <span style="color:#0077aa">byte</span> b1 <span style="color:#a67f59">=</span> <span style="color:#986801">100</span><span style="color:#999999">;</span> <span style="color:#708090">// 编译通过，100没有超过byte的范围，编译器隐式将100转换为byte</span> <span style="color:#0077aa">byte</span> b2 <span style="color:#a67f59">=</span> <span style="color:#986801">257</span><span style="color:#999999">;</span> <span style="color:#708090">// 编译失败，257超过了byte的数据范围，有数据丢失</span> </code></span></span>

​

2.3.7.2 强制类型转换（显式）

强制类型转换：当进行操作时，代码需要经过一定的格式处理，不能自动完成。特点：数据范围大的到数据范围小的。

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">long</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">100L</span><span style="color:#999999">;</span> b <span style="color:#a67f59">=</span> a<span style="color:#999999">;</span> <span style="color:#708090">// int-->long，数据范围由小到大，隐式转换</span> a <span style="color:#a67f59">=</span> <span style="color:#999999">(</span><span style="color:#0077aa">int</span><span style="color:#999999">)</span>b<span style="color:#999999">;</span> <span style="color:#708090">// long-->int, 数据范围由大到小，需要强转，否则编译失败</span> <span style="color:#0077aa">float</span> f <span style="color:#a67f59">=</span> <span style="color:#986801">3.14F</span><span style="color:#999999">;</span> <span style="color:#0077aa">double</span> d <span style="color:#a67f59">=</span> <span style="color:#986801">5.12</span><span style="color:#999999">;</span> d <span style="color:#a67f59">=</span> f<span style="color:#999999">;</span> <span style="color:#708090">// float-->double，数据范围由小到大，隐式转换</span> f <span style="color:#a67f59">=</span> <span style="color:#999999">(</span><span style="color:#0077aa">float</span><span style="color:#999999">)</span>d<span style="color:#999999">;</span> <span style="color:#708090">// double-->float, 数据范围由大到小，需要强转，否则编译失败</span> a <span style="color:#a67f59">=</span> d<span style="color:#999999">;</span> <span style="color:#708090">// 报错，类型不兼容</span> a <span style="color:#a67f59">=</span> <span style="color:#999999">(</span><span style="color:#0077aa">int</span><span style="color:#999999">)</span>d<span style="color:#999999">;</span> <span style="color:#708090">// int没有double表示的数据范围大，需要强转，小数点之后全部丢弃</span> <span style="color:#0077aa">byte</span> b1 <span style="color:#a67f59">=</span> <span style="color:#986801">100</span><span style="color:#999999">;</span> <span style="color:#708090">// 100默认为int，没有超过byte范围，隐式转换</span> <span style="color:#0077aa">byte</span> b2 <span style="color:#a67f59">=</span> <span style="color:#999999">(</span><span style="color:#0077aa">byte</span><span style="color:#999999">)</span><span style="color:#986801">257</span><span style="color:#999999">;</span> <span style="color:#708090">// 257默认为int，超过byte范围，需要显示转换，否则报错</span> <span style="color:#0077aa">boolean</span> flag <span style="color:#a67f59">=</span> <span style="color:#0184bb">true</span><span style="color:#999999">;</span> a <span style="color:#a67f59">=</span> flag<span style="color:#999999">;</span> <span style="color:#708090">// 编译失败：类型不兼容</span> flag <span style="color:#a67f59">=</span> a<span style="color:#999999">;</span> <span style="color:#708090">// 编译失败：类型不兼容</span> </code></span></span>

​

注意事项：

1. 不同数字类型的变量之间赋值, 表示范围更小的类型能隐式转换成范围较大的类型
2. 如果需要把范围大的类型赋值给范围小的, 需要强制类型转换, 但是可能精度丢失
3. 将一个字面值常量进行赋值的时候, Java 会自动针对数字范围进行检查
4. 强制类型转换不一定能成功，不相干的类型不能互相转换

2.3.8 类型提升

不同类型的数据之间相互运算时，数据类型小的会被提升到数据类型大的。

1. int与long之间：int会被提升为long

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">long</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> c <span style="color:#a67f59">=</span> a <span style="color:#a67f59">+</span> b<span style="color:#999999">;</span> <span style="color:#708090">// 编译出错: a + b==》int + long--> long + long 赋值给int时会丢失数据</span> <span style="color:#0077aa">long</span> d <span style="color:#a67f59">=</span> a <span style="color:#a67f59">+</span> b<span style="color:#999999">;</span> <span style="color:#708090">// 编译成功：a + b==>int + long--->long + long 赋值给long </span> </code></span></span>

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   ​

2. byte与byte的运算

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">byte</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">byte</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> <span style="color:#0077aa">byte</span> c <span style="color:#a67f59">=</span> a <span style="color:#a67f59">+</span> b<span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>c<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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   ​

结论：byte和byte都是相同类型, 但是出现编译报错. 原因是, 虽然 a 和 b 都是byte, 但是计算a + b会先将 a 和 b 都提升成int, 再进行计算, 得到的结果也是int, 这是赋给 c, 就会出现上述错误.

由于计算机的 CPU 通常是按照 4 个字节为单位从内存中读写数据. 为了硬件上实现方便, 诸如 byte 和 short 这种低于 4 个字节的类型, 会先提升成 int, 再参与计算

正确的写法：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">byte</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">byte</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> <span style="color:#0077aa">byte</span> c <span style="color:#a67f59">=</span> <span style="color:#999999">(</span><span style="color:#0077aa">byte</span><span style="color:#999999">)</span><span style="color:#999999">(</span>a <span style="color:#a67f59">+</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>c<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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【类型提升小结:】

1. 不同类型的数据混合运算, 范围小的会提升成范围大的.
2. 对于 short, byte 这种比 4 个字节小的类型, 会先提升成 4 个字节的 int , 再运算.

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2.4 字符串类型

在Java中使用String类定义字符串类型，比如：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java">String s1 <span style="color:#a67f59">=</span> <span style="color:#50a14f">"hello"</span><span style="color:#999999">;</span> String s2 <span style="color:#a67f59">=</span> <span style="color:#50a14f">" world"</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>s1<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>s2<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>s1<span style="color:#a67f59">+</span>s2<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// s1+s2表示：将s1和s2进行拼接</span> </code></span></span>

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在有些情况下，需要将字符串和整形数字之间进行转换：

1. int转成String

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> num <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#708090">// 方法1</span> String str1 <span style="color:#a67f59">=</span> num <span style="color:#a67f59">+</span> <span style="color:#50a14f">""</span><span style="color:#999999">;</span> <span style="color:#708090">// 方法2</span> String str2 <span style="color:#a67f59">=</span> String<span style="color:#999999">.</span><span style="color:#dd4a68">valueOf</span><span style="color:#999999">(</span>num<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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2. String转成int

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java">String str <span style="color:#a67f59">=</span> <span style="color:#50a14f">"100"</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> num <span style="color:#a67f59">=</span> Integer<span style="color:#999999">.</span><span style="color:#dd4a68">parseInt</span><span style="color:#999999">(</span>str<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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本节对只是对字符串进行简单的介绍，大家能够正常使用即可，后序会详细给大家介绍。

3.运算符

3.1 什么是运算符

计算机的最基本的用途之一就是执行数学运算，比如：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> a <span style="color:#a67f59">+</span> b<span style="color:#999999">;</span> a <span style="color:#a67f59"><</span> b<span style="color:#999999">;</span> </code></span></span>

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上述+和<等就是运算符，即：对操作数进行操作时的符号，不同运算符操作的含义不同。

作为一门计算机语言，Java也提供了一套丰富的运算符来操纵变量。Java中运算符可分为以下：算术运算符(+ - * /)、关系运算符(< > ==)、逻辑运算符、位运算符、移位运算符以及条件运算符等

3.2 算术运算符

1. 基本四则运算：加减乘除求模（+，-，*，/，%）

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">+</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 30</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">-</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 10</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">*</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 200</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">/</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 2</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">%</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 0 --->模运算相当于数学中除法的余数</span> </code></span></span>

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   【注意】：

   • 都是二元运算符，使用时必须要有左右两个操作数

   • int/int结果还是int类型，而且会向下取整

     <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">3</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">2</span><span style="color:#999999">;</span> <span style="color:#708090">// 在数学中应该是1.5 但是在Java中输出结果为1 会向下取整，即小数点之后全部舍弃掉了</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">/</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 1</span> <span style="color:#708090">// 如果要得到数学中的结果，可以使用如下方式</span> <span style="color:#0077aa">double</span> d <span style="color:#a67f59">=</span> a <span style="color:#a67f59">*</span> <span style="color:#986801">1.0</span> <span style="color:#a67f59">/</span> b<span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>d<span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// 1.5</span> </code></span></span>

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   • 做除法和取模时，右操作数不能为0

     ​

   • % 不仅可以对整型取模，也可以对double类型取模，但是没有意义，一般都是对整型取模的

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java">System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#986801">11.5</span> <span style="color:#a67f59">%</span> <span style="color:#986801">2.0</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// 1.5</span> </code></span></span>

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   • 两侧操作数类型不一致时，向类型大的提升

     <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java">System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#986801">1</span> <span style="color:#a67f59">+</span> <span style="color:#986801">0.2</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// +的左侧是int，右侧是double，在加之前int被提升为double</span> <span style="color:#708090">// 1.2</span> </code></span></span>

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2. 结合赋值和运算符（+=，-=，*=，/=，%=）

   该种类型运算符操作完成后，会将操纵的结果赋值给左操作数.

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> a <span style="color:#a67f59">+=</span> <span style="color:#986801">2</span><span style="color:#999999">;</span> <span style="color:#708090">// 相当于 a = a + 2</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 输出3</span> a <span style="color:#a67f59">-=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span><span style="color:#708090">// 相当于 a = a - 1</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 输出2</span> a <span style="color:#a67f59">*=</span> <span style="color:#986801">3</span><span style="color:#999999">;</span><span style="color:#708090">// 相当于 a = a * 3</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 输出6</span> a <span style="color:#a67f59">/=</span> <span style="color:#986801">3</span><span style="color:#999999">;</span><span style="color:#708090">// 相当于 a = a / 3</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 输出2</span> a <span style="color:#a67f59">%=</span> <span style="color:#986801">3</span><span style="color:#999999">;</span> <span style="color:#708090">// 相当于 a = a % 2</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 输出2</span> </code></span></span>

   运行项目并下载源码java运行

   【注意】：只有变量才能使用该运算符，常量不能使用。

3. 自增/自减运算符（++，--）

   ++是一种自增的操作符，又分为前置++和后置++，–是一种自增的操作符，又分为前置–-和后置–-。

   1. 前置++

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test1 <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#a67f59">++</span>a<span style="color:#999999">;</span><span style="color:#708090">// ++的操作数是a，是放在a的前面的，就是前置++</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">printf</span><span style="color:#999999">(</span><span style="color:#50a14f">"a = %d , b = %d"</span><span style="color:#999999">,</span>a <span style="color:#999999">,</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 运行结果：a = 11 , b = 11</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

   运行项目并下载源码java运行

   计算口诀：先 +1，后使用
   a原来是10，先 +1，后a变成了11，再使用赋值给b，b得到的也是11，所以计算后，a和b都是11，等价于这样的代码：

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test1 <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> a <span style="color:#a67f59">+=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> a<span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">printf</span><span style="color:#999999">(</span><span style="color:#50a14f">"a = %d , b = %d"</span><span style="color:#999999">,</span>a <span style="color:#999999">,</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 运行结果：a = 11 , b = 10</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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   1. 后置++

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test1 <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> a<span style="color:#a67f59">++</span><span style="color:#999999">;</span><span style="color:#708090">// ++的操作数是a，是放在a的后面的，就是后置++</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">printf</span><span style="color:#999999">(</span><span style="color:#50a14f">"a = %d , b = %d"</span><span style="color:#999999">,</span>a <span style="color:#999999">,</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 运行结果：a = 11 , b = 10</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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   计算口诀：先使用，后 +1
   a原来是10，先使用，把a赋值给b，b变成了10，后a+1变成了10，所以计算后，a=11,b=10，等价于这样的代码：

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test1 <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> a<span style="color:#999999">;</span> a <span style="color:#a67f59">+=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">printf</span><span style="color:#999999">(</span><span style="color:#50a14f">"a = %d , b = %d"</span><span style="color:#999999">,</span>a <span style="color:#999999">,</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 运行结果：a = 11 , b = 10</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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   1. 前置--

   和前置++同理，只是换成了-1
   计算口诀：先 -1，后使用

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test1 <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#a67f59">--</span>a<span style="color:#999999">;</span><span style="color:#708090">// --的操作数是a，是放在a的前面的，就是前置--</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">printf</span><span style="color:#999999">(</span><span style="color:#50a14f">"a = %d , b = %d"</span><span style="color:#999999">,</span>a <span style="color:#999999">,</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 运行结果：a = 9 , b = 9</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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   1. 后置--

   和后置++同理，只是换成了-1
   计算口诀：先使用，后-1

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test1 <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> a<span style="color:#a67f59">--</span><span style="color:#999999">;</span><span style="color:#708090">// --的操作数是a，是放在a的后面的，就是后置--</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">printf</span><span style="color:#999999">(</span><span style="color:#50a14f">"a = %d , b = %d"</span><span style="color:#999999">,</span>a <span style="color:#999999">,</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 运行结果：a = 9 , b = 10</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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   【注意】：

   • 如果混合使用，【前置++】先+1，然后使用变量+1之后的值，【后置++】先使用变量原来的值，表达式 结束时给变量+1
   • 只有变量才能使用自增/自减运算符，常量不能使用，因为常量不允许被修改

3.3 关系运算符

关系远算符有6个：（==，!=，>，<，>=，<=），其结果是true和flase

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> <span style="color:#708090">// 注意：在Java中 = 表示赋值，要与数学中的含义区分</span> <span style="color:#708090">//在Java中 == 表示相等</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">==</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// false</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">!=</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// true</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59"><</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// true</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">></span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// false</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59"><=</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// true</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">>=</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// false</span> </code></span></span>

【注意】：当需要多次判断时，不能连着写，比如：3 < a < 5，Java程序与数学中是有区别的

3.4 逻辑运算符（重点）

逻辑远算符主要由3个：（&&，||，!），运算结果都是boolean类型。

1. 逻辑与 &&

   语法规则：表达式1&&表达式2，左右表达式必须是boolean类型的结果。

   两个表达式都为真，结果才是真，只要有一个是假，结果就是假。

   表达式1	表达式2	结果
   真	真	真
   真	假	假
   假	真	假
   假	假	假

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">2</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">==</span> <span style="color:#986801">1</span> <span style="color:#a67f59">&&</span> b <span style="color:#a67f59">==</span> <span style="color:#986801">2</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 左为真 且 右为真 则结果为真</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">==</span> <span style="color:#986801">1</span> <span style="color:#a67f59">&&</span> b <span style="color:#a67f59">></span> <span style="color:#986801">100</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 左为真 但 右为假 则结果为假</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">></span> <span style="color:#986801">100</span> <span style="color:#a67f59">&&</span> b <span style="color:#a67f59">==</span> <span style="color:#986801">2</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 左为假 但 右为真 则结果为假</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">></span> <span style="color:#986801">100</span> <span style="color:#a67f59">&&</span> b <span style="color:#a67f59">></span> <span style="color:#986801">100</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 左为假 且 右为假 则结果为假</span> </code></span></span>

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2. 逻辑或 ||

   语法规则：表达式1||表达式2，左右表达式必须是boolean类型的结果。

   两个表达式都为假，结果才是假，只要由一个是真，结果就是真。

   表达式1	表达式2	结果
   真	真	真
   真	假	真
   假	真	真
   假	假	假

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">2</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">==</span> <span style="color:#986801">1</span> <span style="color:#a67f59">||</span> b <span style="color:#a67f59">==</span> <span style="color:#986801">2</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 左为真 且 右为真 则结果为真</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">==</span> <span style="color:#986801">1</span> <span style="color:#a67f59">||</span> b <span style="color:#a67f59">></span> <span style="color:#986801">100</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 左为真 但 右为假 则结果也为真</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">></span> <span style="color:#986801">100</span> <span style="color:#a67f59">||</span> b <span style="color:#a67f59">==</span> <span style="color:#986801">2</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 左为假 但 右为真 则结果也为真</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">></span> <span style="color:#986801">100</span> <span style="color:#a67f59">||</span> b <span style="color:#a67f59">></span> <span style="color:#986801">100</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 左为假 且 右为假 则结果为假</span> </code></span></span>

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3. 逻辑非 !

   语法规则：! 表达式

   真变假，假变真。

   表达式	结果
   真	假
   假	真

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#a67f59">!</span><span style="color:#999999">(</span>a <span style="color:#a67f59">==</span> <span style="color:#986801">1</span><span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// a == 1 为true，取个非就是false</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#a67f59">!</span><span style="color:#999999">(</span>a <span style="color:#a67f59">!=</span> <span style="color:#986801">1</span><span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// a != 1 为false，取个非就是true</span> </code></span></span>

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4. 短路求值

   && 和 || 遵守短路求值的规则.

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java">System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#986801">10</span> <span style="color:#a67f59">></span> <span style="color:#986801">20</span> <span style="color:#a67f59">&&</span> <span style="color:#986801">10</span> <span style="color:#a67f59">/</span> <span style="color:#986801">0</span> <span style="color:#a67f59">==</span> <span style="color:#986801">0</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 打印 false</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#986801">10</span> <span style="color:#a67f59"><</span> <span style="color:#986801">20</span> <span style="color:#a67f59">||</span> <span style="color:#986801">10</span> <span style="color:#a67f59">/</span> <span style="color:#986801">0</span> <span style="color:#a67f59">==</span> <span style="color:#986801">0</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 打印 true</span> </code></span></span>

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   我们都知道, 计算10 / 0会导致程序抛出异常. 但是上面的代码却能正常运行, 说明10 / 0并没有真正被求值.

   【注意】：

   1. 对于 && ,如果左侧表达式值为 false, 则表达式结果一定是 false, 无需计算右侧表达式

   2. 对于 ||,如果左侧表达式值为 true, 则表达式结果一定是 true, 无需计算右侧表达式.

   3. & 和 | 如果表达式结果为 boolean 时, 也表示逻辑运算. 但与 && || 相比, 它们不支持短路求值.

      ​

      记忆口诀：

      &&：全真为真，有假必假，遇假则停
      ||：全假为假，有真必真，遇真则停
      !：真变假，假变真

3.5 位运算符

Java 中数据存储的最小单位是字节，而数据操作的最小单位是比特位. 字节是最小的存储单位，每个字节是由8个二进制比特位组成的，多个字节组合在一起可以表示各种不同的数据。

位运算符主要有四个:&，|，~，^，除~是一元运算符外，其余都是二元运算符。

位操作表示按二进制位运算. 计算机中都是使用二进制来表示数据的(01构成的序列), 按位运算就是在按照二进制位 的每一位依次进行计算。

1. 按位与 &: 如果两个二进制位都是 1, 则结果为 1, 否则结果为 0

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">&</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 0000 1010 10的二进制</span> <span style="color:#708090">// 0001 0100 20的二进制</span> <span style="color:#708090">// 0000 0000</span> </code></span></span>

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2. 按位或 |: 如果两个二进制位都是 0, 则结果为 0, 否则结果为 1

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>a <span style="color:#a67f59">|</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 0000 1010</span> <span style="color:#708090">// 0001 0100</span> <span style="color:#708090">// 0001 1110</span> </code></span></span>

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3. .按位取反 ~: 如果该位为 0 则转为 1, 如果该位为 1 则转为 0

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">0xf</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">printf</span><span style="color:#999999">(</span><span style="color:#50a14f">"%x\n"</span><span style="color:#999999">,</span> <span style="color:#a67f59">~</span>a<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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   【注意】：

   • 0x 前缀的数字为 十六进制 数字. 十六进制可以看成是二进制的简化表示方式. 一个十六进制数字对应 4 个二进制位.
   • 0xf 表示 10 进制的 15, 也就是二进制的 1111
   • printf 能够格式化输出内容, %x 表示按照十六进制输出.
   • \n 表示换行符

4. 按位异或 ^: 如果两个数字的二进制位相同, 则结果为 0, 相异则结果为 1

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">0x1</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">0x2</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">printf</span><span style="color:#999999">(</span><span style="color:#50a14f">"%x\n"</span><span style="color:#999999">,</span> a <span style="color:#a67f59">^</span> b<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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   【注意】：如果两个数相同，则异或的结果为0

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3.6 移位运算符（了解）

移位运算符有三个:<<，>>，>>>，都是二元运算符，且都是按照二进制比特位来运算的。

1. 左移 <<: 最左侧位不要了, 最右侧补 0

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">0x10</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">printf</span><span style="color:#999999">(</span><span style="color:#50a14f">"%x\n"</span><span style="color:#999999">,</span> a <span style="color:#a67f59"><<</span> <span style="color:#986801">1</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 运行结果(注意, 是按十六进制打印的)</span> <span style="color:#708090">//20</span> </code></span></span>

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   【注意】：向左移位时，丢弃的是符号位，因此正数左移可能会编程负数

2. 右移 >>: 最右侧位不要了, 最左侧补符号位(正数补0, 负数补1)

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">0x10</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">printf</span><span style="color:#999999">(</span><span style="color:#50a14f">"%x\n"</span><span style="color:#999999">,</span> a <span style="color:#a67f59">>></span> <span style="color:#986801">1</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 运行结果(注意, 是按十六进制打印的)</span> <span style="color:#708090">//8</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">0xffff0000</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">printf</span><span style="color:#999999">(</span><span style="color:#50a14f">"%x\n"</span><span style="color:#999999">,</span> b <span style="color:#a67f59">>></span> <span style="color:#986801">1</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 运行结果(注意, 是按十六进制打印的)</span> <span style="color:#708090">//ffff8000</span> </code></span></span>

1. 无符号右移 >>>: 最右侧位不要了, 最左侧补 0.

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">0xffffffff</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">printf</span><span style="color:#999999">(</span><span style="color:#50a14f">"%x\n"</span><span style="color:#999999">,</span> a <span style="color:#a67f59">>>></span> <span style="color:#986801">1</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 运行结果(注意, 是按十六进制打印的)</span> <span style="color:#708090">//7ffffff</span> </code></span></span>

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【注意】：

1. 左移 1 位, 相当于原数字 * 2. 左移 N 位, 相当于原数字 * 2 的N次方.
2. 右移 1 位, 相当于原数字 / 2. 右移 N 位, 相当于原数字 / 2 的N次方.
3. 由于计算机计算移位效率高于计算乘除, 当某个代码正好乘除 2 的N次方的时候可以用移位运算代替.
4. 移动负数位或者移位位数过大都没有意义.

3.7 条件运算符

条件运算符只有一个:表达式1 ? 表达式2 : 表达式3

当表达式1 的值为true时, 整个表达式的值为表达式2的值;

当 表达式1 的值为false 时, 整个表达式的值为表达式3的值.

也是 Java 中唯一的一个三目运算符, 是条件判断语句的简化写法.

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#708090">// 求两个整数的最大值</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> max <span style="color:#a67f59">=</span> a <span style="color:#a67f59">></span> b <span style="color:#a67f59">?</span> a <span style="color:#a67f59">:</span> b<span style="color:#999999">;</span> </code></span></span>

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【注意】：

1. 表达式2和表达式3的结果要是同类型的，除非能发生类型隐式类型转换

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> c <span style="color:#a67f59">=</span> a <span style="color:#a67f59">></span> b<span style="color:#a67f59">?</span> <span style="color:#986801">1</span> <span style="color:#a67f59">:</span> <span style="color:#986801">2.0</span><span style="color:#999999">;</span> </code></span></span>

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2. 表达式不能单独存在，其产生的结果必须要被使用。

   ​

3.8 运算符的优先级

在一条表达式中，各个运算符可以混合起来进行运算，但是运算符的优先级不同，比如：*和/的优先级要高于+和-，有些情况下稍不注意，可能就会造成很大的麻烦。

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#708090">// 求a和b的平均值</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> c <span style="color:#a67f59">=</span> a <span style="color:#a67f59">+</span> <span style="color:#999999">(</span>b <span style="color:#a67f59">-</span> a<span style="color:#999999">)</span> <span style="color:#a67f59">>></span> <span style="color:#986801">1</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>c<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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上述表达式中，由于+的优先级要高于>>, 因此a先和b-a的结果做加法，整体为20，最后再进行右移，因此结果 为10。

【注意】：运算符之间是有优先级的. 具体的规则我们不必记忆. 在可能存在歧义的代码中加上括号即可.

4. 程序逻辑控制

4.1 顺序结构

顺序结构比较简单，按照代码书写的顺序一行一行执行。

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"aaa"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"bbb"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"ccc"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">/* 运行结果 aaa bbb ccc */</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

如果调整代码的书写顺序, 则执行顺序也发生变化。

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"aaa"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"ccc"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"bbb"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">/* 运行结果 aaa ccc bbb */</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

4.2 分支结构（选择结构）

它的作用是根据判断的条件是否成立（真或假），来决定后续代码执行顺序。

举例：

如同在岔路口做选择。不同的选择会带来不同的路径及结果。

​

4.2.1 if 语句

1. 语法格式1

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">if</span> <span style="color:#999999">(</span>布尔表达式<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#708090">// 语句</span> <span style="color:#999999">}</span> </code></span></span>

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   如果布尔表达式结果为true，执行if中的语句，否则不执行。

   执行流程：

   ​

   举例：小明，如果这次考试考60分或以上就不挂科。

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> score <span style="color:#a67f59">=</span> <span style="color:#986801">90</span><span style="color:#999999">;</span> <span style="color:#0077aa">if</span> <span style="color:#999999">(</span>score <span style="color:#a67f59">>=</span> <span style="color:#986801">60</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"不挂科"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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2. 语法格式2

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">if</span> <span style="color:#999999">(</span>布尔表达式<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#708090">// 语句</span> <span style="color:#999999">}</span> <span style="color:#0077aa">else</span> <span style="color:#999999">{</span> <span style="color:#708090">// 语句</span> <span style="color:#999999">}</span> </code></span></span>

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   如果布尔表达式结果为true，执行if中的语句，执行else中的语句。

   执行流程：

   ​

   举例：小明，如果这次考试考60分或以上就不挂科，没有考到60分就挂科。

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> score <span style="color:#a67f59">=</span> <span style="color:#986801">90</span><span style="color:#999999">;</span> <span style="color:#0077aa">if</span> <span style="color:#999999">(</span>score <span style="color:#a67f59">>=</span> <span style="color:#986801">60</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"不挂科"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">else</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"挂科"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

   运行项目并下载源码java运行

3. 语法格式3

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">if</span><span style="color:#999999">(</span>布尔表达式<span style="color:#986801">1</span><span style="color:#999999">)</span><span style="color:#999999">{</span> <span style="color:#708090">// 语句1</span> <span style="color:#999999">}</span><span style="color:#0077aa">else</span> <span style="color:#0077aa">if</span><span style="color:#999999">(</span>布尔表达式<span style="color:#986801">2</span><span style="color:#999999">)</span><span style="color:#999999">{</span> <span style="color:#708090">// 语句2</span> <span style="color:#999999">}</span><span style="color:#0077aa">else</span><span style="color:#999999">{</span> <span style="color:#708090">// 语句2</span> <span style="color:#999999">}</span> </code></span></span>

   运行项目并下载源码java运行

   表达式1成立，执行语句1，否则表达式2成立，执行语句2，否则执行语句3

   比如：考虑到学生自尊，不公开分数排名，因此：

   • 分数在 [90, 100] 之间的，为优秀
   • 分数在 [80, 90) 之前的，为良好 分数在 [70, 80) 之间的，为中等
   • 分数在 [60, 70) 之间的，为及格 分数在 [ 0, 60) 之间的，为不及格
   • 错误数据

   按照上述办法通知学生成绩。

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> score <span style="color:#a67f59">=</span> <span style="color:#986801">90</span><span style="color:#999999">;</span> <span style="color:#0077aa">if</span><span style="color:#999999">(</span>score <span style="color:#a67f59">>=</span> <span style="color:#986801">90</span><span style="color:#999999">)</span><span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"优秀"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span><span style="color:#0077aa">else</span> <span style="color:#0077aa">if</span><span style="color:#999999">(</span>score <span style="color:#a67f59">>=</span> <span style="color:#986801">80</span> <span style="color:#a67f59">&&</span> score <span style="color:#a67f59"><</span> <span style="color:#986801">90</span><span style="color:#999999">)</span><span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"良好"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span><span style="color:#0077aa">else</span> <span style="color:#0077aa">if</span><span style="color:#999999">(</span>score <span style="color:#a67f59">>=</span> <span style="color:#986801">70</span> <span style="color:#a67f59">&&</span> score <span style="color:#a67f59"><</span> <span style="color:#986801">80</span><span style="color:#999999">)</span><span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"中等"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span><span style="color:#0077aa">else</span> <span style="color:#0077aa">if</span><span style="color:#999999">(</span>score <span style="color:#a67f59">>=</span> <span style="color:#986801">60</span> <span style="color:#a67f59">&&</span> score <span style="color:#a67f59"><</span> <span style="color:#986801">70</span><span style="color:#999999">)</span><span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"及格"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span><span style="color:#0077aa">else</span> <span style="color:#0077aa">if</span><span style="color:#999999">(</span>score <span style="color:#a67f59">>=</span> <span style="color:#986801">0</span> <span style="color:#a67f59">&&</span> score <span style="color:#a67f59"><</span> <span style="color:#986801">60</span><span style="color:#999999">)</span><span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"不及格"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span><span style="color:#0077aa">else</span><span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"错误数据"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

   运行项目并下载源码java运行

【注意事项】：

• 代码风格

  <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"> <span style="color:#708090">// 风格1-----> 推荐</span> <span style="color:#0077aa">int</span> x <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">if</span> <span style="color:#999999">(</span>x <span style="color:#a67f59">==</span> <span style="color:#986801">10</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#708090">// 语句1</span> <span style="color:#999999">}</span> <span style="color:#0077aa">else</span> <span style="color:#999999">{</span> <span style="color:#708090">// 语句2</span> <span style="color:#999999">}</span> <span style="color:#708090">// 风格2</span> <span style="color:#0077aa">int</span> x <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">if</span> <span style="color:#999999">(</span>x <span style="color:#a67f59">==</span> <span style="color:#986801">10</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#708090">// 语句1</span> <span style="color:#999999">}</span> <span style="color:#0077aa">else</span> <span style="color:#999999">{</span> <span style="color:#708090">// 语句2</span> <span style="color:#999999">}</span> </code></span></span>

  运行项目并下载源码java

  虽然两种方式都是合法的, 但是 Java 中更推荐使用风格1，代码跟紧凑。

• 分号问题

  <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> x <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> <span style="color:#0077aa">if</span> <span style="color:#999999">(</span>x <span style="color:#a67f59">==</span> <span style="color:#986801">10</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"hehe"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#708090">// 运行结果</span> hehe </code></span></span>

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  此处多写了一个 分号, 导致分号成为了if语句的语句体, 而 { } 中的代码已经成为了和一个 if 无关的代码块，所以运行结果是haha，而不是空白。

• 悬垂else问题

  <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> x <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> y <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">if</span> <span style="color:#999999">(</span>x <span style="color:#a67f59">==</span> <span style="color:#986801">10</span><span style="color:#999999">)</span> <span style="color:#0077aa">if</span> <span style="color:#999999">(</span>y <span style="color:#a67f59">==</span> <span style="color:#986801">10</span><span style="color:#999999">)</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"aaa"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">else</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"bbb"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

  运行项目并下载源码java运行

  if else语句中可以不加大括号 . 但是也可以写语句(只能写一条语句). 此时else是和最接近的if匹配. 但是实际开发中我们不建议这么写. 最好加上大括号.

  ​

4.2.2 switch 语句

基本语句

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">switch</span><span style="color:#999999">(</span>表达式<span style="color:#999999">)</span><span style="color:#999999">{</span> <span style="color:#0077aa">case</span> 常量值<span style="color:#986801">1</span><span style="color:#a67f59">:</span><span style="color:#999999">{</span> 语句<span style="color:#986801">1</span><span style="color:#999999">;</span> <span style="color:#999999">[</span><span style="color:#0077aa">break</span><span style="color:#999999">;</span><span style="color:#999999">]</span> <span style="color:#999999">}</span> <span style="color:#0077aa">case</span> 常量值<span style="color:#986801">2</span><span style="color:#a67f59">:</span><span style="color:#999999">{</span> 语句<span style="color:#986801">2</span><span style="color:#999999">;</span> <span style="color:#999999">[</span><span style="color:#0077aa">break</span><span style="color:#999999">;</span><span style="color:#999999">]</span> <span style="color:#999999">}</span> <span style="color:#999999">.</span><span style="color:#999999">.</span><span style="color:#999999">.</span> <span style="color:#0077aa">default</span><span style="color:#a67f59">:</span><span style="color:#999999">{</span> 内容都不满足时执行语句<span style="color:#999999">;</span> <span style="color:#999999">[</span><span style="color:#0077aa">break</span><span style="color:#999999">;</span><span style="color:#999999">]</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

执行流程：

1. 先计算表达式的值
2. 和case依次比较，一旦有响应的匹配就执行该项下的语句，直到遇到break时结束
3. 当表达式的值没有与所列项匹配时，执行default

代码示例：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> String week <span style="color:#a67f59">=</span> <span style="color:#50a14f">"周四"</span><span style="color:#999999">;</span> <span style="color:#0077aa">switch</span> <span style="color:#999999">(</span>week<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">case</span> <span style="color:#50a14f">"周一"</span><span style="color:#a67f59">:</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"埋头苦干，写程序"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">break</span><span style="color:#999999">;</span> <span style="color:#0077aa">case</span> <span style="color:#50a14f">"周二"</span><span style="color:#a67f59">:</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"请求学长帮忙解决bug"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">break</span><span style="color:#999999">;</span> <span style="color:#0077aa">case</span> <span style="color:#50a14f">"周三"</span><span style="color:#a67f59">:</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"今晚烧烤、小龙虾"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">break</span><span style="color:#999999">;</span> <span style="color:#0077aa">case</span> <span style="color:#50a14f">"周四"</span><span style="color:#a67f59">:</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"帮助学妹解决bug"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">break</span><span style="color:#999999">;</span> <span style="color:#0077aa">case</span> <span style="color:#50a14f">"周五"</span><span style="color:#a67f59">:</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"今晚吃鸡"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">break</span><span style="color:#999999">;</span> <span style="color:#0077aa">case</span> <span style="color:#50a14f">"周六"</span><span style="color:#a67f59">:</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"上GitHub交友"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">break</span><span style="color:#999999">;</span> <span style="color:#0077aa">case</span> <span style="color:#50a14f">"周日"</span><span style="color:#a67f59">:</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"郁郁寡欢、准备上课"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">default</span><span style="color:#a67f59">:</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"输入错误"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

【注意事项】：

• 多个case后的常量值不可以重复

• switch的括号内只能是以下类型的表达式：

  • 基本类型：byte、char、short、int，注意不能是long类型
  • 引用类型：String常量串、枚举类型

• break不要遗漏, 否则会失去 “多分支选择” 的效果

  <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> day <span style="color:#a67f59">=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> <span style="color:#0077aa">switch</span><span style="color:#999999">(</span>day<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">case</span> <span style="color:#986801">1</span><span style="color:#a67f59">:</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"星期一"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// break;</span> <span style="color:#0077aa">case</span> <span style="color:#986801">2</span><span style="color:#a67f59">:</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"星期二"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">break</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#708090">// 运行结果</span> 星期一 星期二 </code></span></span>

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• switch不能表达复杂的条件

  <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#708090">// 例如: 如果 num 的值在 10 到 20 之间, 就打印 hehe</span> <span style="color:#708090">// 这样的代码使用 if 很容易表达, 但是使用 switch 就无法表示. </span> <span style="color:#0077aa">if</span> <span style="color:#999999">(</span>num <span style="color:#a67f59">></span> <span style="color:#986801">10</span> <span style="color:#a67f59">&&</span> num <span style="color:#a67f59"><</span> <span style="color:#986801">20</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"hehe"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> </code></span></span>

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• switch虽然支持嵌套, 但是很丑，一般不推荐

  <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> x <span style="color:#a67f59">=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> y <span style="color:#a67f59">=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> <span style="color:#0077aa">switch</span><span style="color:#999999">(</span>x<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">case</span> <span style="color:#986801">1</span><span style="color:#a67f59">:</span> <span style="color:#0077aa">switch</span><span style="color:#999999">(</span>y<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">case</span> <span style="color:#986801">1</span><span style="color:#a67f59">:</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"hehe"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">break</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">break</span><span style="color:#999999">;</span> <span style="color:#0077aa">case</span> <span style="color:#986801">2</span><span style="color:#a67f59">:</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"haha"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">break</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

  运行项目并下载源码java运行

综上, 我们发现,switch的使用局限性是比较大的

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4.3 循环结构

循环结构它是在满足条件的情况下，反复执行某一段代码的计算过程。

举例：

我们要围着操场跑 5 圈，跑圈这个行为就重复了 5 次，也就是循环了 5 次。

​

4.3.1 while 语句

基本格式：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">while</span> <span style="color:#999999">(</span>循环条件<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#708090">// 语句</span> <span style="color:#999999">}</span> </code></span></span>

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执行流程：

​

首先执行判断表达式，表达式的值为false，循环直接结束；表达式的值为true，则执行循环语句，语句执行完后再继续判断，是否进行下一次判断。

代码示例：打印1~10的值

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> i <span style="color:#a67f59">=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> <span style="color:#0077aa">while</span> <span style="color:#999999">(</span>i <span style="color:#a67f59"><=</span> <span style="color:#986801">10</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>i<span style="color:#999999">)</span><span style="color:#999999">;</span> i<span style="color:#a67f59">++</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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4.3.2 for 语句

基本格式：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">for</span> <span style="color:#999999">(</span>表达式<span style="color:#986801">1</span><span style="color:#999999">;</span>表达式<span style="color:#986801">2</span><span style="color:#999999">;</span>表达式<span style="color:#986801">3</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#708090">// 语句</span> <span style="color:#999999">}</span> </code></span></span>

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表达式1：用于循环变量的初始化
表达式2：用于循环结束条件的判断
表达式3：用于循环变量的调整

执行流程：

首先执行表达式1初始化循环变量，接下来就是执行表达式2的判断部分，表达式2的结果如果为false，则循环结束；如果表达式2的结果为true，则执行循环语句，循环语句执行完后，再去执行表达式3，调整循环变量，然后再去表达式2的地方执行判断，表达式2的结果是否为false，决定循环是否继续。
整个循环的过程中，表达式1初始化部分只被执行1次，剩下的就是表达式2、循环语句、表达式3在循环

​

代码示例：打印1~10的值

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">for</span> <span style="color:#999999">(</span><span style="color:#0077aa">int</span> i <span style="color:#a67f59">=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> i <span style="color:#a67f59"><=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> i<span style="color:#a67f59">++</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>i<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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4.3.3 do…while循环

基本格式：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">do</span><span style="color:#999999">{</span> 语句 <span style="color:#999999">}</span><span style="color:#0077aa">while</span><span style="color:#999999">(</span>表达式<span style="color:#999999">)</span><span style="color:#999999">;</span> </code></span></span>

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while和for这两种循环都是先判断，条件如果满足就进入循环，执行循环语句，如果不满足就跳出循环；
而do...while循环则是先直接进入循环体，执行循环内部，然后在执行while后的判断表达式，表达式为真，就会进行下一次，表达式为false，则不再继续循环。

执行流程：

在do...while循环中先执行图上的“语句”，执行完语句，在去执行“判断表达式”，在判断表达式的结果是true，则继续循环，执行循环；判断表达式的结果false，则循环结束。
所以在do...while语句中循环体是至少执行一次的，这是do...while循环比较特殊的地方。

​

代码示例：打印1~10的值

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> i <span style="color:#a67f59">=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> <span style="color:#0077aa">do</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>i<span style="color:#999999">)</span><span style="color:#999999">;</span> i<span style="color:#a67f59">++</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">while</span> <span style="color:#999999">(</span>i <span style="color:#a67f59"><=</span> <span style="color:#986801">10</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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4.3.4 while语句 和 for语句的对比

​

4.4 break和continue

在循环执行的过程中，如果某些状况发生的时候，需要提前终止循环。

• break的作用是用于永久的终止循环，只要break被执行，直接就会跳出循环，继续往后执行。

  代码示例：找到100~200之间的第一个3的倍数

  <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">for</span> <span style="color:#999999">(</span><span style="color:#0077aa">int</span> num <span style="color:#a67f59">=</span> <span style="color:#986801">100</span><span style="color:#999999">;</span> num <span style="color:#a67f59"><=</span> <span style="color:#986801">200</span><span style="color:#999999">;</span> num<span style="color:#a67f59">++</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">if</span> <span style="color:#999999">(</span>num <span style="color:#a67f59">%</span> <span style="color:#986801">3</span> <span style="color:#a67f59">==</span> <span style="color:#986801">0</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>num<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">break</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#708090">// 运行结果 102</span> </code></span></span>

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• continue的作用是跳出本次循环continue后边的代码

  代码示例：找到100~200中的所有3的倍数

  <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">for</span> <span style="color:#999999">(</span><span style="color:#0077aa">int</span> num <span style="color:#a67f59">=</span> <span style="color:#986801">100</span><span style="color:#999999">;</span> num <span style="color:#a67f59"><=</span> <span style="color:#986801">200</span><span style="color:#999999">;</span> num<span style="color:#a67f59">++</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">if</span> <span style="color:#999999">(</span>num <span style="color:#a67f59">%</span> <span style="color:#986801">3</span> <span style="color:#a67f59">==</span> <span style="color:#986801">0</span><span style="color:#999999">)</span><span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>num<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">continue</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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5. 方法

5.0 前言

在编程的过程中，经常会出现一部分代码多次使用的情况，比如计算多边形面积，输出固定格式的文字等。

今天我们就来学习方法。

5.1 方法的概念和使用

5.1.1 什么是方法

方法就是一个代码片段，类似于C语言的“函数”。

方法存在的意义：

• 是能够模块化的组织代码(当代码规模比较复杂的时候)
• 做到代码被重复使用, 一份代码可以在多个位置使用.
• 让代码更好理解更简单.
• 直接调用现有方法开发, 不必重复造轮子

5.1.2 方法的定义

方法的语法格式：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java">修饰符 返回值类型 方法名称<span style="color:#999999">(</span>形参列表<span style="color:#999999">)</span><span style="color:#999999">{</span> <span style="color:#708090">// 方法体</span> <span style="color:#0077aa">return</span> 返回值<span style="color:#999999">;</span> <span style="color:#999999">}</span> </code></span></span>

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代码示例：实现一个两个整数相加的方法

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">int</span> <span style="color:#dd4a68">add</span><span style="color:#999999">(</span><span style="color:#0077aa">int</span> a<span style="color:#999999">,</span> <span style="color:#0077aa">int</span> b<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> c <span style="color:#a67f59">=</span> a <span style="color:#a67f59">+</span> b<span style="color:#999999">;</span> <span style="color:#0077aa">return</span> c<span style="color:#999999">;</span> <span style="color:#999999">}</span> </code></span></span>

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​

【注意事项】：

• 修饰符：现阶段直接使用public static固定搭配
• 返回值类型：如果方法有返回值，返回值类型必须要与返回的实体类型一致，如果没有返回值，必须写成
  void
• 方法名字：采用小驼峰命名
• 参数列表：如果方法没有参数，()中什么都不写，如果有参数，需指定参数类型，多个参数之间使用逗号隔开
• 方法体：方法内部要执行的语句
• 在 Java当中，方法必须写在类当中
• 在 Java当中，方法不能嵌套定义
• 在 Java当中，没有方法声明一说

5.1.3 方法调用的过程

方法调用过程：
调用方法—>传递参数—>找到方法地址—>执行被调方法的方法体—>被调方法结束返回—>回到主调方法继续往下执行

​【注意事项】

• 定义方法的时候, 不会执行方法的代码. 只有调用的时候才会执行.
• 一个方法可以被多次调用

代码示例：计算两个整数相加

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> x <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> y <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> ret <span style="color:#a67f59">=</span> <span style="color:#dd4a68">add</span><span style="color:#999999">(</span>x<span style="color:#999999">,</span> y<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>ret<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">int</span> <span style="color:#dd4a68">add</span><span style="color:#999999">(</span><span style="color:#0077aa">int</span> a<span style="color:#999999">,</span> <span style="color:#0077aa">int</span> b<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> c <span style="color:#a67f59">=</span> a <span style="color:#a67f59">+</span> b<span style="color:#999999">;</span> <span style="color:#0077aa">return</span> c<span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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方法调用过程：

• DeBug

​

• 内存图

  方法是放在方法区中的，被调用的时候，需要进入到栈内存中运行

  ​

  一旦程序遇到return或者方法执行结束，就会把当前方法栈帧就从栈上进行销毁（回收）

5.1.4 形参和实参（重要）

方法的形参相当于数学函数中的自变量，比如：1 + 2 + 3 + … + n的公式为sum(n) =(1 + n) * n / 2

Java中方法的形参就相当于sum函数中的自变量n，用来接收sum函数在调用时传递的值的。形参的名字可以随意取，对方法都没有任何影响，形参只是方法在定义时需要借助的一个变量，用来保存方法在调用时传递过来的值。

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#dd4a68">getSum</span><span style="color:#999999">(</span><span style="color:#986801">10</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 10是实参，在方法调用时，形参n用来保存10</span> <span style="color:#dd4a68">getSum</span><span style="color:#999999">(</span><span style="color:#986801">100</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 100是实参，在方法调用时，形参n用来保存100</span> <span style="color:#999999">}</span> <span style="color:#0077aa">private</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">int</span> <span style="color:#dd4a68">getSum</span><span style="color:#999999">(</span><span style="color:#0077aa">int</span> n<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#708090">// n 是形参</span> <span style="color:#0077aa">return</span> <span style="color:#999999">(</span><span style="color:#986801">1</span> <span style="color:#a67f59">+</span> n<span style="color:#999999">)</span> <span style="color:#a67f59">*</span> n <span style="color:#a67f59">/</span> <span style="color:#986801">2</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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再比如：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#dd4a68">add</span><span style="color:#999999">(</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 2 和 3 是实参，在调用时传给形参a 和 b</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">int</span> <span style="color:#dd4a68">add</span><span style="color:#999999">(</span><span style="color:#0077aa">int</span> a<span style="color:#999999">,</span> <span style="color:#0077aa">int</span> b<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">return</span> a <span style="color:#a67f59">+</span> b<span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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​

5.1.5 没有返回值的方法

方法的返回值是可选的. 有些时候可以没有的，没有时返回值类型必须写成void

代码示例：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> <span style="color:#dd4a68">print</span><span style="color:#999999">(</span>a<span style="color:#999999">,</span>b<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">private</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">print</span><span style="color:#999999">(</span><span style="color:#0077aa">int</span> x<span style="color:#999999">,</span> <span style="color:#0077aa">int</span> y<span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"x = "</span> <span style="color:#a67f59">+</span> x <span style="color:#a67f59">+</span> <span style="color:#50a14f">",y = "</span> <span style="color:#a67f59">+</span> y<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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return问题

• 如果没有返回值的方法要写return，则return后面不能加任何返回值。

  ​

• return后面的语句不会被执行，return表示方法的结束

  ​

5.2 方法的重载

5.2.1 为什么需要方法的重载

​

由于参数类型不匹配, 所以不能直接使用现有的add方法.

一种比较简单粗暴的解决方法如下：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> x <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> y <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#dd4a68">addInt</span><span style="color:#999999">(</span>x<span style="color:#999999">,</span>y<span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">double</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">1.1</span><span style="color:#999999">;</span> <span style="color:#0077aa">double</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">1.2</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#dd4a68">addDouble</span><span style="color:#999999">(</span>a<span style="color:#999999">,</span>b<span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">int</span> <span style="color:#dd4a68">addInt</span><span style="color:#999999">(</span><span style="color:#0077aa">int</span> a<span style="color:#999999">,</span> <span style="color:#0077aa">int</span> b<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">return</span> a <span style="color:#a67f59">+</span> b<span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">double</span> <span style="color:#dd4a68">addDouble</span><span style="color:#999999">(</span><span style="color:#0077aa">double</span> a<span style="color:#999999">,</span> <span style="color:#0077aa">double</span> b<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">return</span> a <span style="color:#a67f59">+</span> b<span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

5.2.2 方法重载的概念

在Java中，如果多个方法的名字相同，参数列表不同，则称该几种方法被重载了。

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> x <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span> y <span style="color:#a67f59">=</span> <span style="color:#986801">20</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#dd4a68">add</span><span style="color:#999999">(</span>x<span style="color:#999999">,</span>y<span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// 调用add(int, int)</span> <span style="color:#0077aa">double</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">1.1</span><span style="color:#999999">;</span> <span style="color:#0077aa">double</span> b <span style="color:#a67f59">=</span> <span style="color:#986801">1.2</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#dd4a68">add</span><span style="color:#999999">(</span>a<span style="color:#999999">,</span>b<span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// 调用add(double, double)</span> <span style="color:#0077aa">double</span> c <span style="color:#a67f59">=</span> <span style="color:#986801">1.3</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#dd4a68">add</span><span style="color:#999999">(</span>a<span style="color:#999999">,</span>b<span style="color:#999999">,</span>c<span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// 调用add(double, double,double)</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">int</span> <span style="color:#dd4a68">add</span><span style="color:#999999">(</span><span style="color:#0077aa">int</span> a<span style="color:#999999">,</span> <span style="color:#0077aa">int</span> b<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">return</span> a <span style="color:#a67f59">+</span> b<span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">double</span> <span style="color:#dd4a68">add</span><span style="color:#999999">(</span><span style="color:#0077aa">double</span> a<span style="color:#999999">,</span> <span style="color:#0077aa">double</span> b<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">return</span> a <span style="color:#a67f59">+</span> b<span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">double</span> <span style="color:#dd4a68">add</span><span style="color:#999999">(</span><span style="color:#0077aa">double</span> a<span style="color:#999999">,</span> <span style="color:#0077aa">double</span> b<span style="color:#999999">,</span> <span style="color:#0077aa">double</span> c<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">return</span> a <span style="color:#a67f59">+</span> c<span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

注意：

1. 方法名必须相同

2. 参数列表必须不同(参数的个数不同、参数的类型不同、类型的次序必须不同)

3. 与返回值类型是否相同无关

   ​

5.2.3 方法签名

在同一个作用域中不能定义两个相同名称的标识符。比如：方法中不能定义两个名字一样的变量，那为什么类中就可以定义方法名相同的方法呢？
方法签名即：经过编译器编译修改过之后方法最终的名字。具体方式：方法全路径名+参数列表+返回值类型，构成方法完整的名字。

5.3 递归

5.3.1 生活中的例子

从前有坐山，山上有座庙，庙里有个老和尚给小和尚将故事，讲的就是：
"从前有座山，山上有座庙，庙里有个老和尚给小和尚讲故事，讲的就是：
“从前有座山，山上有座庙…”
“从前…”

​

上面的两个例子有个共同的特征：自身中又包含了自己，该种思想在数学和编程中非常有用，因为有些时候，我们遇到的问题直接并不好解决，但是发现将原问题拆分成其子问题之后，子问题与原问题有相同的解法，等子问题解决之后，原问题就迎刃而解了。

5.3.2 递归的概念

一个方法在执行过程中调用自身, 就称为 “递归”.
递归相当于数学上的 “数学归纳法”, 有一个起始条件, 然后有一个递推公式.

例如, 我们求 N!
起始条件: N = 1 的时候, N! 为 1. 这个起始条件相当于递归的结束条件.
递归公式: 求 N! , 直接不好求, 可以把问题转换成 N! => N * (N-1)!

递归的必要条件：

1. 将原问题划分成其子问题，注意：子问题必须要与原问题的解法相同
2. 递归出口

代码示例

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#dd4a68">fun</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">fun</span><span style="color:#999999">(</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#dd4a68">fun</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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上述代码就是一个最简单的递归。

但是存在错误：出来栈溢出错误的时候，就说明结束条件不对或者没有结束条件

​

代码示例：递归求 N 的阶乘

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#dd4a68">factor</span><span style="color:#999999">(</span><span style="color:#986801">5</span><span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 120</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">int</span> <span style="color:#dd4a68">factor</span><span style="color:#999999">(</span><span style="color:#0077aa">int</span> n<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">if</span> <span style="color:#999999">(</span>n <span style="color:#a67f59">==</span> <span style="color:#986801">1</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">return</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">return</span> <span style="color:#dd4a68">factor</span><span style="color:#999999">(</span>n <span style="color:#a67f59">-</span> <span style="color:#986801">1</span><span style="color:#999999">)</span> <span style="color:#a67f59">*</span> n<span style="color:#999999">;</span> <span style="color:#708090">// factor()方法调用自己</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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5.3.3 递归执行过程分析

递归的程序的执行过程不太容易理解, 要想理解清楚递归, 必须先理解清楚 “方法的执行过程”, 尤其是 "方法执行结束之后, 回到调用位置继续往下执行.

代码示例：递归求 N 的阶乘

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#dd4a68">factor</span><span style="color:#999999">(</span><span style="color:#986801">5</span><span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">int</span> <span style="color:#dd4a68">factor</span><span style="color:#999999">(</span><span style="color:#0077aa">int</span> n<span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"函数开始, n = "</span> <span style="color:#a67f59">+</span> n<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">if</span> <span style="color:#999999">(</span>n <span style="color:#a67f59">==</span> <span style="color:#986801">1</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"函数结束, n = 1 ret = 1"</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">return</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#0077aa">int</span> ret <span style="color:#a67f59">=</span> n <span style="color:#a67f59">*</span> <span style="color:#dd4a68">factor</span><span style="color:#999999">(</span>n <span style="color:#a67f59">-</span> <span style="color:#986801">1</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"函数结束, n = "</span> <span style="color:#a67f59">+</span> n <span style="color:#a67f59">+</span> <span style="color:#50a14f">" ret = "</span> <span style="color:#a67f59">+</span> ret<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#0077aa">return</span> ret<span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#708090">/* 函数开始, n = 5 函数开始, n = 4 函数开始, n = 3 函数开始, n = 2 函数开始, n = 1 函数结束, n = 1 ret = 1 函数结束, n = 2 ret = 2 函数结束, n = 3 ret = 6 函数结束, n = 4 ret = 24 函数结束, n = 5 ret = 120 120 */</span> </code></span></span>

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执行图：

​

6. 数组

6.1 数组的基本概念

6.1.1 为什么使用数组？

假设现在要存储5个学生的年龄，按照之前掌握的知识点，我们会写出如下代码：声明5个变量存储学生变量

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Test <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> age1<span style="color:#999999">;</span> <span style="color:#0077aa">int</span> age2<span style="color:#999999">;</span> <span style="color:#0077aa">int</span> age3<span style="color:#999999">;</span> <span style="color:#0077aa">int</span> age4<span style="color:#999999">;</span> <span style="color:#0077aa">int</span> age5<span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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​

如果我们有10个学生呢？我们就要声明20个变量，似乎没有什么问题。那如果有100,1000个学生呢，我们就要声明100,1000个变量，这样就有点离谱了，使用数组我们就可以解决一个问题。

6.1.2 什么是数组

数组，是指一组类型相同的数据的集合，数组中每个数据称为元素。数组可以存放任意类型的元素，但同一个数组里存放的元素类型必须一致。数组分为一维数组和多维数组。

数组在内存中是一段连续的空间，比如现实中的车库：

​

在 Java中，包含6个整形类型元素的数组，就相当于上图中连在一起的6个车位，从上图中可以看到：

1. 数组中存放的元素其类型相同

2. 数组的空间是连在一起的

3. 每个空间有自己的编号，起始位置的编号为0，即数组的下标。

6.1.3 数组的创建和初始化

6.1.3.1 数组的创建

基本语法格式：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-Java">T[] 数组名 = new T[N]; </code></span></span>

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• T：表示数组中存放元素的类型
• T[]：表示数组类型
• N：表示数组的长度

代码示例：存储10个人的年龄

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> ages <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#986801">10</span><span style="color:#999999">]</span><span style="color:#999999">;</span> </code></span></span>

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6.1.3.2 数组的初始化

Java 数组初始化主要分为静态初始化以及动态初始化

1. 静态初始化：在创建数组时，直接指定数组中元素的个数

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> ages <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#986801">10</span><span style="color:#999999">]</span><span style="color:#999999">;</span> </code></span></span>

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2. 动态初始化：在创建数组是不直接指定数据元素个数，而直接讲具体的数据内容进行指定

   语法格式：

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java">T<span style="color:#999999">[</span><span style="color:#999999">]</span> 数组名 <span style="color:#a67f59">=</span> <span style="color:#999999">{</span>data1<span style="color:#999999">,</span>data2<span style="color:#999999">,</span><span style="color:#999999">.</span><span style="color:#999999">.</span><span style="color:#999999">.</span><span style="color:#999999">.</span>data<span style="color:#999999">}</span><span style="color:#999999">;</span> </code></span></span>

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   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> ages <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">,</span><span style="color:#986801">4</span><span style="color:#999999">,</span><span style="color:#986801">5</span><span style="color:#999999">}</span><span style="color:#999999">;</span> </code></span></span>

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【注意事项】

• 静态初始化虽然没有指定数组的长度，编译器在编译时会根据{}中元素个数来确定数组的长度。

• 静态初始化时, {}中数据类型必须与[]前数据类型一致。

• 静态初始化可以简写，省去后面的new T[]。

  <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr <span style="color:#a67f59">=</span> <span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">5</span><span style="color:#999999">,</span><span style="color:#986801">4</span><span style="color:#999999">}</span><span style="color:#999999">;</span> <span style="color:#708090">// 注意：虽然省去了new T[], 但是编译器编译代码时还是会还原</span> </code></span></span>

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• 数组也可以按照如下C语言个数创建，不推荐

  <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">int</span> arr<span style="color:#999999">[</span><span style="color:#999999">]</span> <span style="color:#a67f59">=</span> <span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span> <span style="color:#986801">2</span><span style="color:#999999">,</span> <span style="color:#986801">3</span><span style="color:#999999">}</span><span style="color:#999999">;</span> <span style="color:#708090">/* 该种定义方式不太友好，容易造成数组的类型就是int的误解 []如果在类型之后，就表示数组类型，因此int[]结合在一块写意思更清晰 */</span> </code></span></span>

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• 静态和动态初始化也可以分为两步，但是省略格式不可以。

  <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> array1<span style="color:#999999">;</span> array1 <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#986801">10</span><span style="color:#999999">]</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> array2<span style="color:#999999">;</span> array2 <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">10</span><span style="color:#999999">,</span> <span style="color:#986801">20</span><span style="color:#999999">,</span> <span style="color:#986801">30</span><span style="color:#999999">}</span><span style="color:#999999">;</span> <span style="color:#708090">// 注意省略格式不可以拆分, 否则编译失败</span> <span style="color:#708090">//int[] array3;</span> <span style="color:#708090">//array3 = {1, 2, 3};</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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  ​

• 如果没有对数组进行初始化，数组中元素有其默认值

  • 如果数组中存储元素类型为基类类型，默认值为基类类型对应的默认值，比如：

    类型	默认值
    byte	0
    short	0
    int	0
    long	0
    float	0.0f
    double	0.0
    char	/u0000
    boolean	false

  • 如果数组中存储元素类型为引用类型，默认值为null

6.1.4 数组的使用

6.1.4.1 数组中元素访问

数组在内存中是一段连续的空间，空间的编号都是从0开始的，依次递增，该编号称为数组的下标，数组可以通过下标访问其任意位置的元素。比如：

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">,</span><span style="color:#986801">4</span><span style="color:#999999">,</span><span style="color:#986801">5</span><span style="color:#999999">}</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span><span style="color:#986801">0</span><span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span><span style="color:#986801">1</span><span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span><span style="color:#986801">2</span><span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span><span style="color:#986801">3</span><span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span><span style="color:#986801">4</span><span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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【注意事项】：

1. 数组是一段连续的内存空间，因此支持随机访问，即通过下标快速访问数组中任意位置的元素

2. 下标从0开始，介于[0,N) 之间不包含N，N为元素个数，不能越界，否则会报出下标越界异常。

   ​

   抛出了java.lang.ArrayIndexOutOfBoundsException异常. 使用数组一定要下标谨防越界.

6.1.4.2 遍历数组

所谓“遍历” 是指将数组中的所有元素都访问一遍, 访问是指对数组中的元素进行某种操作，比如：打印。

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">,</span><span style="color:#986801">4</span><span style="color:#999999">,</span><span style="color:#986801">5</span><span style="color:#999999">}</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span><span style="color:#986801">0</span><span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span><span style="color:#986801">1</span><span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span><span style="color:#986801">2</span><span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span><span style="color:#986801">3</span><span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span><span style="color:#986801">4</span><span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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上述代码可以起到对数组中元素遍历的目的，但问题是：

1. 如果数组中增加了一个元素，就需要增加一条打印语句

2. 如果输入中有100个元素，就需要写100个打印语句

3. 如果现在要把打印修改为给数组中每个元素加1，修改起来非常麻烦。

通过观察代码可以发现，对数组中每个元素的操作都是相同的，则可以使用循环来进行打印。

1. 循环遍历数组

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">,</span><span style="color:#986801">4</span><span style="color:#999999">,</span><span style="color:#986801">5</span><span style="color:#999999">}</span><span style="color:#999999">;</span> <span style="color:#0077aa">for</span> <span style="color:#999999">(</span><span style="color:#0077aa">int</span> i <span style="color:#a67f59">=</span> <span style="color:#986801">0</span><span style="color:#999999">;</span> i <span style="color:#a67f59"><</span> <span style="color:#986801">5</span><span style="color:#999999">;</span> i<span style="color:#a67f59">++</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span>i<span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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改成循环之后，上述三个缺陷可以全部2和3问题可以全部解决，但是无法解决问题1。那能否获取到数组的长度呢？

【注意】：在数组中可以通过数组对象.length来获取数组的长度

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">,</span><span style="color:#986801">4</span><span style="color:#999999">,</span><span style="color:#986801">5</span><span style="color:#999999">}</span><span style="color:#999999">;</span> <span style="color:#0077aa">for</span> <span style="color:#999999">(</span><span style="color:#0077aa">int</span> i <span style="color:#a67f59">=</span> <span style="color:#986801">0</span><span style="color:#999999">;</span> i <span style="color:#a67f59"><</span> arr<span style="color:#999999">.</span>length<span style="color:#999999">;</span> i<span style="color:#a67f59">++</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span>i<span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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2. 使用for-each遍历数组

语法格式：

​

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">,</span><span style="color:#986801">4</span><span style="color:#999999">,</span><span style="color:#986801">5</span><span style="color:#999999">}</span><span style="color:#999999">;</span> <span style="color:#0077aa">for</span> <span style="color:#999999">(</span><span style="color:#0077aa">int</span> x <span style="color:#a67f59">:</span> arr<span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>x<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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for-each是for循环的另外一种使用方式. 能够更方便的完成对数组的遍历. 可以避免循环条件和更新语句写错.

for-each循环语句的循环变量将会遍历数组中的每个元素，而不是下标值。

3. 数组转字符串输出

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">import</span> java<span style="color:#999999">.</span>util<span style="color:#999999">.</span>Arrays<span style="color:#999999">;</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">,</span><span style="color:#986801">4</span><span style="color:#999999">,</span><span style="color:#986801">5</span><span style="color:#999999">}</span><span style="color:#999999">;</span> String ret <span style="color:#a67f59">=</span> Arrays<span style="color:#999999">.</span><span style="color:#dd4a68">toString</span><span style="color:#999999">(</span>arr<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>ret<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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代码分析：

​

6.2 数组是引用类型

6.2.1 JVM 内存分布

内存是一段连续的存储空间，主要是用来存储程序运行时数据的。比如：

1. 程序运行时代码需要加载到内存
2. 程序运行产生的中间数据要存放在内存
3. 程序中的常量也要保存
4. 有些数据可能需要长时间存储，而有些数据当方法运行结束后就要被销毁。

如果对内存中存储的数据不加区分的随意存储，那对内存管理起来将会非常麻烦。比如：

​

因此 JVM 也对所使用的内存按照功能的不同进行了划分：

​

• 程序计数器：只是一个很小的空间，保存下一条执行的指令的地址
• 虚拟机栈：与方法调用相关的一些信息，每个方法在执行时，都会先创建栈帧，栈帧中包含有：局部变量表、操作数栈、动态链接、返回地址以及其他的一些信息，保存的都是与方法执行时相关的一些信息。比如：局部变量。当方法运行结束后吧，栈帧就被销毁了，即栈帧中保存的数据也被销毁了。
• 本地方法栈：本地方法栈于虚拟机栈的作用类似，只不过保存的内容是方法的局部变量。在有些版本的 JVM 实现中，本地方法栈和虚拟机栈是一起的
• 堆：JVM 所管理的最大内存区域，使用**new创建的对象都是在堆上保存，堆是随着程序开始运行时而创建，随着程序的结束而销毁，堆中的数据只要还有在使用，就不会被销毁**
• 方法区：用于存储已被虚拟机加载的类信息、常量、静态变量、即时编译器编译后的代码等数据。方法编译出的字节码就是保存在这个区域。

6.2.2 基本类型的变量与引用类型变量的区别

基本数据类型的变量，称为基本变量，该变量空间中直接存放的是其所对应的值；

而引用数据类型创建的变量，一般称为对象的引用，其空间中存储的是对象所在空间的地址

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> a <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">}</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

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在上述代码中，a、arr，都是函数内部的变量，因此其空间都在main方法对应的栈帧中分配。
a是内置类型的变量，因此其空间中保存的就是给该变量初始化的值。
arr是数组类型的引用变量，其内部保存的内容可以简单理解成是数组在堆空间中的首地址。

​

上图可以看出，引用变量并不直接存储对象本生，可以简单理解成存储的是对象在堆中空间的起始地址。通过该地址，引用变量便可以去操作对象。有点类似C语言中的指针，但是 Java 中引用要比指针的操作更简单。

6.2.3 引用变量

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr1 <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#986801">3</span><span style="color:#999999">]</span><span style="color:#999999">;</span> arr1<span style="color:#999999">[</span><span style="color:#986801">0</span><span style="color:#999999">]</span> <span style="color:#a67f59">=</span> <span style="color:#986801">1</span><span style="color:#999999">;</span> arr1<span style="color:#999999">[</span><span style="color:#986801">1</span><span style="color:#999999">]</span> <span style="color:#a67f59">=</span> <span style="color:#986801">2</span><span style="color:#999999">;</span> arr1<span style="color:#999999">[</span><span style="color:#986801">2</span><span style="color:#999999">]</span> <span style="color:#a67f59">=</span> <span style="color:#986801">3</span><span style="color:#999999">;</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr2 <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">,</span><span style="color:#986801">4</span><span style="color:#999999">,</span><span style="color:#986801">5</span><span style="color:#999999">}</span><span style="color:#999999">;</span> arr2<span style="color:#999999">[</span><span style="color:#986801">0</span><span style="color:#999999">]</span> <span style="color:#a67f59">=</span> <span style="color:#986801">100</span><span style="color:#999999">;</span> arr2<span style="color:#999999">[</span><span style="color:#986801">1</span><span style="color:#999999">]</span> <span style="color:#a67f59">=</span> <span style="color:#986801">200</span><span style="color:#999999">;</span> arr1 <span style="color:#a67f59">=</span> arr2<span style="color:#999999">;</span> arr1<span style="color:#999999">[</span><span style="color:#986801">2</span><span style="color:#999999">]</span> <span style="color:#a67f59">=</span> <span style="color:#986801">300</span><span style="color:#999999">;</span> arr1<span style="color:#999999">[</span><span style="color:#986801">3</span><span style="color:#999999">]</span> <span style="color:#a67f59">=</span> <span style="color:#986801">400</span><span style="color:#999999">;</span> arr2<span style="color:#999999">[</span><span style="color:#986801">4</span><span style="color:#999999">]</span> <span style="color:#a67f59">=</span> <span style="color:#986801">500</span><span style="color:#999999">;</span> <span style="color:#0077aa">for</span> <span style="color:#999999">(</span><span style="color:#0077aa">int</span> x <span style="color:#a67f59">:</span> arr1<span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>x<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

​

​

6.2.4 认识 null

null在 Java 中表示“空引用”，也就是一个不指向对象的引用

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr <span style="color:#a67f59">=</span> <span style="color:#0077aa">null</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>arr<span style="color:#999999">[</span><span style="color:#986801">0</span><span style="color:#999999">]</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

​

null的作用类似于C语言中的NULL（空指针），都是表示一个无效的内存位置。因此不能对这个内存进行任何读写操作。一旦尝试读写，就会抛出NullPointerException

【注意】：Java 中并没有约定null和 0 下标地址的内存有任何关联。

6.3 数组应用场景

6.3.1 保存数据

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">}</span><span style="color:#999999">;</span> <span style="color:#0077aa">for</span> <span style="color:#999999">(</span><span style="color:#0077aa">int</span> x <span style="color:#a67f59">:</span> arr<span style="color:#999999">)</span> <span style="color:#999999">{</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>x<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

6.3.2 作为方法的参数

1. 参数传基本数据类型

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span> num <span style="color:#a67f59">=</span> <span style="color:#986801">0</span><span style="color:#999999">;</span> <span style="color:#dd4a68">func</span><span style="color:#999999">(</span>num<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"num = "</span> <span style="color:#a67f59">+</span> num<span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#708090">// 0</span> <span style="color:#999999">}</span> <span style="color:#0077aa">private</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">func</span><span style="color:#999999">(</span><span style="color:#0077aa">int</span> x<span style="color:#999999">)</span> <span style="color:#999999">{</span> x <span style="color:#a67f59">=</span> <span style="color:#986801">10</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span><span style="color:#50a14f">"x = "</span> <span style="color:#a67f59">+</span> x<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// 10</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

   运行项目并下载源码java运行

   上述代码我们可以发现func方法中修改了形参x的值，不影响实参的num值。

2. 参数传引用数据类型

   <span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">}</span><span style="color:#999999">;</span> <span style="color:#dd4a68">fun1</span><span style="color:#999999">(</span>arr<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>Arrays<span style="color:#999999">.</span><span style="color:#dd4a68">toString</span><span style="color:#999999">(</span>arr<span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// [1,2,3]</span> <span style="color:#dd4a68">fun2</span><span style="color:#999999">(</span>arr<span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>Arrays<span style="color:#999999">.</span><span style="color:#dd4a68">toString</span><span style="color:#999999">(</span>arr<span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// [99,2,3]</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">fun1</span><span style="color:#999999">(</span><span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr<span style="color:#999999">)</span> <span style="color:#999999">{</span> arr <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">11</span><span style="color:#999999">,</span><span style="color:#986801">22</span><span style="color:#999999">,</span><span style="color:#986801">33</span><span style="color:#999999">,</span><span style="color:#986801">44</span><span style="color:#999999">,</span><span style="color:#986801">55</span><span style="color:#999999">}</span><span style="color:#999999">;</span> <span style="color:#708090">// 修改了形参的指向</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">fun2</span><span style="color:#999999">(</span><span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr<span style="color:#999999">)</span> <span style="color:#999999">{</span> arr<span style="color:#999999">[</span><span style="color:#986801">0</span><span style="color:#999999">]</span> <span style="color:#a67f59">=</span> <span style="color:#986801">99</span><span style="color:#999999">;</span> <span style="color:#708090">// 形参改变了实惨的值</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>

   运行项目并下载源码java运行

   上述代码我们可以发现fun1方法中修改了形参的指向，不影响实参数组的值

   fun2方法内部修改了数组的内容，方法外部的数组内容也发生了改变。因为数组是引用类型，按照引用类型进行传递，是可以修改其中存放的内容的。

【总结】：所谓的“引用”本质只是存了地址。Java 将数组设定为引用类型，这样的话后续进行数组参数传参，其实只是将数组的地址传入函数形参中，这样可以避免对整数数组的拷贝（数组可能比较长，那么拷贝开销就会很大）。

6.3.3 作为方法的返回值

<span style="color:#000000"><span style="background-color:#fafafa"><code class="language-java"><span style="color:#0077aa">public</span> <span style="color:#0077aa">class</span> Main <span style="color:#999999">{</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">void</span> <span style="color:#dd4a68">main</span><span style="color:#999999">(</span>String<span style="color:#999999">[</span><span style="color:#999999">]</span> args<span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> ret <span style="color:#a67f59">=</span> <span style="color:#dd4a68">fun</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span> System<span style="color:#999999">.</span>out<span style="color:#999999">.</span><span style="color:#dd4a68">println</span><span style="color:#999999">(</span>Arrays<span style="color:#999999">.</span><span style="color:#dd4a68">toString</span><span style="color:#999999">(</span>ret<span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#708090">// [1, 2, 3, 4, 5]</span> <span style="color:#999999">}</span> <span style="color:#0077aa">public</span> <span style="color:#0077aa">static</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> <span style="color:#dd4a68">fun</span><span style="color:#999999">(</span><span style="color:#999999">)</span> <span style="color:#999999">{</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span> arr <span style="color:#a67f59">=</span> <span style="color:#0077aa">new</span> <span style="color:#0077aa">int</span><span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">{</span><span style="color:#986801">1</span><span style="color:#999999">,</span><span style="color:#986801">2</span><span style="color:#999999">,</span><span style="color:#986801">3</span><span style="color:#999999">,</span><span style="color:#986801">4</span><span style="color:#999999">,</span><span style="color:#986801">5</span><span style="color:#999999">}</span><span style="color:#999999">;</span> <span style="color:#0077aa">return</span> arr<span style="color:#999999">;</span> <span style="color:#999999">}</span> <span style="color:#999999">}</span> </code></span></span>