一、问题引出

面向对象的编程思想使得代码中创建的class="tags" href="/tags/Lei.html" title=类>类更加具体，他们都有各自的属性，方法。有的时候一些客观事物之前存在一些联系，那么他们在代码中的具体class="tags" href="/tags/Lei.html" title=类>类也存在一些联系。
例如：设计一个动物class="tags" href="/tags/Lei.html" title=类>类

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">public class="token keyword">class class="token class-name">Animal class="token punctuation">{
     class="token keyword">public class="token class-name">String nameclass="token punctuation">;
     class="token keyword">public class="token keyword">int ageclass="token punctuation">;
    class="token keyword">public class="token class-name">Animalclass="token punctuation">(class="token class-name">String nameclass="token punctuation">) class="token punctuation">{
        class="token keyword">thisclass="token punctuation">.name class="token operator">= nameclass="token punctuation">;
    class="token punctuation">}
     class="token keyword">public class="token class-name">Animalclass="token punctuation">(class="token keyword">int ageclass="token punctuation">) class="token punctuation">{
        class="token keyword">thisclass="token punctuation">.age class="token operator">= ageclass="token punctuation">;
    class="token punctuation">}
    class="token keyword">public class="token keyword">void class="token function">eatclass="token punctuation">(class="token punctuation">)class="token punctuation">{
        class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token keyword">thisclass="token punctuation">.nameclass="token operator">+class="token string">"吃东西"class="token punctuation">)class="token punctuation">;
    class="token punctuation">}
class="token punctuation">}

这个动物class="tags" href="/tags/Lei.html" title=类>类有自己的name，age属性和eat方法
我们又想创建一个猫class="tags" href="/tags/Lei.html" title=类>类和狗class="tags" href="/tags/Lei.html" title=类>类

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">class cat class="token punctuation">{
    class="token keyword">public  class="token class-name">String nameclass="token punctuation">;
    class="token keyword">public  class="token keyword">int ageclass="token punctuation">;
    class="token keyword">public class="token function">catclass="token punctuation">(class="token class-name">String nameclass="token punctuation">) class="token punctuation">{
        class="token keyword">thisclass="token punctuation">.name class="token operator">= nameclass="token punctuation">;
    class="token punctuation">}
    class="token keyword">public class="token function">catclass="token punctuation">(class="token keyword">int ageclass="token punctuation">) class="token punctuation">{
        class="token keyword">thisclass="token punctuation">.age class="token operator">= ageclass="token punctuation">;
    class="token punctuation">}
    class="token keyword">public class="token keyword">void class="token function">eatclass="token punctuation">(class="token punctuation">)class="token punctuation">{
        class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token keyword">thisclass="token punctuation">.nameclass="token operator">+class="token string">"吃饭"class="token punctuation">)class="token punctuation">;
    class="token punctuation">}
class="token punctuation">}
class="token keyword">class dogclass="token punctuation">{
    class="token keyword">public  class="token class-name">String nameclass="token punctuation">;
    class="token keyword">public class="token keyword">int ageclass="token punctuation">;

    class="token keyword">public class="token function">dogclass="token punctuation">(class="token keyword">int ageclass="token punctuation">) class="token punctuation">{
        class="token keyword">thisclass="token punctuation">.age class="token operator">= ageclass="token punctuation">;
    class="token punctuation">}

    class="token keyword">public class="token function">dogclass="token punctuation">(class="token class-name">String nameclass="token punctuation">) class="token punctuation">{
        class="token keyword">thisclass="token punctuation">.name class="token operator">= nameclass="token punctuation">;
    class="token punctuation">}
    class="token keyword">public  class="token keyword">void  class="token function">eatclass="token punctuation">(class="token punctuation">)class="token punctuation">{
        class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token keyword">thisclass="token punctuation">.nameclass="token operator">+class="token string">"干饭"class="token punctuation">)class="token punctuation">;
    class="token punctuation">}
class="token punctuation">}

由于大家都是碳基生物，你要吃饭我也要吃饭，你有名字我也有名字，也都有自己的年龄，我们在创建其他class="tags" href="/tags/Lei.html" title=类>类的时候还需要在写一遍他们的属性name，age，和方法eat。并且，从逻辑上来说猫和狗都属于动物。这就造成了大量重复的代码，那有没有什么办法能让我不用把这些属性。方法在写一遍呢？
在这里插入图片描述

继承：我来啦！！！

继承作为class="tags" href="/tags/MianXiangDuiXiangBianCheng.html" title=面向对象编程>面向对象编程的一个非常重要的关键字，在C++，和Java当中都可以使用它来减少代码冗余。顾名思义，它可以让一个子class="tags" href="/tags/Lei.html" title=类>类继承另一个父class="tags" href="/tags/Lei.html" title=类>类，就像儿子继承爸爸的财产一样，继承后的子class="tags" href="/tags/Lei.html" title=类>类可以拥有父class="tags" href="/tags/Lei.html" title=类>类的方法，属性，这样每次在定义动物class="tags" href="/tags/Lei.html" title=类>类完之后，在想要定义狗或者猫class="tags" href="/tags/Lei.html" title=类>类的时候就不用再把多余的代码写一遍了。我们来看看效果。

二、继承extends

2.1 继承的用法

子class="tags" href="/tags/Lei.html" title=类>类extends父class="tags" href="/tags/Lei.html" title=类>类即可

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">class cat class="token keyword">extends animalclass="token punctuation">{
    class="token keyword">public class="token function">catclass="token punctuation">(class="token class-name">String nameclass="token punctuation">) class="token punctuation">{
        class="token keyword">superclass="token punctuation">(nameclass="token punctuation">)class="token punctuation">;
    class="token punctuation">}
    class="token keyword">public class="token function">catclass="token punctuation">(class="token keyword">int ageclass="token punctuation">)class="token punctuation">{
        class="token keyword">superclass="token punctuation">(ageclass="token punctuation">)class="token punctuation">;
    class="token punctuation">}
    class="token keyword">public  class="token keyword">void class="token function">catchMouceclass="token punctuation">(class="token punctuation">)class="token punctuation">{
        class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token keyword">thisclass="token punctuation">.nameclass="token operator">+class="token string">"抓老鼠"class="token punctuation">)class="token punctuation">;
    class="token punctuation">}
class="token punctuation">}
class="token keyword">class dog class="token keyword">extends animalclass="token punctuation">{
    class="token keyword">public class="token function">dogclass="token punctuation">(class="token class-name">String nameclass="token punctuation">) class="token punctuation">{
        class="token keyword">superclass="token punctuation">(nameclass="token punctuation">)class="token punctuation">;
    class="token punctuation">}

    class="token keyword">public class="token function">dogclass="token punctuation">(class="token keyword">int ageclass="token punctuation">) class="token punctuation">{
        class="token keyword">superclass="token punctuation">(ageclass="token punctuation">)class="token punctuation">;
    class="token punctuation">}
    class="token keyword">public  class="token keyword">void class="token function">defenceclass="token punctuation">(class="token punctuation">)class="token punctuation">{
        class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token keyword">thisclass="token punctuation">.nameclass="token operator">+class="token string">"看家"class="token punctuation">)class="token punctuation">;
    class="token punctuation">}
class="token punctuation">}

在上面的代码中，cat，dog被称为子class="tags" href="/tags/Lei.html" title=类>类、派生class="tags" href="/tags/Lei.html" title=类>类，而animal被称为父class="tags" href="/tags/Lei.html" title=类>类或者超class="tags" href="/tags/Lei.html" title=类>类，extends英文意思为扩展，这里可以理解成继承，例如我们写的catclass="tags" href="/tags/Lei.html" title=类>类有了抓老鼠的方法，dogclass="tags" href="/tags/Lei.html" title=类>类有了看家的方法。继承可以让子class="tags" href="/tags/Lei.html" title=类>类拥有父class="tags" href="/tags/Lei.html" title=类>类public修饰的属性和方法，cat、dogclass="tags" href="/tags/Lei.html" title=类>类就继承了animalclass="tags" href="/tags/Lei.html" title=类>类的name、age属性以及eat方法。

2.2 基本语法

使用 extends 指定父class="tags" href="/tags/Lei.html" title=类>类.
子class="tags" href="/tags/Lei.html" title=类>类会继承父class="tags" href="/tags/Lei.html" title=类>类的所有 public 的字段和方法.
对于父class="tags" href="/tags/Lei.html" title=类>类的 private 的字段和方法, 子class="tags" href="/tags/Lei.html" title=类>类中是无法访问的.
子class="tags" href="/tags/Lei.html" title=类>类的实例中, 也包含着父class="tags" href="/tags/Lei.html" title=类>类的实例. 可以使用 super 关键字得到父class="tags" href="/tags/Lei.html" title=类>类实例的引用
子class="tags" href="/tags/Lei.html" title=类>类继承父class="tags" href="/tags/Lei.html" title=类>类之后，还可以声明自己特有的属性和方法，实现功能的拓展。

2.3继承的好处

1：减少代码的冗余，提高复用性。
2：便于功能的拓展
3：为多态提供了前提。

2.4继承性

Java各种class="tags" href="/tags/Lei.html" title=类>类互相继承的属性称作继承性，继承性有一些规定。
1：一个class="tags" href="/tags/Lei.html" title=类>类可以被多个子class="tags" href="/tags/Lei.html" title=类>类继承。
2：Java的单继承性：一个子class="tags" href="/tags/Lei.html" title=类>类只能继承一个父class="tags" href="/tags/Lei.html" title=类>类。比如一个儿子只能有一个亲爸爸，一个爸爸可以有好几个儿子。在C++，和python中支持多继承
3：如果一个class="tags" href="/tags/Lei.html" title=类>类没有说明一个class="tags" href="/tags/Lei.html" title=类>类的父class="tags" href="/tags/Lei.html" title=类>类的话，那么此class="tags" href="/tags/Lei.html" title=类>类继承于class="tags" href="/tags/JAVA.html" title=java>java.lang.Objectclass="tags" href="/tags/Lei.html" title=类>类
4:所以的class="tags" href="/tags/Lei.html" title=类>类(除了class="tags" href="/tags/JAVA.html" title=java>java.lang.Object）都间接或直接的继承class="tags" href="/tags/JAVA.html" title=java>java.lang.Objectclass="tags" href="/tags/Lei.html" title=类>类。那么就意味这所有的class="tags" href="/tags/Lei.html" title=类>类都具有class="tags" href="/tags/JAVA.html" title=java>java.lang.Objectclass="tags" href="/tags/Lei.html" title=类>类声明的功能。