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面向对象编程（进阶）5：关键字：super

article 2025/2/22 16:32:43

目录

5.1 super的理解

在Java类中使用super来调用父类中的指定操作：

注意：

5.2 super的使用场景

5.2.1 子类中调用父类被重写的方法

举例：

总结：

方法前面没有super.和this.

方法前面有this.

方法前面有super.

5.2.2 子类中调用父类中同名的成员变量

举例：

总结：起点不同（就近原则）

变量前面没有super.和this.

变量前面有this.

变量前面super.

特别说明：应该避免子类声明和父类重名的成员变量

5.2.3 子类构造器中调用父类构造器

情景举例1：

情景举例2：

情景举例3：

情景举例4：

情景举例5：

情景举例6：

情景举例7：

情景举例8：

5.3 小结：this与super

1、this和super的意义

this：当前对象

super：引用父类声明的成员

2、this和super的使用格式

this

super

5.4 练习

**练习1：**

**练习2：**

附加题：

练习3：

5.1 super的理解

在Java类中使用super来调用父类中的指定操作：

  • super可用于访问父类中定义的属性
  • super可用于调用父类中定义的成员方法
  • super可用于在子类构造器中调用父类的构造器

注意：

  • 尤其当子父类出现同名成员时，可以用super表明调用的是父类中的成员
  • super的追溯不仅限于直接父类
  • super和this的用法相像，this代表本类对象的引用，super代表父类的内存空间的标识

5.2 super的使用场景

5.2.1 子类中调用父类被重写的方法

  • 如果子类没有重写父类的方法，只要权限修饰符允许，在子类中完全可以直接调用父类的方法；
  • 如果子类重写了父类的方法，在子类中需要通过super.才能调用父类被重写的方法，否则默认调用的子类重写的方法

举例：

<span style="color:#303030"><span style="background-color:#ffffff"><span style="background-color:#282c34"><span style="color:#abb2bf"><code class="language-java"><span style="color:#c678dd">package</span> com<span style="color:#abb2bf">.</span>atguigu<span style="color:#abb2bf">.</span>inherited<span style="color:#abb2bf">.</span>method<span style="color:#abb2bf">;</span>

<span style="color:#c678dd">public</span> <span style="color:#c678dd">class</span> <span style="color:#d19a66">Phone</span> <span style="color:#abb2bf">{</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">sendMessage</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"发短信"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
    <span style="color:#abb2bf">}</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">call</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"打电话"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
    <span style="color:#abb2bf">}</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">showNum</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"来电显示号码"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
    <span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>

<span style="color:#5c6370"><em>//smartphone：智能手机</em></span>
<span style="color:#c678dd">public</span> <span style="color:#c678dd">class</span> <span style="color:#d19a66">SmartPhone</span> <span style="color:#c678dd">extends</span> <span style="color:#d19a66">Phone</span><span style="color:#abb2bf">{</span>
    <span style="color:#5c6370"><em>//重写父类的来电显示功能的方法</em></span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">showNum</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#5c6370"><em>//来电显示姓名和图片功能</em></span>
        <span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"显示来电姓名"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
        <span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"显示头像"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>

        <span style="color:#5c6370"><em>//保留父类来电显示号码的功能</em></span>
        <span style="color:#c678dd">super</span><span style="color:#abb2bf">.</span><span style="color:#61afef">showNum</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//此处必须加super.，否则就是无限递归，那么就会栈内存溢出</em></span>
    <span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
</code></span></span></span></span>

总结：

  • 方法前面没有super.和this.

    • 先从子类找匹配方法，如果没有，再从直接父类找，再没有，继续往上追溯

  • 方法前面有this.

    • 先从子类找匹配方法，如果没有，再从直接父类找，再没有，继续往上追溯

  • 方法前面有super.

    • 从当前子类的直接父类找，如果没有，继续往上追溯

5.2.2 子类中调用父类中同名的成员变量

  • 如果实例变量与局部变量重名，可以在实例变量前面加this.进行区别
  • 如果子类实例变量和父类实例变量重名，并且父类的该实例变量在子类仍然可见，在子类中要访问父类声明的实例变量需要在父类实例变量前加super.，否则默认访问的是子类自己声明的实例变量
  • 如果父子类实例变量没有重名，只要权限修饰符允许，在子类中完全可以直接访问父类中声明的实例变量，也可以用this.实例访问，也可以用super.实例变量访问

举例：

<span style="color:#303030"><span style="background-color:#ffffff"><span style="background-color:#282c34"><span style="color:#abb2bf"><code class="language-java"><span style="color:#c678dd">class</span> <span style="color:#d19a66">Father</span><span style="color:#abb2bf">{</span>
	<span style="color:#c678dd">int</span> a <span style="color:#61afef">=</span> <span style="color:#d19a66">10</span><span style="color:#abb2bf">;</span>
	<span style="color:#c678dd">int</span> b <span style="color:#61afef">=</span> <span style="color:#d19a66">11</span><span style="color:#abb2bf">;</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">Son</span> <span style="color:#c678dd">extends</span> <span style="color:#d19a66">Father</span><span style="color:#abb2bf">{</span>
	<span style="color:#c678dd">int</span> a <span style="color:#61afef">=</span> <span style="color:#d19a66">20</span><span style="color:#abb2bf">;</span>
    
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">test</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#5c6370"><em>//子类与父类的属性同名，子类对象中就有两个a</em></span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"子类的a："</span> <span style="color:#61afef">+</span> a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//20  先找局部变量找，没有再从本类成员变量找</em></span>
        <span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"子类的a："</span> <span style="color:#61afef">+</span> <span style="color:#c678dd">this</span><span style="color:#abb2bf">.</span>a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//20   先从本类成员变量找</em></span>
        <span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"父类的a："</span> <span style="color:#61afef">+</span> <span style="color:#c678dd">super</span><span style="color:#abb2bf">.</span>a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//10    直接从父类成员变量找</em></span>
		
		<span style="color:#5c6370"><em>//子类与父类的属性不同名，是同一个b</em></span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"b = "</span> <span style="color:#61afef">+</span> b<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//11  先找局部变量找，没有再从本类成员变量找，没有再从父类找</em></span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"b = "</span> <span style="color:#61afef">+</span> <span style="color:#c678dd">this</span><span style="color:#abb2bf">.</span>b<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//11   先从本类成员变量找，没有再从父类找</em></span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"b = "</span> <span style="color:#61afef">+</span> <span style="color:#c678dd">super</span><span style="color:#abb2bf">.</span>b<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//11  直接从父类局部变量找</em></span>
	<span style="color:#abb2bf">}</span>
	
	<span style="color:#c678dd">public</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">method</span><span style="color:#abb2bf">(</span><span style="color:#c678dd">int</span> a<span style="color:#abb2bf">,</span> <span style="color:#c678dd">int</span> b<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#5c6370"><em>//子类与父类的属性同名，子类对象中就有两个成员变量a，此时方法中还有一个局部变量a		</em></span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"局部变量的a："</span> <span style="color:#61afef">+</span> a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//30  先找局部变量</em></span>
        <span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"子类的a："</span> <span style="color:#61afef">+</span> <span style="color:#c678dd">this</span><span style="color:#abb2bf">.</span>a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//20  先从本类成员变量找</em></span>
        <span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"父类的a："</span> <span style="color:#61afef">+</span> <span style="color:#c678dd">super</span><span style="color:#abb2bf">.</span>a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//10  直接从父类成员变量找</em></span>

        <span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"b = "</span> <span style="color:#61afef">+</span> b<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//13  先找局部变量</em></span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"b = "</span> <span style="color:#61afef">+</span> <span style="color:#c678dd">this</span><span style="color:#abb2bf">.</span>b<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//11  先从本类成员变量找</em></span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"b = "</span> <span style="color:#61afef">+</span> <span style="color:#c678dd">super</span><span style="color:#abb2bf">.</span>b<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//11  直接从父类局部变量找</em></span>
    <span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">Test</span><span style="color:#abb2bf">{</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">main</span><span style="color:#abb2bf">(</span><span style="color:#d19a66">String</span><span style="color:#abb2bf">[</span><span style="color:#abb2bf">]</span> args<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">Son</span> son <span style="color:#61afef">=</span> <span style="color:#c678dd">new</span> <span style="color:#d19a66">Son</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
		son<span style="color:#abb2bf">.</span><span style="color:#61afef">test</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
		son<span style="color:#abb2bf">.</span><span style="color:#61afef">method</span><span style="color:#abb2bf">(</span><span style="color:#d19a66">30</span><span style="color:#abb2bf">,</span><span style="color:#d19a66">13</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>  
    <span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
</code></span></span></span></span>

总结：起点不同（就近原则）

  • 变量前面没有super.和this.

    • 在构造器、代码块、方法中如果出现使用某个变量，先查看是否是当前块声明的局部变量，
    • 如果不是局部变量，先从当前执行代码的本类去找成员变量
    • 如果从当前执行代码的本类中没有找到，会往上找父类声明的成员变量（权限修饰符允许在子类中访问的）

  • 变量前面有this.

    • 通过this找成员变量时，先从当前执行代码的本类去找成员变量
    • 如果从当前执行代码的本类中没有找到，会往上找==父类声明的成员变量（==权限修饰符允许在子类中访问的）

  • 变量前面super.

    • 通过super找成员变量，直接从当前执行代码的直接父类去找成员变量（权限修饰符允许在子类中访问的）
    • 如果直接父类没有，就去父类的父类中找（权限修饰符允许在子类中访问的）

特别说明：应该避免子类声明和父类重名的成员变量

在阿里的开发规范等文档中都做出明确说明：

5.2.3 子类构造器中调用父类构造器

① 子类继承父类时，不会继承父类的构造器。只能通过“super(形参列表)”的方式调用父类指定的构造器。

② 规定：“super(形参列表)”，必须声明在构造器的首行。

③ 我们前面讲过，在构造器的首行可以使用"this(形参列表)"，调用本类中重载的构造器，
结合②，结论：在构造器的首行，“this(形参列表)” 和 "super(形参列表)"只能二选一。

④ 如果在子类构造器的首行既没有显示调用"this(形参列表)“，也没有显式调用"super(形参列表)”，
则子类此构造器默认调用"super()"，即调用父类中空参的构造器。

⑤ 由③和④得到结论：子类的任何一个构造器中，要么会调用本类中重载的构造器，要么会调用父类的构造器。
只能是这两种情况之一。

⑥ 由⑤得到：一个类中声明有n个构造器，最多有n-1个构造器中使用了"this(形参列表)“，则剩下的那个一定使用"super(形参列表)”。

开发中常见错误：

如果子类构造器中既未显式调用父类或本类的构造器，且父类中又没有空参的构造器，则编译出错。

情景举例1：

<span style="color:#303030"><span style="background-color:#ffffff"><span style="background-color:#282c34"><span style="color:#abb2bf"><code class="language-java"><span style="color:#c678dd">class</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>

<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">B</span> <span style="color:#c678dd">extends</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>

<span style="color:#abb2bf">}</span>

<span style="color:#c678dd">class</span> <span style="color:#d19a66">Test</span><span style="color:#abb2bf">{</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">main</span><span style="color:#abb2bf">(</span><span style="color:#d19a66">String</span><span style="color:#abb2bf">[</span><span style="color:#abb2bf">]</span> args<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">B</span> b <span style="color:#61afef">=</span> <span style="color:#c678dd">new</span> <span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
        <span style="color:#5c6370"><em>//A类和B类都是默认有一个无参构造，B类的默认无参构造中还会默认调用A类的默认无参构造</em></span>
        <span style="color:#5c6370"><em>//但是因为都是默认的，没有打印语句，看不出来</em></span>
    <span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
</code></span></span></span></span>

情景举例2：

<span style="color:#303030"><span style="background-color:#ffffff"><span style="background-color:#282c34"><span style="color:#abb2bf"><code class="language-java"><span style="color:#c678dd">class</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
	<span style="color:#d19a66">A</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"A类无参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">B</span> <span style="color:#c678dd">extends</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>

<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">Test</span><span style="color:#abb2bf">{</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">main</span><span style="color:#abb2bf">(</span><span style="color:#d19a66">String</span><span style="color:#abb2bf">[</span><span style="color:#abb2bf">]</span> args<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">B</span> b <span style="color:#61afef">=</span> <span style="color:#c678dd">new</span> <span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
        <span style="color:#5c6370"><em>//A类显示声明一个无参构造，</em></span>
		<span style="color:#5c6370"><em>//B类默认有一个无参构造，</em></span>
		<span style="color:#5c6370"><em>//B类的默认无参构造中会默认调用A类的无参构造</em></span>
        <span style="color:#5c6370"><em>//可以看到会输出“A类无参构造器"</em></span>
    <span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
</code></span></span></span></span>

情景举例3：

<span style="color:#303030"><span style="background-color:#ffffff"><span style="background-color:#282c34"><span style="color:#abb2bf"><code class="language-java"><span style="color:#c678dd">class</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
	<span style="color:#d19a66">A</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"A类无参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">B</span> <span style="color:#c678dd">extends</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
	<span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"B类无参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">Test</span><span style="color:#abb2bf">{</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">main</span><span style="color:#abb2bf">(</span><span style="color:#d19a66">String</span><span style="color:#abb2bf">[</span><span style="color:#abb2bf">]</span> args<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">B</span> b <span style="color:#61afef">=</span> <span style="color:#c678dd">new</span> <span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
        <span style="color:#5c6370"><em>//A类显示声明一个无参构造，</em></span>
		<span style="color:#5c6370"><em>//B类显示声明一个无参构造，        </em></span>
		<span style="color:#5c6370"><em>//B类的无参构造中虽然没有写super()，但是仍然会默认调用A类的无参构造</em></span>
        <span style="color:#5c6370"><em>//可以看到会输出“A类无参构造器"和"B类无参构造器")</em></span>
    <span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
</code></span></span></span></span>

情景举例4：

<span style="color:#303030"><span style="background-color:#ffffff"><span style="background-color:#282c34"><span style="color:#abb2bf"><code class="language-java"><span style="color:#c678dd">class</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
	<span style="color:#d19a66">A</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"A类无参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">B</span> <span style="color:#c678dd">extends</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
	<span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#c678dd">super</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"B类无参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">Test</span><span style="color:#abb2bf">{</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">main</span><span style="color:#abb2bf">(</span><span style="color:#d19a66">String</span><span style="color:#abb2bf">[</span><span style="color:#abb2bf">]</span> args<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">B</span> b <span style="color:#61afef">=</span> <span style="color:#c678dd">new</span> <span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
        <span style="color:#5c6370"><em>//A类显示声明一个无参构造，</em></span>
		<span style="color:#5c6370"><em>//B类显示声明一个无参构造，        </em></span>
		<span style="color:#5c6370"><em>//B类的无参构造中明确写了super()，表示调用A类的无参构造</em></span>
        <span style="color:#5c6370"><em>//可以看到会输出“A类无参构造器"和"B类无参构造器")</em></span>
    <span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
</code></span></span></span></span>

情景举例5：

<span style="color:#303030"><span style="background-color:#ffffff"><span style="background-color:#282c34"><span style="color:#abb2bf"><code class="language-java"><span style="color:#c678dd">class</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
	<span style="color:#d19a66">A</span><span style="color:#abb2bf">(</span><span style="color:#c678dd">int</span> a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"A类有参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">B</span> <span style="color:#c678dd">extends</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
	<span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"B类无参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">Test05</span><span style="color:#abb2bf">{</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">main</span><span style="color:#abb2bf">(</span><span style="color:#d19a66">String</span><span style="color:#abb2bf">[</span><span style="color:#abb2bf">]</span> args<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">B</span> b <span style="color:#61afef">=</span> <span style="color:#c678dd">new</span> <span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
        <span style="color:#5c6370"><em>//A类显示声明一个有参构造，没有写无参构造，那么A类就没有无参构造了</em></span>
		<span style="color:#5c6370"><em>//B类显示声明一个无参构造，        </em></span>
		<span style="color:#5c6370"><em>//B类的无参构造没有写super(...)，表示默认调用A类的无参构造</em></span>
        <span style="color:#5c6370"><em>//编译报错，因为A类没有无参构造</em></span>
    <span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
</code></span></span></span></span>

情景举例6：

<span style="color:#303030"><span style="background-color:#ffffff"><span style="background-color:#282c34"><span style="color:#abb2bf"><code class="language-java"><span style="color:#c678dd">class</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
	<span style="color:#d19a66">A</span><span style="color:#abb2bf">(</span><span style="color:#c678dd">int</span> a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"A类有参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">B</span> <span style="color:#c678dd">extends</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
	<span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#c678dd">super</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"B类无参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">Test06</span><span style="color:#abb2bf">{</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">main</span><span style="color:#abb2bf">(</span><span style="color:#d19a66">String</span><span style="color:#abb2bf">[</span><span style="color:#abb2bf">]</span> args<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">B</span> b <span style="color:#61afef">=</span> <span style="color:#c678dd">new</span> <span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
        <span style="color:#5c6370"><em>//A类显示声明一个有参构造，没有写无参构造，那么A类就没有无参构造了</em></span>
		<span style="color:#5c6370"><em>//B类显示声明一个无参构造，        </em></span>
		<span style="color:#5c6370"><em>//B类的无参构造明确写super()，表示调用A类的无参构造</em></span>
        <span style="color:#5c6370"><em>//编译报错，因为A类没有无参构造</em></span>
    <span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
</code></span></span></span></span>

情景举例7：

<span style="color:#303030"><span style="background-color:#ffffff"><span style="background-color:#282c34"><span style="color:#abb2bf"><code class="language-java"><span style="color:#c678dd">class</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
	<span style="color:#d19a66">A</span><span style="color:#abb2bf">(</span><span style="color:#c678dd">int</span> a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"A类有参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">B</span> <span style="color:#c678dd">extends</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
	<span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#c678dd">int</span> a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#c678dd">super</span><span style="color:#abb2bf">(</span>a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"B类有参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">Test07</span><span style="color:#abb2bf">{</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">main</span><span style="color:#abb2bf">(</span><span style="color:#d19a66">String</span><span style="color:#abb2bf">[</span><span style="color:#abb2bf">]</span> args<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">B</span> b <span style="color:#61afef">=</span> <span style="color:#c678dd">new</span> <span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#d19a66">10</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
        <span style="color:#5c6370"><em>//A类显示声明一个有参构造，没有写无参构造，那么A类就没有无参构造了</em></span>
		<span style="color:#5c6370"><em>//B类显示声明一个有参构造，        </em></span>
		<span style="color:#5c6370"><em>//B类的有参构造明确写super(a)，表示调用A类的有参构造</em></span>
        <span style="color:#5c6370"><em>//会打印“A类有参构造器"和"B类有参构造器"</em></span>
    <span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
</code></span></span></span></span>

情景举例8：

<span style="color:#303030"><span style="background-color:#ffffff"><span style="background-color:#282c34"><span style="color:#abb2bf"><code class="language-java"><span style="color:#c678dd">class</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
    <span style="color:#d19a66">A</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"A类无参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
    <span style="color:#abb2bf">}</span>
	<span style="color:#d19a66">A</span><span style="color:#abb2bf">(</span><span style="color:#c678dd">int</span> a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"A类有参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">B</span> <span style="color:#c678dd">extends</span> <span style="color:#d19a66">A</span><span style="color:#abb2bf">{</span>
    <span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#c678dd">super</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//可以省略，调用父类的无参构造</em></span>
        <span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"B类无参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
    <span style="color:#abb2bf">}</span>
	<span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#c678dd">int</span> a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
		<span style="color:#c678dd">super</span><span style="color:#abb2bf">(</span>a<span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span><span style="color:#5c6370"><em>//调用父类有参构造</em></span>
		<span style="color:#d19a66">System</span><span style="color:#abb2bf">.</span>out<span style="color:#abb2bf">.</span><span style="color:#61afef">println</span><span style="color:#abb2bf">(</span><span style="color:#98c379">"B类有参构造器"</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
	<span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
<span style="color:#c678dd">class</span> <span style="color:#d19a66">Test8</span><span style="color:#abb2bf">{</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61afef">main</span><span style="color:#abb2bf">(</span><span style="color:#d19a66">String</span><span style="color:#abb2bf">[</span><span style="color:#abb2bf">]</span> args<span style="color:#abb2bf">)</span><span style="color:#abb2bf">{</span>
        <span style="color:#d19a66">B</span> b1 <span style="color:#61afef">=</span> <span style="color:#c678dd">new</span> <span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
        <span style="color:#d19a66">B</span> b2 <span style="color:#61afef">=</span> <span style="color:#c678dd">new</span> <span style="color:#d19a66">B</span><span style="color:#abb2bf">(</span><span style="color:#d19a66">10</span><span style="color:#abb2bf">)</span><span style="color:#abb2bf">;</span>
    <span style="color:#abb2bf">}</span>
<span style="color:#abb2bf">}</span>
</code></span></span></span></span>

5.3 小结：this与super

1、this和super的意义

this：当前对象

  • 在构造器和非静态代码块中，表示正在new的对象
  • 在实例方法中，表示调用当前方法的对象

super：引用父类声明的成员

2、this和super的使用格式

  • this

    • this.成员变量：表示当前对象的某个成员变量，而不是局部变量
    • this.成员方法：表示当前对象的某个成员方法，完全可以省略this.
    • this()或this(实参列表)：调用另一个构造器协助当前对象的实例化，只能在构造器首行，只会找本类的构造器，找不到就报错

  • super

    • super.成员变量：表示当前对象的某个成员变量，该成员变量在父类中声明的
    • super.成员方法：表示当前对象的某个成员方法，该成员方法在父类中声明的
    • super()或super(实参列表)：调用父类的构造器协助当前对象的实例化，只能在构造器首行，只会找直接父类的对应构造器，找不到就报错

5.4 练习

**练习1：**

修改方法重写的练习2中定义的类Kids中employeed()方法，在该方法中调用父类ManKind的employeed()方法，然后再输出“but Kids should study and no job.”

**练习2：**

修改继承中的练习3中定义的Cylinder类，在Cylinder类中覆盖findArea()方法，计算圆柱的表面积。考虑：findVolume方法怎样做相应的修改？

在CylinderTest类中创建Cylinder类的对象，设置圆柱的底面半径和高，并输出圆柱的表面积和体积。

附加题：

在CylinderTest类中创建一个Circle类的对象，设置圆的半径，计算输出圆的面积。体会父类和子类成员的分别调用。

练习3：

1、写一个名为Account的类模拟账户。该类的属性和方法如下图所示。该类包括的属性：账号id，余额balance，年利率annualInterestRate；包含的方法：访问器方法（getter和setter方法），返回月利率的方法getMonthlyInterest()，取款方法withdraw()，存款方法deposit()。

写一个用户程序测试Account类。在用户程序中，创建一个账号为1122、余额为20000、年利率4.5%的Account对象。使用withdraw方法提款30000元，并打印余额。
再使用withdraw方法提款2500元，使用deposit方法存款3000元，然后打印余额和月利率。

提示：在提款方法withdraw中，需要判断用户余额是否能够满足提款数额的要求，如果不能，应给出提示。
运行结果如图所示：

2、创建Account类的一个子类CheckAccount代表可透支的账户，该账户中定义一个属性overdraft代表可透支限额。在CheckAccount类中重写withdraw方法，其算法如下：

<span style="color:#303030"><span style="background-color:#ffffff"><span style="background-color:#282c34"><span style="color:#abb2bf"><code class="language- language-none">如果（取款金额<账户余额），
	可直接取款
如果（取款金额>账户余额），
	计算需要透支的额度
	判断可透支额overdraft是否足够支付本次透支需要，如果可以
		将账户余额修改为0，冲减可透支金额
	如果不可以
		提示用户超过可透支额的限额
</code></span></span></span></span>

要求：写一个用户程序测试CheckAccount类。在用户程序中，创建一个账号为1122、余额为20000、年利率4.5%，可透支限额为5000元的CheckAccount对象。

使用withdraw方法提款5000元，并打印账户余额和可透支额。

再使用withdraw方法提款18000元，并打印账户余额和可透支额。

再使用withdraw方法提款3000元，并打印账户余额和可透支额。

提示：

（1）子类CheckAccount的构造方法需要将从父类继承的3个属性和子类自己的属性全部初始化。

（2）父类Account的属性balance被设置为private，但在子类CheckAccount的withdraw方法中需要修改它的值，因此应修改父类的balance属性，定义其为protected。

运行结果如下图所示：


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