继承:
关键字:extends
格式:
类A extends 类B
类A为父类  类B为子类
功用:
让类之间产生关系===》》多态
提高代码的复用性

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
注意:
切勿滥用继承,必须有所属关系
简单代码举例:

 1 class C
 2 {
 3     void demo1(){}
 4 }
 5 
 6 class B extends C
 7 {
 8     void demo2(){}
 9     //void demo1(){} B和A不必作此声明
10 }
11 
12 class A extends C
13 {
14     //void demo1(){}
15     void demo3(){}
16 }
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