package pkg;

public class p148 {
 public static void main(String[] args) {
  A a1 = new A();
  A a2 = new B();//KEY point
  //创建B对象，转化成A对象，拥有的成员函数:BshowA,AshowD
  B b = new B();
  C c = new C();
  D d = new D();
  
  System.out.println(a1.show(a1));// A shows A
  System.out.println(a1.show(a2));//KEY: A shows A
  System.out.println(a1.show(b));//KEY:  A shows A
  System.out.println(a1.show(c));// A shows A
  System.out.println(a1.show(d));// A shows A
  
  System.out.println("");
  
  System.out.println(a2.show(a1));//B shows A
  System.out.println(a2.show(a2));//B shows A
  System.out.println(a2.show(b));//KEY POINT:使用覆盖后 B shows A（B shows B disappeared）
  System.out.println(a2.show(c));//使用覆盖后B shows A
  System.out.println(a2.show(d));//使用基类的A show D 
  
  System.out.println("");

  System.out.println(b.show(a1));//B shows A
  System.out.println(b.show(a2));//B shows A
  System.out.println(b.show(b));//B shows B
  System.out.println(b.show(c));//B shows B
  System.out.println(b.show(d));//有合适的基类接口:A shows D
 }
}

//****
class A{
 public String show(D obj){
  return("A shows D");
 }
 public String show(A obj){
  return("A shows A");
 }
}
//****
class B extends A{
 public String show(B obj){
  return("B shows B");
 }
 public String show(A obj){
  return("B shows A");
 }
}
//****
class C extends B{
}
//****
class D extends B{
}

OUTPUT:

A shows A
A shows A
A shows A
A shows A
A shows D

B shows A
B shows A
B shows A
B shows A
A shows D

B shows A
B shows A
B shows B
B shows B
A shows D
 

总结：

upcating时:A a2=new B();

1、基类独有的方法，保留；remain

2、相同的方法，子类覆盖基类的；update

3、子类独有方法，消失（转型代价）。lost

4、域/数据：子类、基类均保留，不存在覆盖问题。

5、多态：仅仅针对普通方法才有，静态方法不会被覆盖。p156