java 多态经典例子

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

class D extends B{}

public static void main(String[] args) {
A a1 = new A();
A a2 = new B();
B b = new B();
C c = new C();
D d = new D();
System.out.println(a1.show(b)); //① A and A
System.out.println(a1.show(c)); // ② A and A
System.out.println(a1.show(d)); // ③ A and D
System.out.println(a2.show(b)); // ④ B and A
System.out.println(a2.show(c)); // ⑤ B and A
System.out.println(a2.show(d)); // ⑥ A and D
System.out.println(b.show(b)); // ⑦ B and B
System.out.println(b.show(c)); //⑧ B and B
System.out.println(b.show(d)); // ⑨ A and D
}

输出结果：

A and A
A and A
A and D
B and A
B and A
A and D
B and B
B and B
A and D

分析：

a1对象里面有两个方法

public String show(D obj){
return ("A and D");
}
public String show(A obj){
return ("A and A");
}

传 b时会进行向上转型走show(A obj)，所以return ("A and A");

传c时也会进行向上转型到b，再向上到a走show(A obj)，所以return ("A and A");

传d时直接走show(D obj)，所以 return ("A and D");

a2对象里面有两个方法

public String show(D obj){
return ("A and D");
}

public String show(A obj){
return ("B and A");
}

传 b时会进行向上转型走show(A obj)，所以return ("B and A");

传c时也会进行向上转型到b，再向上到a走show(A obj)，所以return ("B and A");

传d时直接走show(D obj)，所以 return ("A and D");

b里面三个方法

public String show(B obj){
return ("B and B");
}
public String show(A obj){
return ("B and A");
}

public String show(D obj){
return ("A and D");
}

传 b时直接走show(B obj)，所以 return ("B and B");

传c时也会进行向上转型到b，再向上到a走show(B obj)，所以return ("B and B");

传d时直接走show(D obj)，所以 return ("A and D");

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