类的继承和多态细思

类的继承和多态细思

一、前言

  类的继承和多态光靠概念和想象是不够的，我们需要编码去分析之，因此我使用代码来说明一些很难分清的东西。

二、分析代码

A类代码：

package zyr.study.poly;

public class A {
    public A(){
        System.out.println("初始化A...");
    }

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

B类代码：

package zyr.study.poly;

public class B extends A{
    public  B(){
        System.out.println("初始化B...");
    }

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

C类代码：

package zyr.study.poly;

public class C extends B{
    public C(){
        System.out.println("初始化C...");
    }
}

D类代码：

package zyr.study.poly;

public class D extends B{
    public D(){
        System.out.println("初始化D...");
    }
}

main函数：

 1 package zyr.study.poly;
 2 
 3 
 4 public class Main {
 5 
 6     public static void main(String[] args) {  
 7         A a1 = new A();  
 8         System.out.println("-----------------");  
 9         A a2 = new B();  
10         System.out.println("-----------------");  
11         B b = new B();  
12         System.out.println("-----------------");  
13         C c = new C();  
14         System.out.println("-----------------");  
15         D d = new D();  
16         System.out.println("-----------------");  
17           
18         System.out.println("1--" + a1.show(a1));  // a and a
19         System.out.println("2--" + a1.show(a2));  // a and a
20         System.out.println("3--" + a1.show(b));   // a and b
21         System.out.println("4--" + a1.show(c));   // a and b
22         System.out.println("5--" + a1.show(d));   // a and d
23         
24         System.out.println("-----------------");  
25         
26         System.out.println("1--" + a2.show(a1));  // b and a
27         System.out.println("2--" + a2.show(a2));  // b and a
28         System.out.println("3--" + a2.show(b));   // b and b
29         System.out.println("4--" + a2.show(c));   // b and b
30         System.out.println("5--" + a2.show(d));   // a and d
31         
32         System.out.println("-----------------");  
33         
34         System.out.println("1--" + b.show(a1));  // b and a
35         System.out.println("2--" + b.show(a2));  // b and a
36         System.out.println("3--" + b.show(b));   // b and b
37         System.out.println("4--" + b.show(c));   // b and b
38         System.out.println("5--" + b.show(d));   // a and d
39     }  
40 }  

运行结果：

初始化A...
-----------------
初始化A...
初始化B...
-----------------
初始化A...
初始化B...
-----------------
初始化A...
初始化B...
初始化C...
-----------------
初始化A...
初始化B...
初始化D...
-----------------
1--A and A
2--A and A
3--A and B
4--A and B
5--A and D
-----------------
1--B and A
2--B and A
3--B and B
4--B and B
5--A and D
-----------------
1--B and A
2--B and A
3--B and B
4--B and B
5--A and D

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