继承机制的探讨

1 public class Test {
2     public static void main(String[] args) {
3         Base p = new Sub("SubClass");
4         p.printA();
5     }
6 }

 1 class Base {
 2     public String str = "Base class";
 3     
 4     public void printA() {
 5         System.out.println(str);
 6     }
 7 }
 8 
 9 class Sub extends Base{
10     public Sub(String str) {
11         this.str = str;
12     }
13 }

 1 class Base {
 2     public String str = "Base class";
 3     
 4     public void printA() {
 5         System.out.println(str);
 6     }
 7 }
 8 
 9 class Sub extends Base{
10     public String str = "Base class";
11     
12     public Sub(String str) {
13         this.str = str;
14     }
15 }

 1 class Base {
 2     public String str = "Base class";
 3     
 4     public void printA() {
 5         System.out.println(str);
 6     }
 7 }
 8 
 9 class Sub extends Base{
10     public String str = "Base class";
11     
12     public Sub(String str) {
13         this.str = str;
14     }
15     
16     @Override
17     public void printA() {
18         System.out.println(str);
19     }
20 }