对象的上转型对象

1. 子类覆盖了父类的方法和具有同名的属性

package com.sun.util;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Scanner;

class A{
    public int a=1;
    public int b=2;
    public void add(){
        System.out.println("A add");
    }
    public void minus(){
        System.out.println("A minus");
    }
    
}
public class B extends A{
    public int a=3;
    public int b=4;
    public int c=5;
    @Override
    public void add() {
        // TODO Auto-generated method stub
//        super.add();
        System.out.println("B add");
    }
    public void multi(){
        System.out.println("B multi");
    }
    public static void main(String[] args) {
        LinkedList<Integer> list=new LinkedList<>();
        list.add(1, 1);
        A a = new B();
        System.out.println(a.a);    //1
        System.out.println(a.b);    //2
        a.add();
        a.minus();
//        a.multi() ;// 无法访问子类的新增方法
        
        B b =new B();
        System.out.println(b.a);    //3
        System.out.println(b.b);    //4
        System.out.println(b.c);
        b.add();
    }
}

1
2
B add
A minus
3
4
5
B add

1. 子类覆盖了父类的方法和没有同名的属性

package com.sun.util;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Scanner;

class A{
    public int a=1;
    public int b=2;
    public void add(){
        System.out.println("A add");
    }
    public void minus(){
        System.out.println("A minus");
    }
    
}
public class B extends A{
//    public int a=3;
//    public int b=4;
    public int c=5;
    @Override
    public void add() {
        // TODO Auto-generated method stub
//        super.add();
        System.out.println("B add");
    }
    public void multi(){
        System.out.println("B multi");
    }
    public static void main(String[] args) {

        A a = new B();
        System.out.println(a.a);    //1
        System.out.println(a.b);    //2
        a.add();
        a.minus();
//        a.multi() ;// 无法访问子类的新增方法
        
        B b =new B();
        System.out.println(b.a);    //1
        System.out.println(b.b);    //2
        System.out.println(b.c);
        b.add();
    }
}

 

1
2
B add
A minus
1
2
5
B add