java-课堂笔记

### 多态

父类

```java
public class Uncle {
    private String name;
    private int age;
    public void faHongbao() {
        System.out.println("舅舅发红包");
    }
}

子类一：

public class UncleOne extends Uncle{

    public void faHongbao() {
        System.out.println("大舅发红包");
    }
    public void songYan() {
        System.out.println("大舅喜欢送烟");
    }

}

子类二：

public class UncleTwo extends Uncle{
    public void faHongbao() {
        System.out.println("二舅发红包");
    }
}

多态

        UncleOne dajiu = new UncleOne();
        dajiu.faHongbao();  // 大舅发红包
        UncleTwo uncleTwo = new UncleTwo();
        uncleTwo.faHongbao();  // 二舅发红包

向上转型

        Uncle dajiu1 = new UncleOne();
        dajiu1.faHongbao();  // 大舅发红包

向下转型

        Uncle dajiu1 = new UncleOne();
        dajiu1.faHongbao();
      //dajiu1.songYan();   // 会报错 子类独有的方法在发生向上转型的时候无法在父类中使用
        UncleOne temp = (UncleOne) dajiu1;  // 向下转型
        temp.songYan();  // 可以调用子类独有的方法

instanceof

判断对象是否是指定的类型的实例

避免发生错误的类型转换

public class Demo02 {
    public static void main(String[] args) {
        Uncle uncle1 = new UncleOne();
        Uncle uncle2 = new UncleTwo();
        if ( uncle1 instanceof UncleOne ) {
            UncleOne u1 = (UncleOne)uncle1;
            u1.faHongbao();
        }
        if (uncle2 instanceof UncleTwo ){
            UncleTwo u2 = (UncleTwo) uncle2;
            u2.faHongbao();
        }
    }
}