2021-04-25 多态笔记

# 多态

**父类**

```java
public class Uncle {
    public 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("大舅不仅发红包还送烟");
    }
    public void chouyan() {
        System.out.println("大舅喜欢抽烟");
    }

}

子类二：

public class UncleTwo  extends  Uncle{
    public void fahongbao () {
        System.out.println("二舅不仅发红包，还送烟");
    }

}

启动类：

多态

public class Domo {
    public static void main(String[] args) {
        // 多态
         UncleOne uncle1 = new UncleOne();
         uncle1.fahongbao();
        
        UncleTwo uncle2 = new UncleTwo(); // 向上转型
        uncle2.fahongbao();

用 父类接受子类的对象，只能调用父类中出现过的方法，子类的扩展的独有方法无法调用

public class Domo {
    public static void main(String[] args) {
     UncleOne uncle1 = new UncleOne();
     uncle1.chouyan(); 不能调用

向上转型

UncleTwo uncle2 = new UncleTwo(); // 向上转型
uncle2.fahongbao();

向下转型

UncleOne uncle1 = new UncleOne();
UncleOne u1 = (UncleOne) uncle1 ; // 向下转型
 u1.chouyan();
  u1.fahongbao();

instanceof

判断对象是否给定的类的实例

作用：避免类型转换时，出现错误，进而引发程序的崩溃

public class Domo01 {
    public static void main(String[] args) {
        Uncle uncle1 = new UncleOne();
        if (uncle1 instanceof UncleTwo) {
            UncleTwo u2 = (UncleTwo) uncle1;
            u2.fahongbao();
        }
        if (uncle1 instanceof  UncleOne) {
            UncleOne u1 = (UncleOne) uncle1 ;
            u1.chouyan();
            u1.fahongbao();
        }
    }
}