java关键字-instanceof

主要作用

A instanceof B主要判断以下三个内容（即左边的类型A能否转换为右边的B）：

• 判断对象A是否是B类的对象；
• 判断对象A实现了B接口；
• 判断对象A是否继承了B类；

A表示对象，B表示接口/类，如果A对象为B的对象，或者间接/直接实现B接口，或者间接/直接继承B类，则返回true，否则返回false

代码举例

Animal接口

package com.cyy.instance;

public interface Animal {
    public void eat();
}

Animal接口的两个实现类：Cat和Dog

package com.cyy.instance;

public class Cat implements Animal{
    @Override
    public void eat() {
        System.out.println("猫吃鱼");
    }
}

package com.cyy.instance;

public class Dog implements Animal{
    @Override
    public void eat() {
        System.out.println("狗吃骨头");
    }
}

cat的两个实现类：BlackCat和WhiteCat

package com.cyy.instance;

public class BlackCat extends Cat{
}


package com.cyy.instance;

public class WhiteCat extends Cat{
}

WhiteCat的实现类：WhiteBlackCat

package com.cyy.instance;

public class WhiteBlackCat extends WhiteCat{
}

测试

package com.cyy.instance;

public class Test {
    public static void main(String[] args) {
        Dog dog = new Dog();
        Cat cat = new Cat();
        WhiteCat whiteCat = new WhiteCat();
        BlackCat blackCat = new BlackCat();
        WhiteBlackCat whiteBlackCat = new WhiteBlackCat();

        //1.判断对象cat是否是Cat类的对象
        boolean b1 = cat instanceof Cat;
        System.out.println(b1);//true

        //2.判断dog和cat是否实现了Animal接口
        boolean b2 = cat instanceof Animal;
        boolean b3 = dog instanceof Animal;
        System.out.println(b2);//true
        System.out.println(b3);//true

        //3.判断whiteCat和blackCat是否继承了Cat
        boolean b4 = whiteCat instanceof Cat;
        boolean b5 = blackCat instanceof Cat;
        System.out.println(b4);//true
        System.out.println(b5);//true

        //4.间接实现接口，判断whiteCat和blackCat是否可以转换为Animal
        boolean b6 = whiteCat instanceof Animal;
        boolean b7 = blackCat instanceof Animal;
        System.out.println(b6);//true
        System.out.println(b7);//true

        //间接继承类：判断
        boolean b8 = whiteBlackCat instanceof Cat;
        System.out.println(b8);//true
    }
}