Java中的instanceof和isInstance

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原文链接：https://blog.youkuaiyun.com/wbxaut/article/details/85262194

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本文详细解析了Java中instanceof操作符与Class类isInstance方法的区别，通过多个实例展示了两者在判断对象类型及转换时的行为特性，尤其强调了它们在处理空值时的不同表现。

1. instanceof 是一个操作符

if(a instanceof B){
}

表示：a 是不是 B 这种类型

2. isInstance是Class类的一个方法

if(B.Class.isInstance(a)){
};

表示：a 是否能强转为 B 类型

3. 代码

interface CFather {

}
class DFather {

}
class C implements CFather {

}
class D extends DFather {

}

public class WangboTest1 {
    public static void main(String[] args) {
        C c = new C();
        D d = new D();

        // obj instanceof class:这个对象是不是这种类型.
        // 测试1:一个对象是本身类的一个对象
        // true
        System.out.print("c instanceof C: ");
        System.out.println(c instanceof C);
        // true
        System.out.print("d instanceof D: ");
        System.out.println(d instanceof D);

        // 测试2:一个对象是本身类父类(父类的父类)和接口的一个对象
        // true
        System.out.print("c instanceof CFather: ");
        System.out.println(c instanceof CFather);
        // true
        System.out.print("d instanceof DFather: ");
        System.out.println(d instanceof DFather);

        // 测试3:所有对象都是object
        // true
        System.out.print("c instanceof Object: ");
        System.out.println(c instanceof Object);
        // true
        System.out.print("d instanceof Object: ");
        System.out.println(d instanceof Object);

        // 测试4:凡是null相关的都是false
        // false
        System.out.print("null instanceof Object: ");
        System.out.println(null instanceof Object);
        // false
        System.out.print("null instanceof CFather: ");
        System.out.println(null instanceof CFather);
        // false
        System.out.print("null instanceof DFather: ");
        System.out.println(null instanceof DFather);


        // class.isInstance(obj):这个对象能不能被转化为这个类
        // 测试1:一个对象是本身类的一个实例
        // true
        System.out.print("C.class.isInstance(c): ");
        System.out.println(C.class.isInstance(c));
        // true
        System.out.print("D.class.isInstance(d): ");
        System.out.println(D.class.isInstance(d));

        // 测试2:一个对象能被转化为本身类所继承的类(父类的父类)和实现的接口(接口的父接口)强转
        // true
        System.out.print("CFather.class.isInstance(c): ");
        System.out.println(CFather.class.isInstance(c));
        // true
        System.out.print("DFather.class.isInstance(d): ");
        System.out.println(DFather.class.isInstance(d));

        // 测试3:所有对象都能被Object强转
        // true
        System.out.print("Object.class.isInstance(c): ");
        System.out.println(Object.class.isInstance(c));
        // true
        System.out.print("Object.class.isInstance(d): ");
        System.out.println(Object.class.isInstance(d));

        // 测试4:凡是和null相关的都是false
        // false
        System.out.print("Object.class.isInstance(null): ");
        System.out.println(Object.class.isInstance(null));
        // false
        System.out.print("D.class.isInstance(null): ");
        System.out.println(D.class.isInstance(null));
        // false
        System.out.print("C.class.isInstance(null): ");
        System.out.println(C.class.isInstance(null));
    }
}

结果

c instanceof C: true
d instanceof D: true
c instanceof CFather: true
d instanceof DFather: true
c instanceof Object: true
d instanceof Object: true
null instanceof Object: false
null instanceof CFather: false
null instanceof DFather: false
C.class.isInstance(c): true
D.class.isInstance(d): true
CFather.class.isInstance(c): true
DFather.class.isInstance(d): true
Object.class.isInstance(c): true
Object.class.isInstance(d): true
Object.class.isInstance(null): false
D.class.isInstance(null): false
C.class.isInstance(null): false