java中instanceof与Class的等价性代码示例

时间:2021-10-27 06:49:04

本文研究的主要是javainstanceofClass的等价性的相关问题,具体如下。

java 中的instanceof 运算符是用来在运行时指出对象是否是特定类的一个实例。instanceof通过返回一个布尔值来指出,这个对象是否是这个特定类或者是它的子类的一个实例。

实例1(instanceof)

接口Person

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public interface Person {
public void eat();
}

实现类People

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public class People implements Person {
private int a=0;
 @Override
 public void eat() {
 System.out.println("======"+a);
 
 }
 
}

子类xiaoming:

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public class xiaoming extends People {
private String name;
 
@Override
public void eat() {
 System.out.println("+++++++++");
}
}

主函数

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public static void main(String[] args) {
 People p=new People();
 xiaoming x=new xiaoming();
 System.out.println(p instanceof Person);
 System.out.println(p instanceof xiaoming); -----2
 System.out.println(x instanceof Person);
 System.out.println(x instanceof People);
 }

注意:上面2处的代码在编译时不会报错。

运行结果:

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true
false
true
true

实例2

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package com.test.class_obj;
class Base {
}
class Derived extends Base {
}
public class FamilyVsExactType {
    static void test(Object x) {
        System.out.println("Testing x of type " + x.getClass().getSimpleName());
        System.out.println("-----------------------------------------");
        System.out.println("x instanceof Base " + (x instanceof Base));
        System.out.println("x instanceof Derived " + (x instanceof Derived));
        System.out.println("-----------------------------------------");
        System.out.println("Base.isInstance(x) " + Base.class.isInstance(x));
        System.out.println("Derived.isInstance(x) " +
                Derived.class.isInstance(x));
        System.out.println("-----------------------------------------");
        System.out.println("x.getClass() == Base.class " +
                (x.getClass() == Base.class));
        System.out.println("x.getClass() == Derived.class " +
                (x.getClass() == Derived.class));
        System.out.println("x.getClass().equals(Base.class)) " +
                (x.getClass().equals(Base.class)));
        System.out.println("x.getClass().equals(Derived.class)) " +
                (x.getClass().equals(Derived.class)));
        System.out.println("*****************************************");
        System.out.println("*****************************************");
    }
    public static void main(String[] args) {
        test(new Base());
        test(new Derived());
    }
}

输出内容如下:

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Testing x of type Base
-----------------------------------------
x instanceof Base true
x instanceof Derived false
-----------------------------------------
Base.isInstance(x) true
Derived.isInstance(x) false
-----------------------------------------
x.getClass() == Base.class true
x.getClass() == Derived.class false
x.getClass().equals(Base.class)) true
x.getClass().equals(Derived.class)) false
*****************************************
*****************************************
Testing x of type Derived
-----------------------------------------
x instanceof Base true
x instanceof Derived true
-----------------------------------------
Base.isInstance(x) true
Derived.isInstance(x) true
-----------------------------------------
x.getClass() == Base.class false
x.getClass() == Derived.class true
x.getClass().equals(Base.class)) false
x.getClass().equals(Derived.class)) true
*****************************************
*****************************************
 
Process finished with exit code 0

通过以上测试可以得出以下结论:

  • instanceof() 和 isInstance() 生成的结果相同
  • equals() 和 == 生成的结果相同
  • 父类可以是子类的实例,但子类不可以是父类的实例
  • Class 对象比较时,不考虑继承

总结

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原文链接:http://blog.csdn.net/heatdeath/article/details/79099861