Java 深入学习（26） —— instanceof 与 Class 的等价性

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：

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

《Java 编程思想》