对象通用超类Java instanceof

Question

Can someone provide an explanation for when (x instancef Object) will not equal true? Here's the full question?

Object is the universal superclass in Java, a superclass of all other classes. However, the expression x instanceof Object does not always equal true. Devise a declaration for x to demonstrate this?

Answer 1

null is not an instance of any class, and therefore returns false to the use of the instanceof operator:

Object x = null;
if (!x instanceof Object) {
    System.out.println ("we get here since X is null");
}

Answer 2

Clearly the null answer is the "correct one". But here is a pedantic alternative:

public class Test {

  private class Object {}

  public static void main(String[] args) {
    java.lang.Object s = new String("foo");    
    System.out.println((s instanceof Object));
  }
}

Prints

false

Here we abuse that fact that Object does not always refer to java.lang.Object .

Answer 3

如果x为null ， instanceof将返回false 。

Answer 4

Duplicate of When is “obj instanceof Object” false in Java?

If Object a=null; then a instanceof Object returns false