为什么不按照它听起来的方式工作？

Question

In Java, the instanceof operator will only return false if comparing null . If you test a reference variable that isn't of the required type, the program fails to compile. Does this operator effectively only test if a reference is null or not? Or is there another use I'm not thinking of?

If not, why not call it isNull or something more descriptive?

Answer 1

I'm not sure how you came to your conclusion, but the instanceof operation will result in false when the runtime type of the first operand does not match the type specified by the second operand.

For example,

Object t1 = "Hello, World!";
System.out.println(t1 instanceof Number);

… prints "false" because t1 is a String , not a Number or one of its subclasses.

Can you please include your example of an instanceof operation that failed to compile? Most likely, this is due to a syntax error.

Answer 2

It fails to compile if the comparison can never be true. For example:

"string" instanceof Number

This will not compile because the compiler knows that a string can never be a number. The operator does exactly what one would expect it to do: it tests if the operand is an instance of the specified type.

Returning false when testing a null reference is meant to make the code simpler and more readable. It prevents you from needing to do this:

if (obj != null && obj instanceof Number) {
    ...
}

Read this to learn about the operator: http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.20.2

Answer 3

It's useful when you have a superclass type reference and you want to check if the object is an instance of some subclass:

eg

public class Sup { }

public class Sub1 extends Sup { }

public class Sub2 extends Sup { }

public void someMethod(Sup sup) {
    if(sup instanceof Sub1) {
        //....
    }
}

Answer 4

instanceof returns true when you use it agains an instance of the class at its right, or any derived from it. It will return false otherwise.

class A { /*...*/ }
class B extends A { /*...*/ }

A objA = new A();
B objB = new B();

boolean test = objA instanceof A;   // test is true
test = objA instanceof B;   // test is false
test = objB instanceof A;   // test is true
test = objB instanceof B;   // test is true

The code above will not raise any exception. The following will do, however:

objB = (B) objA;

Hope this helps.

Answer 5

if(MyObj instanceof SomeClass)

• If left side objet is null= return false
• if left side is not instance of rigth side class then return false
• if left side is instance if right side then return true

Please note, that instanceof usage can be avoided with polymorphism in most cases.