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I would like to understand how instanceof works.

Consider the following code:

class A { }
class B extends A { }
class C extends B { }
class D extends C { }

public class TestClass{
    public static void main(String args[]){
        B b = new C();   //1
        A a = b;        //2
        if (a instanceof A) System.out.println("A"); //3
        if (a instanceof B) System.out.println("B"); //4
        if (a instanceof C) System.out.println("C"); //5
        if (a instanceof D) System.out.println("D"); //6
    }
}

Correct me if I'm wrong here, in order for instanceof to return true, the IS-A condition must be satisfied. If you take a look at line //1. At runtime, the program knows that the object denoted by reference "a" is of type C. Therefore, shouldn't only condition at line //5 be in the output? Why are A and B also in the output?

D not being displayed is because the object is not an instance of D and so there's no confusion there. But I don't understand why A and B are displayed in the console.

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  • 6
    a is an A because C extends B extends A. a is an B because C extends B. a is a C because, well, it is. a is not a D because D extends C and a is a C. Commented Feb 4, 2017 at 10:56
  • I am upvoting the question to remedy the down-votes. Commented Feb 4, 2017 at 11:30
  • @LajosArpad see here. In short, upvote because you think this post is useful and deserves an upvote. Do not upvote to "reverse downvotes". Commented Feb 5, 2017 at 19:10
  • @Jeese James, An object of a class is instanceof all the super classes in the hierarchy (till Object.java) and also for all the interfaces implemented by it and by the super classes in the hierarchy. I have included the various cases for instanceof. Hopefully it will be helpful. Commented Feb 6, 2017 at 4:08

4 Answers 4

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Since C extends B and B extends C, a is an instanceof A.

Since C extends B, a is an instanceof B.

It is trivial that a is an instanceof C.

Let me put this into context. Let's suppose that you have three classes, Animal, CarnvorousAnimal and Lion. CarnivorousAnimal extends Animal and Lion extends CarnivorousAnimal. When you instantiate a Lion, your Lion will be an instanceof Lion trivially, but it will be an instanceof CarnivorousAnimal, since every Lion is also a CarnivorousAnimal. Since your object is a CarnivorousAnimal, it will be an Animal as well, because every CarnivorousAnimal is an Animal as well.

When you extend your classes, your subclass will be a more exact specification. Something more specific is the instance of its more general definition, a subset, if you will.

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1

As mentioned here

An object of subclass type is also a type of parent class.

The java rule is

A SuperClass reference variable can hold the object of SubClass.

So your Class A is holding reference of Class B and Class B is holding reference of Class C.

Animal                             Class A

DomesticAnimal extends Animal      Class B extends A

Dog extends DomesticAnimal         Class C extends B

From the above table we can say Dog is an Domestic Animal and also is an Animal

An animal is also a Animal

simply Class A can be a instance of Class A

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You could upvote my (earlier, equivalent) answer, as I have upvoted yours.
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As a matter of fact each reference of a certain class must reference to an object instance to that class.

That means that you can do things like B b = new C(); because C is a subclass of B, therefore every instance of C are also instance of B.

Think of those as a matrioska doll. Every inner doll is a super-class, and the outer is a derived class.

Everytime you instantiate an object of a certain type, you duplicate the doll, but to duplicate a big doll you must also duplicate the smaller dolls that are inside.

The same way, you can think of the object as a block of memory which has everything is in its class, plus everything is in its super class, and so on till you get up to the Object class.

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2 Comments

This is very unlear. "instance of C are also instance of C" - what? "instantiate an object of a certain type, you duplicate the doll" - what?
The first one is a type, which i corrected now. The second is an analogy, maybe not expressed in the best way i guess, but it should work.
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All the java objects are instanceof Object.java So any Object when used with instanceof keyword returns true for Object. This is because all the java classes are derived from Object.java ( all classes are implicitly sub claases or child classes of Object.java class ).

Keyword instanceof returns true for

  1. an object for its class

  2. and for all the upper level parent classes

  3. and for all the interfaces implemented by the class of the object and by all the classes in upper hierarchy of the class. Also true is returned for all the higher level interfaces (interfaces extended by the ones which are implemented)

    public class GrandParent implements IGrandParent { .... }

    public class Parent implements IParent{ .... }

    public class Child implements IChild { .... }

    public interface IgrandParent extends ISpecial1,ISpecial2 { ..... }

Now we see Child.java inherits from ISpecial1, ISpecial2, GrandParent, Parent, IParent, IGrandParent and IChild and hence instanceof for objcet of Child.java will evaluate to true for all above classes and interface, including fir Child.java.

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