Creation date: 14 March, 2011

Use instanceof carefully

interface MyInterface{}
class A{}
public class B{
   public static void main(String[] args) {
     B b = new B();
     if (b instanceof MyInterface){
       System.out.println("b is MyInterface");
     }
   }
}

This code will compile. Althougt the condition b instanceof MyInterface will not succeed the code will compile. But when we add the following condition:

if (b instanceof A){
  System.out.println("b is A");
}

compilation fails. The class B must be on the same path of inheritance tree with class A. In order to make such comparision class B must be subclass of A, or class A must be subclass of B. If we add extends A to the declaration of class B the condition b instanceof A will return true. If we add extends B to the declaration of class A, the condition b instanceof A will not succeed, but in both cases compilation will succeed.

For interfaces there are not such restriction. You can check an instance of any class against any interface.

Author: Jafar N.Aliyev (Jsoft)

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