Java Downcasting of extending class

Question

Why this code is not working? When i casting, I get ClassCastExeption Exception in thread "main" java.lang.ClassCastException: A cannot be cast to B at HelloWorld.main

public class HelloWorld
{
 public static void main(String[] args)
  {
    B b1 = (B)new A();
    b1.a();
   }
}

public class A
{
  public void a (){
  System.out.println("A.a");
  }
  public void b (){
  System.out.println("A.b");
  }
}

public class B extends A{ 
}

I suggest you to read the basics of inheritance and how it works you cannot cast parent to child.... — Martin Čuka, Apr 01 '17 at 21:55
You're declaring B is a kind of A. That doesn't mean A is a kind of B. Then you're saying 'this A is a B'. Compiler complains. — pvg, Apr 01 '17 at 21:56
You _probably_ wanted it the other way round: `A b1 = new B(); b1.a();` — Alex Shesterov, Apr 01 '17 at 21:57
Yes, its upcasting. And what about downcasting in this example? — A. Kashnikov, Apr 01 '17 at 22:01
https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science) — Jazzwave06, Apr 01 '17 at 22:04

Martin Čuka · Accepted Answer · 2017-04-01T22:20:26.513

4

Casting can't always be done in both ways.

If you are creating an Animal, by calling "new Animal()", you are creating an Object that is an Animal, but it cannot be downcasted to Dog. Because it's not a dog.

However when you create a Dog by calling "new Dog()", then you can upcast it to "Animal" and downcast it back to "Dog".

Dog dog = new Dog();         
Animal animal = dog;         //upcasting to Animal
if(animal instanceof Dog){ // testing if the Animal is a Dog       
    Dog dog2 = (Dog) animal; //downcast
}

edited Apr 01 '17 at 22:20

answered Apr 01 '17 at 22:14

Martin Čuka

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Aha! Thank you! Downcasting possible only after upcasting! Now its clear! – A. Kashnikov Apr 01 '17 at 22:21
@A.Kashnikov And should almost never be used anyway. – chrylis -cautiouslyoptimistic- Apr 01 '17 at 22:24
right ! he should exercise more about inheritance concept in java to understand this well. – Oghli Apr 01 '17 at 22:25
@chrylis, yes. But its interesting to know such details – A. Kashnikov Apr 01 '17 at 22:27

Java Downcasting of extending class

1 Answers1