Following code shows ClassCastException

[Minal Silimkar-Urankar]

    

class A{
      A(){
      }
      void met(){
      }
}


class B extends A{
      B(){
      }
      void met(){
      }
}


class Test{
   public static void main(String[] args){
          B b=(B)new A();
          b.met();
   }
}

Line below gives runtime error ClassCastException, it does not give any error or warning at compile time
B b=(B)new A();

[Meet Gaurav]

you cannot do this. Actually wat you are trying to do here ?

A is a Super class of B

You are trying something like this
Exception aa = new Throwable(); \\ Impossible in Java

[Sagar Rohankar]

Down casting is not allowed in Java.
Like,

B b = new A(); // Oops.. error..

You cant assign parent object to child reference, you cant put big cup size sugar materiel into a small cup.

Now,
You can compile this program, by telling the compiler, like..

B b=(B)new A(); // I want to covert it, so I use casting .. and compiler says OK,  husshh...

Now, when you call.

b.met(); 

As the down-cast is invalid since an object created as type A doesn't necessarily support all the operations type B does. Thus throws exception..

[Ankit Garg]

Let me give a simple example to show what Sagar is saying.

class Animal
{
    void walk()
    {
    }
}
class Dog extends Animal
{
    void bark()
    {
        System.out.println("woof woof");
    }
}

Now if you do this

Dog dog = (Dog)new Animal();

If this doesn't throw exception, then you will try this

dog.bark();

But dog reference actually points to an Animal. And an animal cannot bark. So the Cast itself will fail...

[Casttro Francis]

Meet Gaurav is exactly rite. Downcasting is not possible in Java..

You are trying something like this
Exception aa = new Throwable(); \\ Impossible in Java

Throwable is a super class of Exception.

[Chandra Bhatt]

Just adding one brick to the wall

Animal animal = new Dog();
Dog dog = (Dog)animal; //down casting allowed because animal reference may refer to Dog object
dog.makeSound(); //ok

Animal animal = new Animal();
Dog dog = (Dog)animal;//fails obviously at runtime

Because it is run time decision/assignment what dog reference will refer to?
Compiler needs to keep silent until this assignment happens in real(at runtime).

[Corrected:] Changed up casting to down casting in comment

[Campbell Ritchie]

Chandra Bhatt wrote:

Animal animal = new Dog();
Dog dog = (Dog)animal; //upcasting allowed because animal reference may refer to Dog object
dog.makeSound(); //ok

You mean downcasting, surely?

[Vikas Kapoor]

Chandra,

Animal animal = new Dog();
Dog dog = (Dog)animal; //upcasting allowed because animal reference may refer to Dog object
dog.makeSound(); //ok

I guess you mean DOWN CASTING here.

[Vikas Kapoor]

One More thing Polymorphism and Inheritance is always tricky. So Practice, Practice and More Practice...

[Chandra Bhatt]

Yeah I meant down casting.

Thanks for correction.

[Minal Silimkar-Urankar]

Chandra Bhatt wrote:

  
Animal animal = new Dog();  
Dog dog = (Dog)animal; //upcasting allowed because animal reference may refer to Dog object  
dog.makeSound(); //ok  
 

As you shown in your code, using sub class reference if I try to refer to super class using casting it will work. But code I tried gives class cast exception.

[Chandra Bhatt]

minal silimkar wrote:

Chandra Bhatt wrote:

  
Animal animal = new Dog();  
Dog dog = (Dog)animal; //upcasting allowed because animal reference may refer to Dog object  
dog.makeSound(); //ok  
 

As you shown in your code, using sub class reference if I try to refer to super class using casting it will work. But code I tried gives class cast exception.

Using sub class reference that actually points to super class object, you can only call methods that have been overridden by the subclass.
In other words methods specific to sub class can not be invoked in this way.
In fact the dog reference points to Animal object at run time, if Animal class doesn't have makeSound(), it fails.

[Minal Silimkar-Urankar]

Originally posted by Chandra Bhatt

Using sub class reference that actually points to super class object, you can only call methods that have been overridden by the subclass. In other words methods specific to sub class can not be invoked in this way.

If you check code I originally posted, I am invoking overriden method in main, which gives runtime error ClassCastException.

[Chandra Bhatt]

It is going to fail if the sub class reference refers to super class object at run time, giving you ClassCastException.
Compiler forgives us doing down casting, thinking the reference variable may refer to sub class object.

[Vikas Kapoor]

minal silimkar wrote:

    

class A{
      A(){
      }
      void met(){
      }
}


class B extends A{
      B(){
      }
      void met(){
      }
}


class Test{
   public static void main(String[] args){
          B b=(B)new A();
          b.met();
   }
}

Line below gives runtime error ClassCastException, it does not give any error or warning at compile time
B b=(B)new A();

Minal,
If we simplify your main method's content,

A a = new A(); // Line 1
B b = (B) a; // Line 2
b.met();// Line 3

In line 1 we have created a reference 'a' of type A which refers to Object of type A. In line 2 we have created a reference 'b' and we are trying to DOWNCAST the reference of type A to type B. As A and B are in the same hierarchy compiler won't complain about DOWNCASTING. Now the current situation is a reference b of type B refers to object of type A. And compiler comes to know about this fact at runtime because at compile time it is still thinking that it would point to correct object type (optimistic ) . but when it comes to know about the fact that it is pointing to wrong object type it fails (crashes with broken heart). Now tell me , will below work?

A a = new B();
B b = (B) a;
a.met();

What would be the output?

[Minal Silimkar-Urankar]

So it gives "Run Time Error"

Thanks Vishal for detailed explanation