Difference between casting and Superclass-Subclass referencing

[Mohnish Khiani]

class A
{
}
class B extends A
{
}
class C
{
public static void main(String a[])
{
A o1=new B();
A o2=(A)(new B());
}
}

What difference would be there in the working of o1 and o2 objects?

[Andrey Kozhanov]

No difference. Both objects are of type A, containing instances of type B. Actually, casting 'new B()' to A does nothing, because 'B is-an A'.

[Rob Spoor]

In fact, my Eclipse would give an "unnecessary cast" warning for line 12.

[Mohnish Khiani]

That means whichever objects and instance variables can be accessed by o1 can also be accessed by o2....am I right?

[Jesper de Jong]

Yes, lines 11 and 12 do exactly the same, and o1 and o2 will behave exactly the same. The cast in line 12 is unnecessary, it's better to leave it out.

[Mohnish Khiani]

okay,so is this also correct :

B obj=(B)(new A());

[Jesper de Jong]

No, that line means something entirely different and will give you a ClassCastException when you try to run it.

It will be more clear if you give the classes more descriptive names than A and B. Consider this:

class Animal {
  // ...
}

class Dog extends Animal {
  // ...
}

class Cat extends Animal {
  // ...
}

Now, you can ofcourse do this, because a Dog is an Animal, and a Cat is an Animal:

Animal fifi = new Dog();
Animal tommy = new Cat();

But the other way around doesn't work; an Animal is not always a Dog:

Dog fifi = new Animal();  // Error!

[Mohnish Khiani]

thank you..I got it