Assigning base class object to sub class reference ?

[arulraj michealraj]

Hi ,

I have one doubt in the following code.
class A
{
}
class B extends A.

1. A objA=new A();
2. A obj=new B();
3. B objB=new A();

The first two line will work without giving any error. but for line 3, it will say compile time error.


May i know the reason why this is giving an error?

PLEASE HELP ME

Thanks
Arulraj

[Jesper de Jong]

The most important thing to remember about inheritance is that it is an "is a" relationship. So, an instance of a subclass is an instance of its superclass too.

Just like a dog is an animal, and a cow is an animal. Ofcourse it only works one way; if a dog is an animal, it doesn't mean that an animal is a dog.

In your code, an instance of B is an instance of A, so line #2 is no problem. However, an A is not necessarily a B, so line #3 doesn't work.

[arulraj michealraj]

Thanks.............

[Ilja Preuss]

And the technical reason is that Java is a statically typed language - that is, the compiler uses the type information to check at compile time whether a method call is possible.

Imagine that class B declared a new method that isn't available in class A. Consequently, you could call that method on any B reference. If such a reference could point to an instance of A, the compiler couldn't know whether the object actually *knows* the method you are trying to call.

Google for "Liskov Substitution Principle" for more on this.