How it works

Why it's not wrong if I was replaced by A and wrong if we replace I by B .Can you explain this

The given class structure is like this:

interface I { }
class A implements I { }
class B extends A { }

Would this statement compile?

A obj = new A();

Would this statement compile?

B obj = new A();

What do you think?

Hi,

From my understanding

A obj = new A();

compile without error

B obj = new A();

Throws a compile time error bcoz A is not B .

A obj = new B();

This works bcoz B is a A.

Perfect! Since A implements I, we can further extend it like this:

I obj = new A(); // compiles
A obj = new A(); // compiles
B obj = new A(); // does not compile

When it comes to arrays, the hierarchy remains the same:

I[] obj = new A[]{...}; // compiles
A[] obj = new A[]{...}; // compiles
B[] obj = new A[]{...}; // does not compile

So, as you can see above, when I is replaced with A it works, and when it is replaced with B it does NOT work - which answers the question you have asked.