Vfrdtyky

Let's say I have the following interface and classes defined:

public interface I { void a(); }



public class A implements I {

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

}



public class B implements I {

    public void a() { System.out.println("B"); }

    public void b() { System.out.println("C"); }

}

And then I run the following code:

public class Main {

    public static void main(String args) {

        A a = new A();

        B b = new B();

        I i;

        i = a;

        i.a();    // prints "A"

        i = b;

        i.a();    // prints "B"

        i.b();    // 1st problem: i can't seem to find method b. Why?

        b = i;    // 2nd problem: b can't be assigned to i although i references an object of class B?

        b = (B)i; //              why does this work fine...

        a = (A)i; // 3rd problem: ...but this here doesn't?

    }

}

So here are my questions:

First Problem

Why can't i.b() be called?

i points to the same object as b, an object of class B which does have a method b.

So why does i.a() call the right method (the one that prints out "B") but i.b() doesn't resolve at all?

Does the fact that i was declared as being of type I (an interface) have anything to do with that? Does this mean that in an assignment X x = new Y() where Y extends X, one can only ever call methods on x that are already declared in X, and not just specific to Y?

Second Problem

Why can't b be assigned to i although i references an object of class B? b and i already reference the same object, don't they? So why does it cause an error if I try to assign b to i - the end result of which should be identical to the state of the program before that assignment, unless I'm missing something significant.

Third Problem

Why can I cast i to type B now although I couldn't assign b to i earlier, and why doesn't casting i to A work?

I'm assuming my confusion is somehow rooted in an unclear distinction between the reference variables and the objects they're referencing, as well as the differences between the types of these variables and objects. I just can't quite explain these occurrences - and in particular the first problem confuses me a lot.

For the first problem:
You can use the interface reference to call only the methods it declares

For the second problem:
You can use interface reference to invoke methods in the classes that implement the interface. However, there is no use to assign interface reference to a class reference since interface reference doesn't have any methods that can be invoked.

for the third problem:
You have assigned previously

        i=b 

and hence

     b=(B)i 

works fine.

However,
a=(A)i
wouldn't work because i stores b and not a

First of all, learn Java (and/or OO (object oriented)) programming...

1. Variable i is a reference to an object instance that implements interface I. Method b() was not declared in interface I, thus it is not visible through i.b().
   
   To be able to call it, i needs to be casted, EG: ((B) i).b()
   


2. Variable b is a reference to an object that is an instance of class B, and cannot be assigned to any reference that itself is not declared as an instance of B.
   
   Again, a cast needed, EG: b = (B) i
   


3. Class B is not a child of class A. They both implement interface I, but A is not parent of B.

It's not a problem at all but It's behavior of inheritance and polimorphism.

Please note that when you

I i = new A();

Left hand side (I) will tells compiler which all methods it can call using that reference.

Right hand side (A) will tells the Runtime which method should execute using that method call

So in your case

1 Problem

you can not call b() since b() is not there in inteface I

2 Problem

you are casting interface to object b and then calling b() so its working fine.