Assigning interface to interface

Question

Hi All,
I have a question in interface. It may be a silly doubt, but I am not sure what I am missing out.
I have 2 interfaces I1 and I2, totally unrelated. I create variables for these 2 interfaces i1 and i2 respectively.
Can I do like this?
i1 = (I1) i2;
I tried it and compiled the program. My program was running perfectly fine.
I again tried using java's standard API interfaces.
Comparable c1 = null;
Collection c2 = null;
c1 = (Comparable) c2;
This time also I didn't get any compile time or run time error. I know that I can compile time error if i try using c1 on some method of Collection like c1.isEmpty().
However what's the use of such kind of assignment between two unrelated interfaces. In what circumstances, do we need to make such kind of assignments?
Thanks,
kunalrock.

Answer 1

kunalrock wrote:
Yes. That's true for class variables assignment. No, what I described is true for assigning to variables of interface type.
interface I1 {}
interface I2 {}
class C1 extends ... {}
class C2 extends ... {}
I1 i1;
I2 i2;
C1 c1;
C2 c2;
i1 = i2; // 1
i1 = c1; // 2
c1  = c2; // 3
c1 = i1; // 4Here are the 4 possible scenarios
#1 is a compile-time error if there's no cast, unless I2 extends I1. With the cast, it will always compile, because the compiler can't know if the object that i2 points to will implement I1. With the cast, it is a ClassCastException (CCE) at runtime if the object that i2 points to does not implement I1.
#2 is a compile-time error if there's no cast, unless C1 implements I1. With the cast, it will always compile, because the comipler can't know if the object that c1 points to will implement I1, EXCEPT if C1 is final, then the compiler knows that if C1 does not implement I1, the cast cannot succeed. With the cast, it is a CCE at runtime if the object that c1 points to does not implement I1
#3 If C2 is a subclass of C1, it's legal as-is. If C1 is a subclass of C2, it's legal at compile time IF there's a cast. With the cast, if the object that c2 points to is not a C1 or a subclass of it, then you'll get a CCE
#4 is legal at compile time if C1 implements I1 or if you cast. It's a CCE at runtime if you cast and the object that I1 points to is not a subclass of C1.