casting

Here class A and B is-NOT-a I

then why A[] is casted to I[]

The rule of thumb about interfaces is, at the compile time, you can cast between any non-final class and an interface, even if there was not an is-a relationship.

Here's an example:

interface I { }
class B { }

I obj = (I) new B();

The above line compiles. It however fails at the run time with a ClassCastException due to the fact that B is-not-a(n) I.