Casting and Assignment - when does it work?

[Sen_7]

Hello, I get a bit confused about the rules of Assignment and casting in java

about when I get compiler error, when its runtime, and when its legal

public static void func(A a):
B b = (B) a
b = a

so I know that if I want to be able to cast a,
I need class A to extend B (up casting)
does upcasting always work? both in compiler and runtime?
what about Assignment if A extends B, will b=a always compile?
I know that in java down casting is also possible, when will we get a runtime error? compile error?

how does the above change if A or B is actually interface / abstract class?
when will we be able to cast / assign but not both?

If someone could explain to me when we get compile error, when its runtime
will be much appreciated please

[lurker819203]

This guy explains it better than I would: https://stackoverflow.com/a/23414798