java interface references downcasting -

    interface {}      interface b{}      interface c{}      a = new a(){};     b b = (b)a;     c c = (c)b; 

even though these interfaces not related , above code seems compile ,if change these interfaces classes , doesn't compile obvious reasons . why interfaces behave way

the interfaces unrelated, there classes implement 3 interfaces, there instances can cast around merrily.

the reason not work classes given class can extend 1 superclass, there cannot class extends both class a , class b.

another spin on final classes. cannot cast, example, string map because there can no subclasses of string implement map. can cast date or arraylist map because there subclasses fit.

note reasoning applies @ compile-time. invalid cast still fail @ run-time. compile-time type-checking supposed catch invalid casts as possible, not cases can detected static type checking (at least not in java). unless compiler can guarantee cast makes no sense, allow it.