Why not compile error on "incompatible interface cast"?

Question

simeple code here
class IncompInterfaceTest {
void doTest() {
ClassB b = new ClassB();
InterfaceA a = (InterfaceA)b;
interface InterfaceA{}
class ClassB{}
javac issues "ClassCastException" on runtime rather than compile
error.
as i know incompatible type casting may be caught at runtime and
compiler shows the error.
but why compiler doesn't do it's job for interfaces? it's just so
clear to be compile error to me.
ps. in a case ClassB is final class, compiler issues a error.
(a thread I started in google groups)
http://groups.google.co.kr/group/comp.lang.java.programmer/browse_thread/thread/c9c12a8864672436?hl=ko#

Answer 1

sun9h0st wrote:
thanks for reply.
I think i might know something wrong about class, object concept, so I wish you tell me more about it.
As I though the code below means
ClassB b = new ClassC();'b' can be an Object of ClassB as a subclass of ClassC.
'b' IS an Object of ClassC.b is not an object.
b is a variable that can point to an instance of ClassB or to an instance of any subclass of ClassB.
>
so
b instanceof ClassB == true // as it can be
b instanceof ClassC == true // as it is
If b is not null, the first line will always be true.
If b points to a ClassC or a subclass, the second one will be true.
It means actual Object(something behind 'b' pointing, i don't know what i call it) is from ClassC to me. (somewhat like c pointer,
'b' is a bowl that called Object and actuall instance is refered by 'b')No idea what you're saying here.
if b IS an Object of ClassB
shouldn't "b instanceof ClassC" is false?If we have C extends B, then...
B b1 = new B();
B b2 = new C();
b1 instanceof B // true
b1 instanceof C // false
b2 instanceof B // true
b2 instanceof C // true
I Think compiler should be known that instance 'b' hold. No, it doesn't, and it's good that it doesn't, as it keeps the language simpler and more consistent.
Also, b doesn't hold an instance. It holds a reference to an instance.