Re: why not compile error on "incompatible interface cast"?

From:

Lew <com.lewscanon@lew>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 26 Jul 2008 20:03:27 -0400

Message-ID:

<ybqdnTAB9ITSJhbVnZ2dnUVZ_vGdnZ2d@comcast.com>

gg9h0st wrote:

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 [sic] know incompatible type casting may be caught at runtime and
compiler shows the error.

Patricia Shanahan wrote:

In general, given a non-final class and an interface, the compiler has
to allow for the possibility that the class has a subclass that
implements the interface.

From the JLS section 5.5:

The detailed rules for compile-time legality of a casting conversion of a
value of compile-time reference type S to a compile-time reference type T
are as follows:

....

If S is a class type:

....

* If T is an interface type:
o If S is not a final class (??8.1.1), then, if there exists a supertype X of
T, and a supertype Y of S, such that both X and Y are provably distinct
parameterized types, and that the erasures of X and Y are the same, a
compile-time error occurs. Otherwise, the cast is always legal at compile time
(because even if S does not implement T, a subclass of S might).

Take S as ClassB and T as InterfaceA.

--
Lew