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

From:
Patricia Shanahan <pats@acm.org>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 26 Jul 2008 13:22:53 -0700
Message-ID:
<g6g12u$1p36$1@ihnp4.ucsd.edu>
gg9h0st wrote:

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.


In theory, the compiler could apply flow analysis in your test case to
conclude that b references the new ClassB() result at the point of the
cast.

Without flow analysis, the compiler only knows that you are casting a
ClassB reference to InterfaceA. Its value might be a pointer to a ClassC
object, where extends ClassB and implements InterfaceA

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.

Patricia

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