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RE: Type soundness issues in Java

Thanks for all the answers. I would like to share three observations:

1. Verification is delayed as far as possible
---------------------------------------------
It seems that verification is, presumably for optimization, delayed as far
as possible. This seems to explain the observed behavior. If one forces
I.class to be loaded, e.g. by introducing a class B similar to A and
invoking one of its methods via I as below, the
IncompatibleClassChangeError is still raised at the same point.

public class Main {

        public static void main(String args[]) {
                A a = new A();
                I i = new B();
                i.m(); // force I.class to be loaded,
		       //  but might not be linked with A yet
                System.out.println("I've been started");
		...
	}
}




2. Type soundness violations that are not exploited go unnoticed
----------------------------------------------------------------
The second issue was brought to my attention by Christian Horn. If we do
not exploit the type soundness violation ---but still use the value of i---
than the violation will never be detected. The following Main2 should be
compiled against I.java and A.java from the previous posting and then
A.java recompiled with "implements I" removed:


public class Main2 {
        public static void main(String args[]) {
                A a=new A();
                I i;
                i=a; // type soundness violation
                A aa;
                aa=(A)i; // we use "i", but in a sound way
                aa.m();
        }
}


This issue is not due to an optimization by javac. It might --of course--
be optimized away by a JVM:

Method void main(java.lang.String[])
   0 new #1 <Class A>
   3 dup
   4 invokespecial #4 <Method A()>
   7 astore_1
   8 aload_1
   9 astore_2
  10 aload_2
  11 checkcast #1 <Class A>
  14 astore_3
  15 aload_3
  16 invokevirtual #6 <Method void m()>
  19 return



3. The abuse of exceptions to signal not implemented methods is worse
---------------------------------------------------------------------
Whereas the above type soundness issue in conjunction with binary
compatibility is definitely interesting, I am personally more worried about
the bad example set by the abuse of exceptions to signal unimplemented
methods in JCF.


Cheers,

Martin

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