[TYPES] Java generics unsoundness?

[Eugene Vigdorchik]

 Hello, Ejiro,

As far as I can tell (I'm working on IntelliJ IDEA, and we have our own type system implementation) the compiler is wrong here.
Though bridge methods are not mentioned explicitly in JLS, certain restrictions are imposed by language specification due to their
existance. The code you provide should be rejected because of JLS section 8.4.8.3 stating it is an error to have two inherited
methods (A.compareTo and Comparable.compareTo)  with same erasure that do not override each other.

Hope this helps,
Eugene.

> -----Original Message-----
> From: types-list-bounces at lists.seas.upenn.edu 
> [mailto:types-list-bounces at lists.seas.upenn.edu] On Behalf Of 
> Eijiro Sumii
> Sent: Sunday, October 01, 2006 5:19 AM
> To: types at cis.upenn.edu
> Cc: Eijiro Sumii
> Subject: [TYPES] Java generics unsoundness?
> 
> [ The Types Forum, 
> http://lists.seas.upenn.edu/mailman/listinfo/types-list ]
> 
> Dear all,
> 
> Hiromasa Kido, an undergraduate student in the University of 
> Tokyo, has found the problem below in the implementation of 
> generics in Java.
>  Another student, Kouta Mizushima, in Tsukuba University, 
> reported that it also reproduces in Eclipse (which has its 
> own compiler).
> 
> ----------------------------------------------------------------------
> C:\WINDOWS\Temp>c:\progra~1\java\jdk1.6.0\bin\java -version 
> java version "1.6.0-rc"
> Java(TM) SE Runtime Environment (build 1.6.0-rc-b100) Java 
> HotSpot(TM) Client VM (build 1.6.0-rc-b100, mixed mode, sharing)
> 
> C:\WINDOWS\Temp>type B.java
> class A{
>        public int compareTo(Object o){
>                return 0;
>        }
> }
> 
> class B extends A implements Comparable<B>{
>        public int compareTo(B b){
>                return 0;
>        }
> 
>        public static void main(String[] argv){
>                System.out.println(new B().compareTo(new Object()));
>        }
> }
> 
> C:\WINDOWS\Temp>c:\progra~1\java\jdk1.6.0\bin\javac -Xlint B.java
> 
> C:\WINDOWS\Temp>c:\progra~1\java\jdk1.6.0\bin\java B 
> Exception in thread "main" java.lang.ClassCastException: 
> java.lang.Object cannot  be cast to B
>         at B.compareTo(B.java:7)
>         at B.main(B.java:13)
> 
> C:\WINDOWS\Temp>
> ----------------------------------------------------------------------
> 
> Here is my understanding (confirmed by Atsushi Igarashi) of the
> problem: after erasure (i.e., replacing every type variable 
> with the Object class), an auxiliary method (called bridged 
> method) like
> 
>   public int compareTo(Object x){
>     return compareTo((B)x);
>   }
> 
> is created by the compiler in class B.  However, this 
> additional method happens to override A.compareTo and raises 
> an unexpected ClassCastException.
> 
> We have already submitted a bug report to Sun.  My question 
> is: Is this a bug in the compiler, or in the design of the 
> language?  On one hand, it is a compiler bug because the 
> bridge method, inserted by the compiler, is doing the harm.  
> On the other hand, it would be hard for any compilers to 
> implement generic interfaces without bridge methods (unless 
> we modify the present JVM).  In my understading, such 
> overriding as above is not forbidden in the current language 
> specification (page 227 and page 478 perhaps).
> 
> Does any expert in this list have a word on this matter...?
> 
> Thanks in advance,
> 
> Eijiro Sumii
> http://www.kb.ecei.tohoku.ac.jp/~sumii/
> 
> 
>