[TYPES] Java generics unsoundness?

[Pete Kirkham]

The section on override-equivalent signatures,
<http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.4.2>,
indicates the effect is intentional - the erasure of B#compareTo(B)
overrides A#compareTo(Object) as well as Comparable<T>#compareTo(T)
since B#compareTo(B)  is override equivalent to A#compareTo(Object) by
erasure.

Since B's method does override A's method, but takes a takes a B, the
object undergoes assignment conversion
<http://java.sun.com/docs/books/jls/third_edition/html/conversions.html#5.2
>, and being incompatible, a ClassCastException is thrown. This is
achieved via the bridging method.

So I don't believe it's a bug in the compilers, but rather one of
several design features of a Java generics that are intended to help
backwards compatibility. In this case the non-generic version of B
would have been:

class B extends A implements Comparable {
      public int compareTo(Object o){
              B b = (B)o;
              // it's part of the contract of Comparable to throw CCE
if presented
              // with the wrong type
              return 0;
      }
}

Though actually it was common practice prior to 1.5 to add a bridge
method manually so the type-specific method was called when the type
of the argument was know.


Pete