6908259: Covariance bug for multiple interface inheritence

Maurizio Cimadamore Maurizio.Cimadamore at Sun.COM
Tue Feb 9 02:10:19 PST 2010

David M. Lloyd wrote:
> I've produced a patch for javac bug #6908259 which appears to fix the 
> issue, at first blush.  Before engaging in more rigorous testing and 
> figuring out how to do this jtreg thing, can someone in the know have 
> a glance over this patch and see if it makes any sense?
> See http://bugs.sun.com/view_bug.do?bug_id=6908259 for a detailed 
> description of the problem.
> My attached patch tries to work around the issue by detecting when two 
> incompatible methods are both overridden, and if so, it does an extra 
> check to see if the overriding method is a valid covariation (?) of 
> both methods.
We also have a patch that is currently applied to JDK 7:


The fix looks a bit more complex than yours (it adds a new method in 
check that finds the common covariant overrider in the superclass 
hiererachy) - in fact there could be situations in which the last method 
in the hierarchy does NOT define the common overrider, as in one of the 
regression tests attached above:

class T6294779b {

 interface I1<E> {
 List<E> m();

 interface I2<E> {
 Queue<E> m();

 interface I3<E> {
 LinkedList<E> m();

 interface I4<E> extends I1<E>, I2<E>, I3<E> {}

In this case, I4 is well-formed as one of the implementing interfaces 
--- namely I3 --- is defining a method which is compatible with all 
other signatures from I1,I2.

> - DML

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