Generics oddness

[Brian Goetz]

The number of generic type parameters is one of the many axes on which 
two methods are judged to be similar enough to be the same method.

On 5/29/2012 11:07 AM, Ben Evans wrote:
> Hi,
>
> At Sunday's Lambdas Hackday, we discovered an issue with Apache
> Cassandra's use of Google Guava.
>
> Specifically, Guava defines an Ordering<T>  class which implements Comparator<T>.
>
> Ordering<T>  defines reverse(), which is of course now present on
> Comparator<T>, and causes javac to complain:
>
> This reduces to the following test case:
>
> public class Ordering<T>  implements Comparator<T>  {
>      @Override
>      public int compare(T o1, T o2) {
>          // ...
>      }
>
>      public<S extends T>  Ordering<S>  reverse() {
>          // ...
>      }
> }
>
> public class OrderingMain<T>  {
>          Ordering<T>  myOrdering;
>
>          public Ordering<T>  get() {
>              return myOrdering.reverse();
>          }
> }
>
> This compiles fine under JDK 7.
>
> Under JDK 8 Ordering<T>  compiles fine. OrderingMain<T>  fails with this error:
>
> hackday1/OrderingMain.java:20: error: reference to reverse is
> ambiguous, both method reverse() in Comparator and method<S>reverse()
> in Ordering match
>              return myOrdering.reverse();
>                               ^
>    where T#1,S,T#2 are type-variables:
>      T#1 extends Object declared in interface Comparator
>      S extends T#2 declared in method<S>reverse()
>      T#2 extends Object declared in class Ordering
> hackday1/OrderingMain.java:20: error: incompatible types
>              return myOrdering.reverse();
>                                       ^
>    required: Ordering<T>
>    found: Comparator<T>
>    where T is a type-variable:
>      T extends Object declared in class OrderingMain
> 2 errors
>
> I've stared at this extensively over the last couple of days, and come
> to the conclusion that I don't understand this behaviour.
>
> Is this a bug? If not, can someone untangle what's happening here for me?
>
> Thanks,
>
> Ben
>