Re: Collection of distinc objects

"Mike Schilling" <>
Fri, 29 Dec 2006 13:04:43 GMT
"D?ejm" <no.spam@no.spam> wrote in message news:en24ki$cpd$

Ok. It was simple. Sorry for bothering. It should work I think:

public class ReferenceComparator <T> implements Comparator <T>
public int compare( T o1, T o2 )
 if ( o1 == o2 )
  return 0;
 else if ( System.identityHashCode( o1 ) < System.identityHashCode( o2 ) )
  return -1;
  return 1;

This isn't foolproof. It's possible (though unlikely) for two distinct
objects to return the same value for identityHashCode(), and now your
comparator is illegal, since compare(o1,o2) == compare(o2,o1) == 1. This
can lead to strange behavior in TreeSet.

I can see two solutions which would always work:

1. Build a Set implementation on top of IdentityHashMap, where adding a
member to the set does a put to the underlying map with that member as the
key, and checking whether the set contains an object checks whether the map
contains that key. See java.util.HashSet for details (it's built exactly
this way on top of HashMap)

2. Create a wrapper class like this:

    public class Wrapper {
        private Object obj;

        public Wrapper(Object o) {
            obj = o;

        public boolean equals(Object o) {
            return (o instanceof Wrapper && ((Wrapper)o).obj == obj;

        public int hashCode() :
            return obj == null ? 0 : obj.hashCode();

Now make a Set of Wrappers.

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