AbstractAccessReferenceMap.addDirectReference not invariant

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Implementation of addDirectReference may return a different value in case of 
concurrent calls for the same "direct" value.

1.   public <T> K addDirectReference(T direct) {
2.      if ( dtoi.keySet().contains( direct ) ) {
3.         return dtoi.get( direct );
4.      }
5.      K indirect = getUniqueReference();
6.      itod.put(indirect, direct);
7.      dtoi.put(direct, indirect);
8.      return indirect;
9.   }

If two threads concurrently call addDirectReference, the indirect value 
returned may be different (can be verified by putting a sleep within the above 
method). As a result, itod will contain both the indirect values whereas dtoi 
will contain only the last one. As the indirect value is returned, one of the 
two threads will have a value that cannot be obtained anymore from 
addDirectReference.

Although this is not strictly a bug, stating that the class is thread-safe 
leads to believe that its methods are invariant unless told otherwise. 
Multi-threaded scenarios where is expected that a given direct id always 
returns the same indirect id will experience erratic behaviour from this call, 
which will "mostly" return the same id but not always.

Whether this is considered a bug or not is yours to decide. If it's not, then 
please put the proper way to use addDirectReference and getDirectReference into 
the documentation.

Original issue reported on code.google.com by filippo....@gmail.com on 11 Apr 2014 at 4:48