FULL PRODUCT VERSION :
java version "1.6.0_01"
Java(TM) SE Runtime Environment (build 1.6.0_01-b06)
Java HotSpot(TM) Client VM (build 1.6.0_01-b06, mixed mode, sharing)


EXTRA RELEVANT SYSTEM CONFIGURATION :
None...platform-independent problem

A DESCRIPTION OF THE PROBLEM :
The equals() method is flawed for extremely large sets (> Integer.MAX_VALUE elements).

The size() contract for Collections is this:

    /**
     * Returns the number of elements in this collection. If this collection
     * contains more than <tt>Integer.MAX_VALUE </tt> elements, returns
     * <tt>Integer.MAX_VALUE</tt>.
     *
     * @return the number of elements in this collection
     */
    int size();

but the AbstractSet equals method does this:

    public boolean equals(Object o) {
    if (o == this)
        return true;

    if (!(o instanceof Set))
        return false;
    Collection c = (Collection) o;
    if (c.size() != size())
        return false;
        try {
            return containsAll(c);
        } catch (ClassCastException unused) {
            return false;
        } catch (NullPointerException unused) {
            return false;
        }
    }

If I have two Sets, each with at least MAX_VALUE elements, and one is a proper superset of the other, then equals is asymmetric.

So, if:
    set1 = {0, .., MAX_INT} and set2 = {1, ..., MAX_INT}
then:
    set1.equals(set2)
but
   !set2.equals(set1)

Here's a fix:

  public boolean equals(Object o) {
    if (o == this)
      return true;

    if (!(o instanceof Set)) {
      return false;
    }
    @SuppressWarnings("unchecked") // o instanceof Set ==> o is a Collection
    Collection<?> c = (Collection) o;
    int cSize = c.size();
    if (cSize != size()) {
      return false;
    }
    try {
      boolean result = containsAll(c);
      if (cSize == Integer.MAX_VALUE) {
        result &= c.containsAll(this);
      }
      return result;
    } catch (ClassCastException unused) {
      return false;
    } catch (NullPointerException unused) {
      return false;
    }
  }

It appears that AbstractMap has the same problem.

Just to head off questions like "What are you doing, creating sets with more than MAX_INT elements!?!!?" ...I'm creating sets that generate their elements dynamically, so I'm not really allocating MAX_INT objects. :-)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
See description.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I expect equals() to always be symmetric.
ACTUAL -
equals() is asymmetric under the conditions as per the Description field, above.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
No error message...just incorrect behaviour.

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
Subclasses of AbstractSet / AbstractMap can override equals() with the fix given in the Description.