TreeSet and equals function

Question

Hi,

There is a Java bean object which has implemented equals function based on certain criteria (Criteria A). I have a requirement to identify unique objects based on another criteria (Criteria B). Since the equals function uses criteria A, I can not use HashSet. So I thought of using TreeSet with my custom Comparator which is based on criteria B. My question is, Is it allowed to do like this? Any issues with this approach?

Thank you.

Answer 1

Here is some guide from Oracle Java:

Note that the ordering maintained by a set (whether or not an explicit comparator is provided) must be consistent with equals if it is to correctly implement the Set interface. (See Comparable or Comparator for a precise definition of consistent with equals.) This is so because the Set interface is defined in terms of the equals operation, but a TreeSet instance performs all key comparisons using its compareTo (or compare) method, so two keys that are deemed equal by this method are, from the standpoint of the set, equal. The behavior of a set is well-defined even if its ordering is inconsistent with equals; it just fails to obey the general contract of the Set interface.

I think in terms of technical, no, you don't have any problems. But, in terms of coding, readability and maintainability, you have to be careful, because other people can misuse or misunderstand what you are doing

Answer 2

If you perform search often and add elements rarely, consider keeping them in a List sorted by Criteria B and using Collections.binarySearch.