Question

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