Unordered pair in Java

Question

I need a Java generic class to represent unordered pairs of any type. Meanwhile I see two solutions:

• HashSet to store the pair elements
• a Pair class with overriden hashCode and equals (to make Pair(a, b) and Pair(b, a) equal).

Does it make sense? What would you suggest?

Answer 1

In your place I would roll out my own class. As long as you are interested in sets of only two objects, using HashMap, HashSet (which, incidentally, uses a HashMap internally anyway) or any other class designed for sets of arbitrary cardinality is a waste of resources and adds unneeded complexity.

Just create your own class with proper equals() and hashCode() implementations. Having a contains() operation, or even implementing parts of the Set interface, might also make sense.

One important note: make sure you document your class extensively - at least specify whether equals() performs an identity or an equality comparison for the contained objects, and what is the meaning of a null contained reference...