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?

What is your reason to write a container of `unordered paris of any type`? — Andrew Logvinov, Dec 22 '12 at 09:12
possible duplicate of [What is the equivalent of the C++ Pair in Java?](http://stackoverflow.com/questions/156275/what-is-the-equivalent-of-the-c-pairl-r-in-java) — JB Nizet, Dec 22 '12 at 09:17
@AndrewLogvinov You can consider for example edges of unordered graph. `Edge(v1, v2)` equals to `Edge(v2, v1)`. — Michael, Dec 22 '12 at 09:47

score 3 · Answer 1 · answered Dec 22 '12 at 10:08

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...

Unordered pair in Java

1 Answers1