Question

I need to have a Set (HashSet) such that if I insert a pair (a, b) and if (b, a) is already in the set, the insertion would just be ignored. How to do this in Java?

Many thanks!

Answer 1

Well, it depends on the hashCode() and equals() method of your Pair class. They need to ignore order.

Set itself is a good example of a class which ignores order for equality–you can look at the code of AbstractSet. If the order of the pair doesn’t matter even outside of equality comparison, you can just store HashSets (each with two elements) in your set. It would be best to wrap it in a datatype:

 public class UnorderedPair<T> {
     private final Set<T> set;

     public UnorderedPair(T a, T b) {
          set = new HashSet<T>();
          set.add(a);
          set.add(b);
     }

     public boolean equals(Object b) {
         //...delegate to set
     }

     public int hashCode() {
         return set.hashCode();
     }
}