how to write write a set for unordered pair in Java

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

Answer 1

As none of the answers mentions this approach, I'd like to add this approach:

public class UnorderedPair {
    final T first, second;

    public UnorderedPair(T first, T second) {
        this.first = first;
        this.second = second;
    }

    @Override
    public boolean equals(Object o) {
        if (!(o instanceof UnorderedPair))
            return false;
        UnorderedPair up = (UnorderedPair) o;
        return (up.first == this.first && up.second == this.second) ||
                (up.first == this.second && up.second == this.first);
    }

    @Override
    public int hashCode() {
        int hashFirst = first.hashCode();
        int hashSecond = second.hashCode();
        int maxHash = Math.max(hashFirst, hashSecond);
        int minHash = Math.min(hashFirst, hashSecond);
        // return Objects.hash(minHash, maxHash);
        // the above line also works but I tend to avoid this to avoid unnecessary auto-boxing
        return minHash * 31 + maxHash;
    }
}

Note that the general contract of the hashCode() and equals() should be followed:

• If you are overriding either hashCode() or equals() you usually have to also override the other method.
• If the equals return true for 2 objects, then the hashCode() method must return the same int for both the objects.
• If the hashCode() returns same int for 2 objects, then it's not necessarily true that the equals() method return true.

The above implementation of equals() and hashCode() method ensures this.