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

final class Pair {
  private final Set elements = new LinkedHashSet();
  Pair(T a, T b) {
    elements.add(a);
    if (!elements.add(b))
      throw new IllegalArgumentException();
  }
  @Override
  public int hashCode() {
    return elements.hashCode();
  }
  @Override
  public boolean equals(Object obj) {
    if (obj == this)
      return true;
    if (!(obj instanceof Pair))
      return false;
    return elements.equals(((Pair) obj).elements);
  }
}