Question

Say I have objects A,B,C,D. They can contain references to one another, for example, A might reference B and C, and C might reference A. I want to create segments but dont want to create them twice, so I don’t want segment A C and segment C A, just 1 of them. So I want to keep a list of created segments, ex: A C, and check if I already have an A C or C A and skip it if so.

Is there a data structure that can do this?

Thanks

if(list.contains(a,b)
{
   //dont add
}

Answer 1

you may introduce something like

class PairKey<T extends Comparable<T>> { 
  final T fst, snd;
  public PairKey(T a, T b) {
    if (a.compareTo(b) <=0 ) {
      fst = a;
      snd = b;
    } else {
      fst = b;
      snd = a;
    }
  }

  @Override
  public int hashCode() {
    return a.hashCode() & 37 & b.hashCode();
  }

  @Override
  public boolean equals(Object other) {
    if (other == this) return true;
    if (!(other instanceOf PairKey)) return false;
    PairKey<T> obj = (PairKey<T>) other;
    return (obj.fst.equals(fst) && obj.snd.equals(snd));
  }
}

then you may put edges into HashSet < PairKey < ? extends Comparable> > and then check if the given pair is already there.

You will need to make your vertexes comparable, so it will be possible to treat PairKey(A,B) equal to PairKey(B,A)

And then HashSet will do the rest for you, e.g you will be able to query

pairs.contains(new PairKey(A,B)); 

and if pairs contain either PairKey(A,B) or PairKey(B,A) – it will return true.

hashCode implementation might be slightly different, may be IDE will generate something more sophisticated.

Hope that helps.