Allow null values for Graphs.Edge

[michaelamaura]

As far as I can tell the io.lacuna.bifurcan.Graphs.Edge<V, E> class serves as a standard implementation for IEdge<V, E>.

As in many graph instances the edges won't have a special value, it should be allowed to set the value to null. But now, the #hashCode method will throw a NullPoinrterException.

It should either be changed to:

@Override
public int hashCode() {
  if (hash == -1) {
    hash = from.hashCode() ^ to.hashCode() ^ (vale == null ? 0 : value.hashCode());
  }
  return hash;
}

or to:

@Override
public int hashCode() {
  if (hash == -1) {
    hash = Objects.hash(from, to, value);
  }
  return hash;
}

If either from or to is null a NullPointerException should be thrown by the constructor.

[ztellman]

Sorry for the late response, I've been traveling. Thanks for catching this, since from or to could also potentially be null I've changed it to this:

public int hashCode() {
      if (hash == -1) {
        hash = Objects.hashCode(from) ^ Objects.hashCode(to) ^ Objects.hashCode(value);
      }
      return hash;
    }

[michaelamaura]

Hey, great thanks for fixing this :-)

BTW: Great library, has been really helpful on a project I'm working on :-)

Just out of interest: Can you tell me the advantage from xoring the different hashCodes over calling Objects#hash? Is it the array allocation?

[ztellman]

There is some overhead with Objects.hash, but I believe my original rationale was to have equivalent hashes for undirected edges. On reflection, however, that doesn't also hold true for the equality check, so it probably makes sense to have DirectedEdge and UndirectedEdge classes.