`DirectedGraph#edge(V, V)` can be improved by removing the `Optional` allocation

[michaelamaura]

Currently, DirectedGraph#edge relies on Map#get(K)which returns an optional.

This could be improved by using Map#get(K, V)with a default value - especially in highly frequently used graph traversals this could save man allocations.

So instead of:

  @Override
  public E edge(V from, V to) {

    Map m = out.get(from).orElseThrow(() -> new IllegalArgumentException("no such edge"));
    Object e = m.get(to, DEFAULT);

    if (e == DEFAULT) {
      throw new IllegalArgumentException("no such edge");
    } else {
      return (E) e;
    }
  }

It could be something like:

  @Override
  public E edge(V from, V to) {

    Map m = out.get(from, DEFAULT_MAP)
    if (m == DEFAULT) {
      throw new IllegalArgumentException("no such edge");
   }

    Object e = m.get(to, DEFAULT);

    if (e == DEFAULT) {
      throw new IllegalArgumentException("no such edge");
    } else {
      return (E) e;
    }
  }

[ztellman]

The JVM's escape analysis exists for exactly these sorts of situations, and should prevent any actual allocations from occurring. I'm willing to consider benchmarks that prove real overhead here, but otherwise I'm going to err on the side of keeping the code simpler.

[ztellman]

Closing, pending justifying benchmarks.