How do I initialize vertex locations in a custom JUNG layout?

Question

How do I initialize a new layout with vertex positions known beforehand?

I have created a custom JUNG layout class:

public class CustomLayout extends AbstractLayout {

    AbstractLayout subLayout = null;

    final int WIDTH = 500;
    final int HEIGHT = 500;

    public CustomLayout(Graph<Vertex, Edge> graph, Transformer<Vertex, Point2D> init) {
        super(graph, init);

        for (Vertex v : this.getGraph().getVertices()) {
            // Assign each vertex a random initial position.
            setLocation(v, new Point2D.Double(random * WIDTH, random * HEIGHT);
        }

        subLayout = new FRLayout(this.getGraph(), ...?, null);
        // How do I pass each vertices prior positions?

    }

}

Answer 1

That's what the initializer (Transformer<V,Point2D>) is for. This transformer should return the initial position of any vertex you pass into it. I see that FRLayout doesn't accept an initializer in its constructor, but looks like you can call setInitializer().

In response to your question, Transformer is a generic interface that, when given an input, produces a corresponding output. The implementation can be whatever you want. It could serve up a set of statically defined positions, or calculate positions on the fly. This gives you a lot of flexibility.

You might notice that the interface bears a passing similarity to Map<K,V>, and in fact a very simple wrapper implementation could be made as follows:

  public class MapTransformer<K,V> implements Transformer<K,V> {

    private final Map<K,V> map;

    public MapTransformer(Map<K,V> map) {
      this.map = map;
    }

    public V transform(K key) {
      return map.get(key);
    }

  }

So you could put your positions in a map and then wrap that map in the above MapTransformer.