Java: Weighted Directed Graph where the vertices are OBJECTS

Question

I need to create a weighted digraph. The implementations I've seen represent the vertices as simple integers, and the edges are represented with adjacency lists. However I need vertices that are represented by objects of a custom class. With this graph I need to perform typical operations (specifically, Dijkstra).

My idea was to hash somehow represent the objects as integers but I wouldn't know how to come up with a hash function that can be converted both ways (ie vertex to int and int to vertex).

Answer 1

If you really want to represent vertices as (unique) integers in the graph, you could use an ArrayList to map from vertices to your objects and a HashMap<MyObject, Integer> to map back. You could wrap this pair in your own class providing getIndex() and getElement() (as sketeched below), which may improve readability.

That said, why not just use http://jgrapht.org or a similar library?

public class IndexMap<T> {
  private List<T> elements = new ArrayList<>();
  private Map<T, Integer> indices = new HashMap<>();

  public int add(T element) {
    int index = elements.size();
    elements.add(element);
    indices.put(element, index);
    return index;
  }

  public int getIndex(T element) {
    Integer index = indices.get(element);
    return index == null ? -1 : index;
  }

  public T getElement(int index) {
    return elements.get(index);
  }

If you don't need to (re)use a pre-existing integer-based graph and can't use a library, you could alternatively implement your own Vertex class than stores an object:

 class Vertex<T> {
   T element;
   List<Vertex<T>> edges = new ArrayList<>();

   Vertex(T element) {
     this.element = element;
   }

   void addEdge(Vertex<t> to) {
     edges.add(to);
   }

   ...
 }