public class GraphOperations {

    public static int hopDistance(Graph graph, Vertex source, Vertex target){

        Set<Vertex> searchTree = new HashSet<Vertex>();
        searchTree.add(source);
        int count = 0;

        while (true){

          for (Vertex v : searchTree){

            if (v.equals(target)){
              return count;
            }
            else if (v.equals(searchTree[-1])){
              searchTree.addAll(getNeighbours(graph, v));
              count++;
            }

          }
        }

    }