public class Edge {
    public Vertex vertex1, vertex2; // undirected edge -- no significance
    public double weight; // the distance btwn the two vertices -- used
        // as the cost for shortest-path / MST problems

    public Edge(Vertex v1, Vertex v2, double wt) {
        vertex1 = v1;
        vertex2 = v2;
        weight = wt;
    }

    // returns the other vertex that's a member of the edge.  If v is 
    // not in the edge, throw an IllegalArgumentException.
    Vertex opposite(Vertex v) {
      if (v.equals(vertex1)) {
        return vertex2;
      } else if (v.equals(vertex2)) {
        return vertex1;
      } else {
        throw new IllegalArgumentException();
      }
    }
    
    public String toString() {
      return(vertex1.toString() + "<->" + vertex2.toString());
    }
    
}