import java.util.Hashtable;
import java.util.Enumeration;

/** Class representing an undirected graph composed
  * of nodes.  The node class is a top-level class
  * nested within the Graph2 class.
  */
public class Graph2 {
   private Hashtable nodeList = new Hashtable();

   public void addNode( int x, int y ) {
     // the use of "this." is not required here
     this.new Node( x, y );
   }

   public String toString() {
      StringBuffer sb = new StringBuffer( "[ " );
      Enumeration e = nodeList.elements();
      while ( e.hasMoreElements() ) {
         sb.append( e.nextElement().toString()
                    + " " );
      }
      sb.append( "]" );
      return sb.toString();
   }

   public static void main( String[] args ) {
      System.out.println( "creating the graph" );
      Graph2 g = new Graph2();
      System.out.println( "adding nodes" );
      g.addNode( 4, 5 );
      g.addNode( -6, 11 );
      System.out.println( g );
   }

   private class Node {
      private int x, y;
      public Node( int x, int y ) {
         this.x = x;
         this.y = y;
         // the use of "Graph2.this." is not
         // required here
         if ( ! Graph2.this.nodeList
                      .containsKey( key() ) ) {
            nodeList.put( key(), this );
         }
      }

      public Object key() {
         return x + "," + y;
      }

      public String toString() {
         return "(" + x + "," + y + ")";
      }
   } // end of Node class
}