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 Graph1 class.
  */
public class Graph1 {
   private Hashtable nodeList = new Hashtable();

   public void addNode( int x, int y ) {
      Node n = new Node( x, y );
      if ( ! nodeList.containsKey( n.key() ) ) {
         nodeList.put( n.key(), n );
      }
   }

   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" );
      Graph1 g = new Graph1();
      System.out.println( "adding nodes" );
      g.addNode( 4, 5 );
      g.addNode( -6, 11 );
      System.out.println( g );
   }

   /** The class representing nodes within the graph */
   private static class Node {
      private int x, y;

      public Node( int x, int y ) {
         this.x = x;
         this.y = y;
       }

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

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