// Purpose.  Iterator design pattern

// Take traversal-of-a-collection functionality out of the collection and
// promote it to "full object status".  This simplifies the collection, allows
// many traversals to be active simultaneously, and decouples collection algo-
// rithms from collection data structures.

// 1. Design an internal "iterator" class for the "collection" class
// 2. Add a createIterator() member to the collection class
// 3. Clients ask the collection object to create an iterator object
// 4. Clients use the first(), isDone(), next(), and currentItem() protocol

import java.util.*;

class IntSet {
   private Hashtable ht = new Hashtable();

   // 1. Design an internal "iterator" class for the "collection" class
   public static class Iterator {
      private IntSet      set;
      private Enumeration e;
      private Integer     current;
      public Iterator( IntSet in ) { set = in; }
      public void    first()       { e = set.ht.keys();  next(); }
      public boolean isDone()      { return current == null; }
      public int     currentItem() { return current.intValue(); }
      public void    next()        {
         try { current = (Integer) e.nextElement(); }
         catch (NoSuchElementException e) { current = null; }
   }  }

   public void      add( int in )     { ht.put( new Integer( in ), "null" ); }
   public boolean   isMember( int i ) { return ht.containsKey(new Integer(i)); }
   public Hashtable getHashtable()    { return ht; }
   // 2. Add a createIterator() member to the collection class
   public Iterator  createIterator()  { return new Iterator( this ); }
}

class IteratorDemo {
   public static void main( String[] args ) {
      IntSet set = new IntSet();
      for (int i=2; i < 10; i += 2) set.add( i );
      for (int i=1; i < 9; i++)
         System.out.print( i + "-" + set.isMember( i ) + "  " );

      // 3. Clients ask the collection object to create many iterator objects
      IntSet.Iterator it1 = set.createIterator();
      IntSet.Iterator it2 = set.createIterator();

      // 4. Clients use the first(), isDone(), next(), currentItem() protocol
      System.out.print( "\nIterator:    " );
      for ( it1.first(), it2.first();  ! it1.isDone();  it1.next(), it2.next() )
         System.out.print( it1.currentItem() + " " + it2.currentItem() + "  " );

      // Java uses a different collection traversal "idiom" called Enumeration
      System.out.print( "\nEnumeration: " );
      for (Enumeration e = set.getHashtable().keys(); e.hasMoreElements(); )
         System.out.print( e.nextElement() + "  " );
      System.out.println();
}  }

// 1-false  2-true  3-false  4-true  5-false  6-true  7-false  8-true
// Iterator:    8 8  6 6  4 4  2 2
// Enumeration: 8  6  4  2