迭代器模式Iterator

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 ); }
	}

public	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();
	}  }