// 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