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Iterator Design Pattern in Java

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 algorithms 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
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;
        }

        public void  next() {
            try {
                current = (Integer)e.nextElement();
            } catch (NoSuchElementException e) {
                current = null;
            }
        }
    }

    public void add(int in) {
        ht.put(in, "null");
    }

    public boolean isMember(int i) {
        return ht.containsKey(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();
    }
}

Output

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

Code examples

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