Intent

It is belongs to behavioral design patterns catalog.
Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

Also known as

Cursor

Advantage of Iterator Pattern

It supports variations in the traversal of a collection.

It simplifies the interface to the collection.

Structure

Participants

1. Iterator

defines an interface for accessing and traversing elements.

2. ConcreteIterator

implements the Iterator interface.

keeps track of the current position in the traversal of the aggregate.

3. Aggregate

defines an interface for creating an Iterator object.

4. ConcreteAggregate

implements the Iterator creation interface to return an instanceof the proper ConcreteIterator.

Collaborations

A ConcreteIterator keeps track of the current object in the aggregate and can compute the succeeding object in the traversal.

Source code

Step 1: Create a Iterator interface.

/**
 * 
 * ItemIterator interface.
 * 
 */
public interface ItemIterator {

  boolean hasNext();

  Item next();
}

Step 2: Create Item class (iterating list of items).

public class Item {

  private ItemType type;
  private String name;

  public Item(ItemType type, String name) {
    this.setType(type);
    this.name = name;
  }

  @Override
  public String toString() {
    return name;
  }

  public ItemType getType() {
    return type;
  }

  public final void setType(ItemType type) {
    this.type = type;
  }
}

Step 3 : Create enum for ItemTypes

public enum ItemType {

  ANY, WEAPON, RING, POTION

}

Step 4: Create TreasureChest class which holds a collection of items.

import java.util.ArrayList;
import java.util.List;

/**
 * 
 * TreasureChest, the collection class.
 * 
 */
public class TreasureChest {

  private List<Item> items;

  /**
   * Constructor
   */
  public TreasureChest() {
    items = new ArrayList<>();
    items.add(new Item(ItemType.POTION, "Potion of courage"));
    items.add(new Item(ItemType.RING, "Ring of shadows"));
    items.add(new Item(ItemType.POTION, "Potion of wisdom"));
    items.add(new Item(ItemType.POTION, "Potion of blood"));
    items.add(new Item(ItemType.WEAPON, "Sword of silver +1"));
    items.add(new Item(ItemType.POTION, "Potion of rust"));
    items.add(new Item(ItemType.POTION, "Potion of healing"));
    items.add(new Item(ItemType.RING, "Ring of armor"));
    items.add(new Item(ItemType.WEAPON, "Steel halberd"));
    items.add(new Item(ItemType.WEAPON, "Dagger of poison"));
  }

  ItemIterator iterator(ItemType itemType) {
    return new TreasureChestItemIterator(this, itemType);
  }

  /**
   * Get all items
   */
  public List<Item> getItems() {
    List<Item> list = new ArrayList<>();
    list.addAll(items);
    return list;
  }

}

Step 5: Let's create an Iterator implementation class TreasureChestItemIterator.java

import java.util.List;

/**
 * 
 * TreasureChestItemIterator
 *
 */
public class TreasureChestItemIterator implements ItemIterator {

  private TreasureChest chest;
  private int idx;
  private ItemType type;

  /**
   * Constructor
   */
  public TreasureChestItemIterator(TreasureChest chest, ItemType type) {
    this.chest = chest;
    this.type = type;
    this.idx = -1;
  }

  @Override
  public boolean hasNext() {
    return findNextIdx() != -1;
  }

  @Override
  public Item next() {
    idx = findNextIdx();
    if (idx != -1) {
      return chest.getItems().get(idx);
    }
    return null;
  }

  private int findNextIdx() {

    List<Item> items = chest.getItems();
    boolean found = false;
    int tempIdx = idx;
    while (!found) {
      tempIdx++;
      if (tempIdx >= items.size()) {
        tempIdx = -1;
        break;
      }
      if (type.equals(ItemType.ANY) || items.get(tempIdx).getType().equals(type)) {
        break;
      }
    }
    return tempIdx;
  }
}

Step 6: Create App class to test Iterator design pattern.

public class App {

  /**
   * Program entry point
   * 
   * @param args command line args
   */
  public static void main(String[] args) {
    TreasureChest chest = new TreasureChest();

    ItemIterator ringIterator = chest.iterator(ItemType.RING);
    while (ringIterator.hasNext()) {
      System.out.println(ringIterator.next());
    }

    System.out.println("----------");

    ItemIterator potionIterator = chest.iterator(ItemType.POTION);
    while (potionIterator.hasNext()) {
      System.out.println(potionIterator.next());
    }

    System.out.println("----------");

    ItemIterator weaponIterator = chest.iterator(ItemType.WEAPON);
    while (weaponIterator.hasNext()) {
      System.out.println(weaponIterator.next());
    }

    System.out.println("----------");

    ItemIterator it = chest.iterator(ItemType.ANY);
    while (it.hasNext()) {
      System.out.println(it.next());
    }
  }
}