Iterator Design Pattern Example in Java

In this article, we will learn how to use and implement the Iterator pattern in Java with an example.

Intent

Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

Iterator Design Pattern Example in Java

The main class in our example is the treasure chest that contains items.

public class TreasureChest {

  private final List<Item> items;

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

  public Iterator<Item> iterator(ItemType itemType) {
    return new TreasureChestItemIterator(this, itemType);
  }

  public List<Item> getItems() {
    return new ArrayList<>(items);
  }
}

public class Item {

  private ItemType type;
  private final 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;
  }
}

public enum ItemType {

  ANY, WEAPON, RING, POTION

}

The iterator interface is extremely simple.

public interface Iterator<T> {

  boolean hasNext();

  T next();
}

In the following example, we iterate through the ring-type items found in the chest.

var itemIterator = TREASURE_CHEST.iterator(ItemType.RING);
while (itemIterator.hasNext()) {
  LOGGER.info(itemIterator.next().toString());
}
// Ring of shadows
// Ring of armor

Applicability

Use the Iterator pattern

• to access an aggregate object's contents without exposing its internal representation
• to support multiple traversals of aggregate objects
• to provide a uniform interface for traversing different aggregate structures

Real-world examples

• java.util.Iterator
• java.util.Enumeration

Related Design Pattern Examples

• Chain of responsibility
• Command Design Pattern
• Iterator Design Pattern
• Mediator Design Pattern
• Memento Design Pattern
• Observer Design Pattern
• State Design Pattern
• Strategy Design Pattern
• Template Method Design Pattern
• Delegation Pattern

Credits

• Design Patterns: Elements of Reusable Object-Oriented Software
• Head First Design Patterns: A Brain-Friendly Guide