title	category	language	tag
Iterator	Behavioral	en	Gang of Four

Also known as

Cursor

Intent

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

Explanation

Real-world example

Treasure chest contains a set of magical items. There multiple types of items such as rings, potions, and weapons. The items can be browsed by type using an iterator the treasure chest provides.

In plain words

Containers can provide a representation agnostic iterator interface to provide access to the elements.

Wikipedia says

In object-oriented programming, the iterator pattern is a design pattern in which an iterator is used to traverse a container and access the container's elements.

Programmatic Example

The main class in our example is the TreasureChest 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);
  }
}

Here's the Item class:

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());
}

Program output:

Ring of shadows
Ring of armor

Class diagram

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.

Tutorials

• How to Use Iterator?

Known uses

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

Credits

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