136 lines
3.6 KiB
Markdown
136 lines
3.6 KiB
Markdown
---
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layout: pattern
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title: Iterator
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folder: iterator
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permalink: /patterns/iterator/
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categories: Behavioral
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tags:
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- Gang of Four
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---
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## Also known as
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Cursor
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## Intent
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Provide a way to access the elements of an aggregate object
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sequentially without exposing its underlying representation.
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## Explanation
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Real world example
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> 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.
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In plain words
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> Containers can provide a representation agnostic iterator interface to provide access to the elements.
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Wikipedia says
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> 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.
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**Programmatic Example**
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The main class in our example is the treasure chest that contains items.
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```java
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public class TreasureChest {
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private final List<Item> items;
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public TreasureChest() {
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items = List.of(
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new Item(ItemType.POTION, "Potion of courage"),
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new Item(ItemType.RING, "Ring of shadows"),
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new Item(ItemType.POTION, "Potion of wisdom"),
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new Item(ItemType.POTION, "Potion of blood"),
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new Item(ItemType.WEAPON, "Sword of silver +1"),
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new Item(ItemType.POTION, "Potion of rust"),
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new Item(ItemType.POTION, "Potion of healing"),
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new Item(ItemType.RING, "Ring of armor"),
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new Item(ItemType.WEAPON, "Steel halberd"),
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new Item(ItemType.WEAPON, "Dagger of poison"));
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}
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public Iterator<Item> iterator(ItemType itemType) {
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return new TreasureChestItemIterator(this, itemType);
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}
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public List<Item> getItems() {
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return new ArrayList<>(items);
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}
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}
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public class Item {
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private ItemType type;
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private final String name;
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public Item(ItemType type, String name) {
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this.setType(type);
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this.name = name;
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}
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@Override
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public String toString() {
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return name;
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}
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public ItemType getType() {
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return type;
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}
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public final void setType(ItemType type) {
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this.type = type;
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}
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}
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public enum ItemType {
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ANY, WEAPON, RING, POTION
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}
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```
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The iterator interface is extremely simple.
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```java
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public interface Iterator<T> {
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boolean hasNext();
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T next();
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}
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```
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In the following example we iterate through the ring type items found in the chest.
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```java
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var itemIterator = TREASURE_CHEST.iterator(ItemType.RING);
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while (itemIterator.hasNext()) {
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LOGGER.info(itemIterator.next().toString());
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}
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// Ring of shadows
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// Ring of armor
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```
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## Class diagram
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## Applicability
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Use the Iterator pattern
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* to access an aggregate object's contents without exposing its internal representation
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* to support multiple traversals of aggregate objects
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* to provide a uniform interface for traversing different aggregate structures
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## Real world examples
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* [java.util.Iterator](http://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html)
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* [java.util.Enumeration](http://docs.oracle.com/javase/8/docs/api/java/util/Enumeration.html)
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## Credits
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* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
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* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
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