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Ilkka Seppälä
2020-08-29 21:29:15 +03:00
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iterator/README.md
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@ -9,29 +9,34 @@ tags:
---
## Also known as
Cursor
## Intent
Provide a way to access the elements of an aggregate object
sequentially without exposing its underlying representation.
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.
> 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.
> 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.
> 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 treasure chest that contains items.
The main class in our example is the `TreasureChest` that contains items.
```java
public class TreasureChest {
@ -60,7 +65,11 @@ public class TreasureChest {
return new ArrayList<>(items);
}
}
```
Here's the `Item` class:
```java
public class Item {
private ItemType type;
@ -92,7 +101,7 @@ public enum ItemType {
}
```
The iterator interface is extremely simple.
The `Iterator` interface is extremely simple.
```java
public interface Iterator<T> {
@ -110,19 +119,26 @@ var itemIterator = TREASURE_CHEST.iterator(ItemType.RING);
while (itemIterator.hasNext()) {
LOGGER.info(itemIterator.next().toString());
}
// Ring of shadows
// Ring of armor
```
Program output:
```java
Ring of shadows
Ring of armor
```
## Class diagram
![alt text](./etc/iterator_1.png "Iterator")
## 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
* 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