[Specical] Design Pattern - Behavioral Patterns - Iterator Pattern

2007

Section 4, Chapter 3

Iterator Pattern

Concept

Iterators allow sequential access of elements in a collection of objects without exposing its underlying code.

Use

If you have a list object and you wish to move through each element in the list sequentially and maintain your current place in the list, an iterator seems appropriate.

Design

• Aggregate: a collection or aggregate object of some type whose elements we wish to iterate through.
  
• Iterator: a tool to move through the aggregate elements and keep track of the progress.
  

Illustration

class List{  private ArrayList _listItems = new ArrayList();  public Iterator CreateIterator()  {    return new Iterator(this);  }    public int Count  {    get{ return _listItems.Count; }  }  public void Append(object item)  {    _listItems.Add(item);  }    public void Remove(object item)  {    _listItems.Remove(item);  }  public void RemoveAt(int index)  {    _listItems.RemoveAt(index);  }  public object this[int index]  {    get{ return _listItems[index]; }    set{ _listItems[index] = value; }  }}class Iterator{  private List _collection;  private int _currentIndex = 0;  private int _stepCount = 1;  public Iterator(List collection)  {    this._collection = collection;  }  public int Step  {    get{ return _stepCount; }    set{ _stepCount = value; }  }  public object First()  {    _currentIndex = 0;    return _collection[_currentIndex];  }    public object Next()  {    _currentIndex += _stepCount;    if(!IsDone())      return _collection[_currentIndex];    else      return null;  }    public object Last()  {    _currentIndex = _collection.Count - 1;    return _collection[_currentIndex];  }  public object Current()  {    return _collection[_currentIndex];  }  public bool IsDone()  {    return _currentIndex >= _collection.Count ? true : false ;  }}

Iterator skipIterator = list.CreateIterator();skipIterator.Step = 3;

for(object item = skipIterator.First(); !skipIterator.IsDone(); item = skipIterator.Next())