[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())