Iterator Design Pattern

Introduction:
Sometimes you want to access a collection of objects in a sequential way without even expose how things is going inside your implementation . Iterator Design Pattern Provides you with a Skelton of how to design your classes to solve this issue.
Example:

IteratorDesignPattern

static void Main(string[] args)
{
ListAggregate a = new ListAggregate();
a[0] = “Item A”;
a[1] = “Item B”;
a[2] = “Item C”;
a[3] = “Item D”;
// Create Iterator and provide aggregate
ListIterator i = new ListIterator(a);
Console.WriteLine(“Iterating over collection:”);
string item = i.FirstItem();
while (item != null)
{
Console.WriteLine(item);
item = i.NextItem();
}
// Wait for user
Console.ReadKey();
}
class ListIterator : Iterator
{
private ListAggregate _aggregate;
private int _current = 0;
// Constructor
public ListIterator(ListAggregate aggregate)
{
this._aggregate = aggregate;
}
// Gets first iteration item
public override string FirstItem()
{
return (string)_aggregate[0];
}
// Gets next iteration item
public override string NextItem()
{
string ret = null;
if (_current < _aggregate.Count – 1)
{
ret = (string)_aggregate[++_current];
}
return ret;
}
// Gets current iteration item
public override string CurrentItem()
{
return (string)_aggregate[_current];
}
// Gets whether iterations are complete
public override bool HasMoreItems()
{
return _current >= _aggregate.Count;
}
}
class ListAggregate : Aggregate
{
private ArrayList _items = new ArrayList();
public override Iterator CreateIterator()
{
return new ListIterator(this);
}
// Gets item count
public int Count
{
get { return _items.Count; }
}
// Indexer
public string this[int index]
{
get { return (string)_items[index]; }
set { _items.Insert(index, value); }
}
}
abstract class Iterator
{
public abstract string FirstItem();
public abstract string NextItem();
public abstract bool HasMoreItems();
public abstract string CurrentItem();
}
abstract class Aggregate
{
public abstract Iterator CreateIterator();
}

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