What is Iterator

1
What is Iterator

• Category Behavioral
• Generic Way to Traverse Collection
• Not Related to Collection Implementation Details
• Not Related to the direction/fashion with which
  the collection is traversed

2
How to Use Iterator

• Implementation Details
• Have a class (Class A) with which iteration makes
  sense
• Create a class (Class B) that implements the
  java.util.Iterator interface
• Class B should manage the iteration state over
  Class A for a single client.
• Usage Details
• Call constructing iterator() method on collection
  object.
• Use Iterator object returned to iterate elements
  (next(), hasNext())

3
How to Use Iterator

• Example Iterator implementation (note similar to
  actual Java implementation)
• // Note Some features are not implemented
• public class VectorIterator implements
  java.util.Iterator
• private Vector collection
• private int cursor0
• public VectorIterator(Vector toIterate)
• collection toIterate
• public boolean hasNext()
• return (cursor lt collection.size())
• public void next()
• cursor
• Object next collection.get(cursor)

4
How to Use Iterator

• Traditional Looping
• Vector aVector new Vector()
• // load vector with elements
• // Implementation omitted
• for(int i0 iltaVector.size()i)
• Object anElem aVector.get(i)

• Iterator Looping
• Vector aVector new Vector()
• // load vector with elements
• // Implementation omitted
• Iterator elements aVector.iterator()
• while(elements.hasNext())
• Object anElem elements.next()

5
Class Diagram

• From (Stephen Stelting, Olav Maassen, Applied
  Java Patterns)

6
Why Iterator

• Hides details of Collection Traversal
• Avoids the ugly, and sometimes bug-prone
  index/cursor management (int i)
• The iterator pattern can be used to disperse the
  cost of expensive object construction

7
In the Real World

• Java has built in Iterator Support for the
  Collections Library (java.util.Iterator)
• Vector, ArrayList, LinkedList, HashSet, TreeSet,
  etc (Collection.iterator() method).
• Iterator may be traversing an Array, a
  LinkedList, or even a binary tree, but the code
  using the library knows no difference.
• Developers can reuse the Iterator interface for
  their own libraries/implementations

8
In the Real World

• Iterator can be used to defer expensive reads
  such as database calls and file parsing until it
  is actually necessary.
• In combination with the factory pattern,
  collection type choices can be completely
  abstracted from the program, and into
  configuration files.

9
Advanced Java Examples

• public class CollectionsExample
• public static void main(String args)
• Collection collectionA null
• collectionA createCollection(args)
• iterateCollection(collectionA)
• public static Collection createCollection(String
  args)
• Collection collection null
• if(args0.equals(Vector))
• // returns a index based list walking iterator
• collection new Vector()
• else if(args0.equals(TreeSet))
• // returns a tree walking iterator.
• collection new TreeSet()
• return collection