• An aggregate object like a list needs to be accessed, but you don't want clients to see (or depend on) the implementation of the collection.
• You might have multiple ways to traverse a collection, and you don't want to bloat the collection interface with all the methods to navigate it.
• We want to write generic code to traverse multiple types of aggregate objects

• Iterator
• ConcreteIterator
• Aggregate
• ConcreteAggregate

• It supports variations in the traversal of an aggregate
• Iterators simplify the Aggregate interface
• More than one traversal can be pending on an aggregate

• Who controls the iteration? - client - external iteration, iterator - internal iteration.
• Who defines the traversal algorithm? - it can be controlled by the client as well
• How robust is the iterator? - what if the aggregate changes?
• Additional Iterator operations - can supply method for more direct and quick access
• Iterators may have privileged access - Iterators can be inner classes in Java
• Iterators for composites - may usually be internal iterators
• Null iterators - makes travelling a composite easier - it is always done.

public interface Enumeration {
  public Object hasMoreElements();
  public Object nextElement();
}

public interface Iterator {
  public boolean hasNext();
  public Object next();
  public void remove();
}