The iterator design pattern provides a way to access elements of an aggregate object sequentially without exposing its underlying representation. It hides implementation details and allows for different traversals while separating traversal code from the list itself. The pattern involves an iterator interface with methods like first(), next(), and isDone(), and an aggregate that creates an iterator and returns it. Iterators can be external, controlled by the client, or internal, controlling iteration themselves through anonymous functions or closures.

1. Iterator
Design
Pa.ern
Stewart
Gleadow
University
of
Melbourne

2. Intent
• “Provide
a
way
to
access
the
elements
of
an
aggregate
object
sequen>ally
without
exposing
its
underlying
representa>on”
–
GoF

3. Mo3va3on
• Hide
implementa3on
details
• Allow
for
different
traversals
• Separate
traversal
code
from
the
list
itself

4. Basic
Structure
List Iterator
count() first()
add(Item) next()
remove(Item) isDone()
Item get(int index) Item currentItem()
• Provide
List
to
Iterator
(3ghtly
coupled)
• Iterator
keeps
a
Cursor
to
current
loca3on

5. Structure
Aggregate Iterator
createIterator() first()
next()
isDone()
Item currentItem()
ConcreteAggregate ConcreteIterator
createIterator()
return new ConcreteIterator(this)

6. Types
of
Iterators
• External:
client
controls
the
itera3on
• Internal:
iterator
controls
itera3on
• anonymous
func3ons
• closures
/
blocks

7. Applicability
• List
processing
&
selec3on
• Tree
traversals.
eg.
depth
first
• Syntax
trees
-­‐>
Interpreter?

8. Concurrency
• hasNext()
or
!isDone()
implies
subsequent
call
to
currentItem()
will
pass
• Iterators
get
a
copy
of
the
data?
• Iterators
keep
track
of
modifica3ons?

9. External
(Java)
Iterator<MyType> iter = list.iterator();!
while(iter.hasNext())!
!System.out.println(iter.next());!
// OR!
for(MyType item : list)!
!System.out.println(item);!

10. External
(Ruby)
for item in list!
!puts item!
end!
// OR!
list.each { |item| puts item }!

11. Internal
(Java)
public abstract class Internal<T> {!
!private Iterator<T> iter;!
public void traverse() {!
for(T item : iter)!
process(item);!
}!
protected abstract process(T item);!
}!

12. Internal
(Java)
Iterator<T> iter = list.iterator();!
Internal<T> internal = new Internal<T>(iter)!
{!
!protected void process(T item) {!
! ! !if(condition)!
! ! ! !System.out.println(item);!
!}!
};!

13. Internal
(Ruby)
list.each { |item| puts item if condition }!