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 }!