Iterator Matt G. Ellis

Intent Metsker: Provide a way to access elements of a collection sequentially. GoF: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation

Motivation for Iterator Accessing each element in a collection is done often. Collections should be interchangeable Users of your collection shouldn’t have to know how it works to use it.

Type Safe Collections public List getPromotionalFireworks()  This method returns a List.  What type of objects are in the List?  Are you sure?  Would you bet your life on it?

Type Safe Collections public FireworksList getPromotinalFireworks()  This method returns a FireworksList.  What objects are in the FireworksList?  Are you sure?  Would you bet your life on it?

Type Safe Collections

Challenge 28.1 Fill in the missing code in ShowFireworkList to instantiate the iterator.

Challenge 28.1

Maintaining Type Safety May wish to inherit the underlying collection Dangerous idea, but it has its benefits How much information do you want reveal?

Iterator and Composites Composites can be thought of as a kind of collection, it may make sense to want to use an Iterator with a Composite. Due to their possibly cyclical nature, special care must be taken when writing the Iterator for a Composite (to prevent an infinite loop)

Composites at Oozinoz The Process of Building Fireworks is represented as a composite

Composites at Oozinoz

Composite Iterator Need two different types of Iterators. Iteration over a ProcessSequence is different than iteration over a ProcessStep However, we wish to provide a way of interchanging these Iterators.

Composite Iterator

CompositeIterator vs. LeafIterator The CompositeIterator class needs to create new "subiterators" as it traverses over the children of a composite object. For each child, a CompositeIterator object must create either a new leaf node iterator or a new composite iterator, depending on the type of the child. LeafIterators do not need to create "subiterators"

UML for CompositeIterator

Getting the Right Iterator The ProcessComposite class can implement iterator() to return an instance of CompositeIterator. The ProcessStep class can implement iterator() method to return an instance of LeafIterator.

Challenge 28.2 What pattern are you applying if you let classes in the ProcessComponent hierarchy implement iterator() to create instances of an appropriate iterator class?

Using the Iterator

Building the Iterator

LeafIterator

Some Notes This part is the simple one Note that the depth of an iterator is different from the depth of a node in a tree. The depth of an iterator depends on the depth of its subiterator. Leaf iterators do not have subiterators, so their depth is always 0.

CompositeIterator

Depth of a CompositeIterator is one plus the depth of its subiterator

CompositeIterator The CompositeIterator uses a peek object to facilitate the implementation of hasNext(). It can be difficult to tell whether a (possibly cyclic) composite has a next node. A simple workaround is to have hasNext() search for the next value, and report whether or not this search was successful.

CompositeIterator Check the composite bellow the current child. Move to the next child if necessary.

Challenge 28.3 We want to support a showInterior method on our Iterator. To do so we add a boolean called showInterior to ComponentIterator What changes do we have to make in CompositeIterator so that next returns only leaf nodes when showInterior is false?

Challenge 28.3

Thread Safety Modifying the Underlying collection that is being iterated over will result in odd and problematic behavior. Mutex can be used to overcome this by preventing access to a collection when an it is being access by multiple threads. Most Iterators are fail-fast in that they detect that the underlying collection has changed and raise an exception

Thread Safety When multiple threads may be accessing one data source at the same time, use java’s synchronized method to enforce Mutex. Bear in mind that doing so may effect performance of your program. Why is this the case?

Thread Safety What can we do to overcome this? Metsker’s discussion on Thread-Safe Iterators is a good one, but remember that writing multi threaded code is difficult. Understanding the problems that can arise from multiple threads running at the same time is important especially when using an Iterator.

Questions?