Mid-Level Design Patterns: Iteration and Iterators

1. Mid-Level Design Patterns: Iteration and Iterators

2. Objectives • To introduce the problem of collection iteration • To present a case study about how to design an iteration mechanism • To introduce the Iterator design pattern as an object-oriented realization of the best iteration mechanism design alternative • To present the structure, behavior, and uses of the Iterator pattern

3. Topics • Collections and iteration • Iteration mechanisms • Iteration mechanism design alternatives • Selecting a design alternative • Iteration mechanism robustness • The Iterator pattern

4. Collections and Iteration A collection is an object that holds or contains other objects. Iteration over a collection or collection iteration is traversal and access of each element of a collection.

5. Iteration Mechanisms • An iteration mechanism is a language feature or a set of operations that allow clients to access each element of a collection. • For example: • Java and Visual Basic have for-loop constructs that support iteration over collections • Several Java collections have size() and get() operations that allow element access in a standard for-loop

6. Iteration Mechanism Operations • Initialize—Prepare the collection for traversal • Access currentelement—Provide client access to the current element • Advance currentelement—Move on to the next element in the collection • Completiontest—Determine whether traversal is complete

7. Other Iteration Mechanism Requirements • Informationhiding—The internal structure of the collection must not be exposed. • Multiple simultaneousiteration—It must be possible to do more than one iteration at a time. • Collection interfacesimplicity—The collection interface must not be cluttered up with iteration controls. • Flexibility—Clients should have flexibility during collection traversal.

8. Iteration Mechanism Design Alternatives: Residence Iteration mechanism residence: • Programming language—As in Java or Visual Basic • Depends on the language • Collection—A built-in iteration mechanism resides in the collection • Iterator—An external entity housing the iteration mechanism • An iterator is an entity that provides serial access to each elements of an associated collection.

9. Iteration Mechanism Design Alternatives: Control Iteration mechanism control: • External iterationcontrol—The iteration mechanism provides access to collection elements as directed by the client; the client calls the iteration control operations. • Internal iterationcontrol—The iteration mechanism accepts operations from clients that it applies to elements of the collection; the iteration mechanism calls the iteration control operations.

10. Iteration Mechanism Design Alternatives: Summary

11. Built-In Internal Control: Implementation printObject( o : Object ) { print( o ) } … Collection c … c.apply( printObject )

12. Built-In Internal Control: Evaluation • Hides collection internals  • Does not complicate the collection interface  • Multiple simultaneous iteration is not easy  • Client has little control over iteration—no flexibility 

13. Built-In External Control: Implementation For each kind of iteration desired • Add the iteration control operations (or their equivalents) to the collection • Other operations may be needed to provide flexibility

14. Built-In External Control: Evaluation • Hides collection internals  • Greatly complicates the collection interface  • Multiple simultaneous iteration is not easy  • Client has control over iteration—adequate flexibility 

15. Iterators with Internal Control • Hides collection internals  • Does not unduly complicate the collection interface  • Multiple simultaneous iteration is allowed  • Client has little control over iteration—no flexibility 

16. Iterators with External Control • Hides collection internals  • Does not unduly complicate the collection interface  • Multiple simultaneous iteration is allowed  • Client has control over iteration—adequate flexibility  • Iterators with external control is clearly the best design alternative—this is the basis for the Iterator design pattern.

17. Change During Iteration • What should happen when a collection is changed during iteration? • Requirements for a coherent iteration mechanism specification: • Faulttolerance—The program should not crash. • Iterationtermination—Iteration should halt. • Completetraversal—Elements always present should not be missed during traversal. • Singleaccess—No element should be accessed more than once.

18. Robust Iteration Mechanism • A robust iteration mechanism is one that conforms to some coherent specification of behavior when its associated collection changes during iteration. • Making an iteration mechanism robust may be difficult or expensive.

19. Iterator Pattern The Iterator pattern is an object-oriented design pattern for externally controlled iterators.

20. An Analogy Consider a warehouse full of items that a client must process one by one. • Don’t allow clients into the warehouse (information hiding) • Clerks are like iterators • Clerks can fetch each item for clients (external control) • Clerks can be instructed by the client and then process each element on their behalf (internal control)

21. Iterator Structure

22. Iterator Behavior

23. When to Use Iterators • Whenever traversing any collection • Helps hide information • Helps decouple code from particular collections (increases changeability) • Simplifies collection interfaces • Supports multiple concurrent iterations • Problems with iterators • Robustness

24. Summary 1 • Collection iteration is an important task that can be realized by an iteration mechanism in many ways: • Programming language feature • Built-in to the collection with internal or external control • Iterator with internal or external control • If an iteration mechanism is not part of the programming language, the best design alternative is an iterator with external control.

25. Summary 2 • The Iterator pattern is a model for an object-oriented implementation of an iterator with external control. • The Iterator pattern is well-known and widely implemented; it offers many advantages and should be the used whenever a collection is traversed.