Iterator Pattern
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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.
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This document discusses object-oriented programming and compares old procedural programming techniques using structures to the newer object-oriented approach using classes. Specifically:
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The slides may not be much useful standalone, but I have uploaded them for reference.
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Iterator Pattern
- 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 }!