Iterator - Harsh4999/Design-Patterns GitHub Wiki

Iterator

USE

• Whenever we have an aggregate object and we want to give access to its elements inside it in a sequential manner we use an iterator to iterate over it in order to access the internal members.
• Iterator helps us hide the internal data structure of aggregate objects internal data structures.
• Iterators are state full and if the underline collection changes while we are iterating the iterator becomes invalid. Sensitive to underline changes

UML

• Aggregate: gives out iterator reference by createiterator() method.
• Concrete Aggregate: implementation of aggregate
• Iterator: Is typically an interface which is used by client to iterate
• Concrete Iterator: Implementation of Iterator it defines methods like hasNext(), next() etc.
• Client: Makes aggregate and uses Iterator to iterate

Implementation

• Define Iterator interface provide hasNext(), next()
• Implement Iterator interface
• Concrete iterator needs to maintain some state
• Detecting changes to underlying collection is important to avoid errors.
• Having iterator as inner class helps access internal values.
• Eg: Collection framework Iterator