Design Patterns

1. Design Patterns • A Pattern presents a proven advice in standard format • A design pattern gives advice about a problem in software design. • Attributes of s design Pattern: 1. Name 2. Context 3. Problem 4. Solution

2. What are iterators • Iterators helps to iterate or traverse the collections.[ From Start to End of collection] • Advantages : • Iterators does not expose internal structure [only return elements for use] • More than one iterators can be attached to a single collection.

3. Iterators cont…. Consider a LinkedList LinkedList list = new LinkedList() How you will traverse in Java How you will traverse in C ListIterator Ltr = list.listIterator(); While(Ltr.hasNext() { Object current = Ltr.next(); ………………. } Link Ltr = list.head; While(Ltr != null) { Object current = Ltr.data; Ltr = Ltr.next; } Disadvantages • Internal structure links Exposed to user • Only one way traversal at a time

4. The Iterator Pattern • Intent • Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation • An aggregate object is an object that contains other objects for the purpose of grouping those objects as a unit. It is also called a container or a collection Examples are a linked list and a hash table. • Also Known As Cursor • Motivation • An aggregate object such as a list should allow a way to traverse its elements without exposing its internal structure • It should allow different traversal methods • It should allow multiple traversals to be in progress concurrently

5. Solution • Define an iterator that fetches one element at a time • Each iterator object keeps track of the position of the next element • If there are several aggregate/iterator variations, it is best if the • aggregate and iterator classes realize common interface types.