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Design Patterns

A Pattern presents a proven advice instandard format

A design pattern gives advice about aproblem in software design.

Attributes of s design Pattern:

1. Name

2. Context3. Problem

4. Solution

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What are iterators

Iterators helps to iterate or traverse the

collections.[ From Start to End of

collection] Advantages :

1. Iterators does not expose internal

structure [only return elements for use]

2. More than one iterators can be attached

to a single collection.

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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

1. Internal structure links Exposed to user

2. Only one way traversal at a time

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The Iterator Pattern

Intent

1. Provide a way to access the elements of an aggregate object sequentially

without exposing its underlying representation

2. 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 ora collection Examples are a linked list and a hash table.

3. Also Known As Cursor

Motivation

1. An aggregate object such as a list should allow a way to traverse its

elements without exposing its internal structure

2. It should allow different traversal methods

3. It should allow multiple traversals to be in progress concurrently

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Solution

1. Define an iterator that fetches one element at a time2. Each iterator object keeps track of the position of the next element

3. If there are several aggregate/iterator variations, it is best if the

aggregate and iterator classes realize common interface types.

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Name in Design Pattern Actual Name (linked lists)

Aggregate List

ConcreteAggregate LinkedList

Iterator ListIterator

ConcreteIterator anonymous class implementing

ListIterator

createIterator() listIterator()

next() next()

isDone() opposite of hasNext()

currentItem() return value of hasNext()

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Model View Controller

1. Some programs have multiple editable views

2. Example: HTML Editor

(a) WYSIWYG view(b) structure view

(c) source view

3. Editing one view updates the other

4. Updates seem instantaneous

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1. Model: data structure, no visual representation

2. Views: visual representations

3. Controllers: user interaction

4. Views/controllers update model

5. Model tells views that data has changed

6. Views redraw themselves

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Observer Pattern

Model notifies views when something interesting happens

Button notifies action listeners when something interestinghappens

Views attach themselves to model in order to be notified

Action listeners attach themselves to button in order to be

notified Generalize: Observers attach themselves to subject

Context

1. An object, called the subject, is source of events

2. One or more observer objects want to be notified

when such an event occurs.

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Solution

1. Define an observer interface type. All concrete observers implementit.

2. The subject maintains a collection of observers.

3. The subject supplies methods for attaching and detaching

observers.

4. Whenever an event occurs, the subject notifies all observers.

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Name in Design Pattern Actual

Name (Swing buttons)

Subject JButton

Observer ActionListener

ConcreteObserver the class that implements the ActionListener

interface type

attach() addActionListener()

notify() actionPerformed()

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Strategy Pattern

Context:

A class can benefit from different

variants for an algorithm Clients sometimes want to replace

standard algorithms with custom

versions

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Solution Define an interface type that is an abstraction for the algorithm

Actual strategy classes realize this interface type.

Clients can supply strategy objects

Whenever the algorithm needs to be executed, the context class

calls the appropriate methods of the strategy object

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Name in Design Pattern Actual Name (layout management)

Context Container

Strategy LayoutManager

ConcreteStrategy a layout manager such as BorderLayout

doWork() a method such as layoutContainer

Example : Layout Managers

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Name in Design Pattern Actual Name (sorting)

Context Collections

Strategy Comparator

ConcreteStrategy a class that implements

Comparator

doWork() compare

Example : Sorting