Design Patterns

A Introduction to Object Orientated Design Patterns

Design Patterns - Why? Designing OO software is hard

Designing reusable OO software – harder

Experienced OO designers make good design choices?

New designers tend to fall back on non-OO techniques used before

Experienced designers know something ….

what is it?

Design Patterns - Why? Expert designers know not to solve every problem from first principles

They reuse solutions

Careful –

they reuse ideas NOT exact solutions

These patterns make OO designs more flexible, elegant, and ultimately reusable

Re-Use Code re-use

Don’t reinvent the wheel

Requires clean, elegant, understandable, general, stable code

leverage previous work

Design re-use Don’t reinvent the wheel

Requires a precise understanding of common, recurring designs

leverage previous work

What is a Design Pattern?

A design pattern

abstracts a recurring design structure

comprises a class or classes

dependencies,

structures,

interactions, or

conventions

distills design experience

Some Design Patterns

Strategy

Observer

Model View Controller (MVC)

Strategy Make algorithms interchangeable---”changing the guts”

Choice of implementation at run-time

Use when there are two or more ways of manipulating the same “data”

Strategy Operation()

ConcreteStrategy2 Operation()

ConcreteStrategy1 Operation()

means

generalization

or inheritance

Strategy Example Different ways to move a chess piece

MoveChessPiece

move(pointA, pointB)

TextualMove

move(pointA, pointB)

GraphicalMove

move(pointA, pointB)

NetworkMove

move(pointA, pointB)

Observer

Intent

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Observer

Observer

Observer – In Action

Model-View-Controller (MVC) Pattern

MVC consists of three kinds of objects:

Model – the application object

View – UI (screen presentation)

Controller – defines the way the UI reacts to user inputs

MVC – What Is the Problem?

The same data needs to be accessed when presented in different views: e.g. HTML, JFC/swing, XML

The same data needs to be updated through different interactions

Supporting multiple types of views and interactions should not impact the components that provide the core functionality of the application

MVC – Solution

Separate core model functionality from: the presentation and

control logic

that uses this functionality

Allows multiple views to share the same data model

Makes supporting multiple clients easier to implement, test, and maintain

MVC Basics

Model. The model manages the behavior and data of the application domain, responds to

Requests for information about its state (from the view),

Responds to instructions to change state ( from the controller).

View. The view manages the display of information.

Controller. The controller interprets inputs from the user, informing the model and/or the view to change as appropriate.

Many … Many Views: Two examples are the rows and columns view of a spreadsheet and the pie chart view of some column in the same spreadsheet. We end up with a hierarchy of views.

Many Controllers: examples from Word are normal interaction, short cut keys for Word Perfect users, short cut keys adapted from different keyword layouts.

MVC Structure Model-View-Controller is made up of a

number of smaller design patterns.

The relationship between views and models is the ObserverPattern.

The controllers are StrategyPattern of the views.

Patterns vs “Design”

Patterns are High Level Design

Patterns transcend the “identify classes and associations” approach to design

Instead: learn to recognize patterns in the problem space and translate to the solution

Patterns can capture OO design principles within a specific domain

Patterns provide structure to “design”