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Design Patterns

John T. Bell

Department of Computer ScienceUniversity of Illinois, Chicago

Based on materials from Chapter 15 of “The Object-Oriented Thought Process” by Matt Weisfeld

and “Design Patterns” by Gamma et. Al.

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Design Through Reuse

• Many design problems have been solved before.

• Libraries, Frameworks, COTS, are all approaches to reusing code.

• Architectures and Design Patterns are approaches to reusing concepts and ideas.

• Design Patterns are well-documented solutions to commonly encountered ( OO ) design problems. The classic source is by Gamma et. al.

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THE Classic Design Patterns Book( Available from UIC Library Safari Books )

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Documentation for all Patterns

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Design Pattern Classification

Purpose

Creational Structural Behavioral

Scope Class Factory Method (107) Adapter (139) Interpreter (243)Template Method (325)

Object Abstract Factory (87)Builder (97)Prototype (117)Singleton (127)

Adapter (139)Bridge (151)Composite (163)Decorator (175)Facade (185)Proxy (207)

Chain of Responsibility (223)Command (233)Iterator (257)Mediator (273)Memento (283)Flyweight (195)Observer (293)State (305)Strategy (315)Visitor (331)

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Creational Patterns

( clone( ) method in Java Object )

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Singleton – Ensure Only 1 InstanceAND provide universal access to it.

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Factory Method creates a related object, type determined dynamically

Example: A “Lock” class could contain a “getKey( )” method.ComboLock creates ComboKey, BikeLock creates BikeKey, etc.

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Abstract Factory class produces many things, all designed to work together

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Prototype makes copies of objects using overriding and polymorphism.

• Does not require knowing what type of object will be created when writing code.

• Implemented in Java as Object.clone( ).

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Structural Patterns

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Bridge separates interface from implementation, for 2 new classes

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Design Choice:Overloading or Bridge Pattern ?

A = Character B = DecisionMakerA1/A2 = Player / NPC B1/B2 = UI/AImethodA = makeMove method = getMove

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Adapter is used to connect two pre-existing classes.

Class Approach:

Object Approach:

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Proxy – A “local” stand-in taking the place of a “remote” resource.

Uses: Speedup, Caching, Security,Smart pointers, more.

Note: Client only knows about Subject, which is abstract

Note: Proxy and RealSubject are siblings, both descendants of Subject

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Composite creates hierarchical trees, with grouping nodes and leaf nodes

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Java’s AWT Implements theComposite Design Pattern

Object

java.awt.Component

java.awt.Container

*

LabelButton

WindowPanel JComponent

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Behavioral Patterns – 1 / 2

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Iterator is frequently used with Composite, and other collections

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Iterator Example

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Mediator coordinates between classes that don’t need to know each other

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Behavioral Patterns – 2 / 2

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Memento remembers a state without violating privacy / info hiding

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Observer – Gets notifications when changes occur.

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Observer Implements the Model-View-Controller Architecture, MVC

• Model - Data modelling a system.• View - Observes the data.• Controller - Adjusts the data

and/or the view.

Model

and controllersViews

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Java Swing implements the Observer pattern by adding EventListeners:

• EventListeners ( and descendants ) are Interfaces, implemented by many classes, including user-written classes.

• Listeners can be added to java.awt.Components and descendants thereof.

• Listeners may be low-level, e.g. mouseMotionListener, or high-level, e.g. ActionListener.

• Some listeners require multiple methods. Adapters are classes that provide dummy methods for these types of listeners. ( Override only the needed methods. )

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EventListeners are added to Components

Object

java.awt.Component

java.awt.Container

*

LabelButton

WindowPanel JComponent

EventListeners

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State subclasses finite states

state =

: State : State

: State

Changing the state variable changes how the Handle( ) method operates

Consider using Singleton to ensure only one instance of each State type exists

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The Design Pattern Triangle

Name(Classification)

Description Implementation

Given any one, provide the other two

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Name That Pattern

Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

A. Abstract Factory

B. Factory

C. Factory Method

D. Family

E. Builder

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What Does Command Do ?

A. Encapsulate requests as objects, providing support for undoing, queueing or logging requests.

B. Delegate a request to a subordinate class.

C. Explore a daisy-chain of potential handlers to find the best handler to service a request.

D. Provide a surrogate or placeholder for another object to control access to it.

E. Provide a unified interface to a set of interfaces in a subsystem.

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Name that Pattern

A. Composite

B. Tree

C. Builder

D. Façade

E. Proxy

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What Type of Pattern is Memento?

A. Abstract

B. Behavioral

C. Creational

D. Forgetful

E. Structural

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Open-Ended

• Name and describe a design pattern that you have used in a project, either this semester or at some other time. Provide a UML diagram illustrating its use in your project.

• Which is your favorite design pattern that we have studied this semester? Explain what you like about it.

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Design Patterns in the Java API

• Iterator• Factory Method - Collection.iterator( )• Prototype - Object.clone( )• Composite - Container extends Component• Observer - Component.add______Listener( )• Decorator - ScrollPane

• Singleton - KeyboardScanner• Command - Move