July 28, 2015IAT 2651 Design Patterns

“Gang of Four” July 28, 2015IAT 2652

Design Advice  Program to an Interface, not an implementation  Composition over Inheritance –Build software by composing objects together –Inheritance is often difficult to manage Changes to parent class may mess up child classes July 28, 2015IAT 2653

Interface  If you write code to implement an interface –There is no possibility for parent changes to mess up child classes –Unfortunately, you can’t re-use code via inheritance July 28, 2015IAT 2654

Balance Inheritance  Inheritance is useful where: –There is a strong IS-A relationship –Child class uses all of parent data –Child class does a lot of the same stuff the parent does  Not so good if –Child class ignores a lot of parent data –Child class adds new behaviors and ignores old behaviors July 28, 2015IAT 2655

Design Advice  As with any design problem  It is Rarely the case that the first design is the best  As with any design task, be prepared to do it over July 28, 2015IAT 2656

Design Patterns  The Gang of Four offer the following design patterns, in 3 categories: –Creational Patterns: Patterns of objects that are about creating objects –Structural Patterns: Patterns of composing objects together –Behavioral: Patterns of communication between objects July 28, 2015IAT 2657

Creational: Factory  Factory method –a method in some class generates an instance of a class -- NOT a constructor! class ArmedRocket extends Rocket {... Missile fire() { Missile m = new Missile(xPos, yPos, rotation); return m; } July 28, 2015IAT 2658

Singleton pattern  Make only 1 instance of an object: Ever!  Useful when: there’s some single resource or state you want to manage public class SingletonDemo { private static SingletonDemo instance; private SingletonDemo() { } public static SingletonDemo getInstance() { if (instance == null ) { instance = new SingletonDemo(); } return instance; } July 28, 2015IAT 2659

Behavioral Patterns  Iterator  Provide a way to access the elements of an aggregate object (container, list) without exposing its underlying implemetation  In Java, it’s the Iterator interface Iterator itr = al.iterator(); while(itr.hasNext()) { Object element = itr.next(); } July 28, 2015IAT 26510

Observer  A Subject –Maintains a list of observers, and notifies them whenever subject’s state changes  Used in almost every GUI toolkit/widget set –Slider “Observes” mouse events in its region  Also fundamental to CPU architecture as the Interrupt Service Routine July 28, 2015IAT 26511