jgs CSE 460 Software Analysis and Design Lecture 12: Design Patterns Dr. Javier Gonzalez-Sanchez [email protected] javiergs.engineering.asu.edu | javiergs.com PERALTA 230U Office Hours: By appointment 

jgs 460 00001100 Announcement § Quiz 04 is due Today (midnight) 11:59 PM AZ time. 

jgs 460 00001100 The LEGO Paradigm 

jgs 460 00001100 Patterns are these Blocks § Design patterns are solutions to software design problems you find again and again in real-world application development. § Patterns are about reusable designs and interactions between objects. § The 23 Gang of Four (GoF) patterns are generally considered the foundation for all other patterns (Gamma, Helm, Johnson, and Vlissides). 

jgs 460 00001100 GoF Patterns 

jgs 460 00001100 

jgs 460 00001100 GoF Patterns 

jgs Observer Software Design 

jgs 460 00001100 Observer 

jgs 460 00001100 Observer 

jgs 460 00001100 Observer abstract class Observer { public abstract void update(); } 

jgs 460 00001100 ConcreteObserver class ConcreteObserver extends Observer { private string name; private string observerState; private ConcreteSubject subject; public Subject getSubject() { return subject; } public void setSubject(Subject s) { subject = s; } // Constructor public ConcreteObserver(ConcreteSubject s, string n){ this.subject = s; this.name = n; } public override void update() { observerState = subject.subjectState; println("Observer %s's new state is %s", name, observerState); } } 

jgs 460 00001100 Subject abstract class Subject { private List observers = new List(); public void attach(Observer observer) { observers.add(observer); } public void detach(Observer observer){ observers.remove(observer); } public void notify(){ foreach (Observer o in observers){ o.update(); } } } 

jgs 460 00001100 ConcreteSubject class ConcreteSubject extends Subject { private String subjectState; public String getState () { return subjectState; } public void setState (s) { subjectState = s; } } 

jgs 460 00001100 Main Public class Main { public static void Main() { // Configure Observer pattern ConcreteSubject s = new ConcreteSubject(); ConcreteObserver eyeOne = new ConcreteObserver(s, "X"); ConcreteObserver eyeTwo = new ConcreteObserver(s, "X"); s.Attach(eyeOne); s.Attach(eyeTwo); // Change subject and notify observers s.subjectState = "ABC"; s.notify(); } } 

jgs Java Implementation java.util.* 

jgs 460 00001100 Scenario Student Tutor Classroom 

jgs 460 00001100 Scenario Observer + update() Observable + addObserver() + notifyObservers() + setChanged() Tutor Student Classroom 

jgs 460 00001100 Student import java.util.Observable; import java.util.Observer; public class Student implements Observer { public String answerQuestion (String question) { return "I am thinking about \'" + question +"\'"; } @Override public void update(Observable o, Object arg) { String x = ((Tutor)o).getQuestion(); String y = this.answerQuestion(x); System.out.println(y); } } 

jgs 460 00001100 Tutor import java.util.Observable; public class Tutor extends Observable { private String [] questions = { "2 + 2", "public or private", "functional or not fuctional", "white or black" }; private String theQuestion; public void askQuestion() { theQuestion = questions[((int)(Math.random()*10))%4]; setChanged(); notifyObservers(); } public String getQuestion() { return theQuestion; } } 

jgs 460 00001100 Classroom public class Classroom { public static void main(String[] args) { Student student = new Student(); Tutor tutor = new Tutor(); tutor.addObserver(student); for (int i=0; i<5;i++) tutor.askQuestion(); } } 

jgs 460 00001100 Output 

jgs CSE 460 Software Analysis and Design Javier Gonzalez-Sanchez [email protected] Fall 2020 Disclaimer. These slides can only be used as study material for the class CSE460 at ASU. They cannot be distributed or used for another purpose.