jgs CSE 564 Software Design Lecture 11: Design Patterns - Singleton Dr. Javier Gonzalez-Sanchez [email protected] javiergs.engineering.asu.edu | javiergs.com PERALTA 230U Office Hours: By appointment 

jgs Example 2 Observer 

Javier Gonzalez-Sanchez | CSE564 | Fall 2020 | 4 jgs Scenario JFrame Observable + addObserver() + notifyObservers() + setChanged() Tutor Student Classroom Observer + update() JPanel TutorPanel JPanel 

Javier Gonzalez-Sanchez | CSE564 | Fall 2020 | 5 jgs Student (version 2) import java.util.*; import javax.swing.*; public class Student extends JPanel implements Observer { JLabel label = new JLabel(); public Student () { this.setBackground(Color.lightGray); ImageIcon background = new ImageIcon("src/student.png"); label.setIcon(background); add(label); } 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); label.setText(y); } } 

Javier Gonzalez-Sanchez | CSE564 | Fall 2020 | 6 jgs Tutor (version 2) import java.util.Observable; public class Tutor extends Observable { private String theQuestion; public void askQuestion() { theQuestion = questions[((int)(Math.random()*10))%4]; setChanged(); notifyObservers(); } public void askQuestion(String s) { theQuestion = s; setChanged(); notifyObservers(); } public String getQuestion() { return theQuestion; } } 

Javier Gonzalez-Sanchez | CSE564 | Fall 2020 | 7 jgs TutorPanel import javax.swing.*; import javax.awt.*; public class TutorPanel extends JPanel implements ActionListener{ JTextField question = new JTextField(15); JButton ok = new JButton("send"); Tutor tutor; public TutorPanel(Tutor tutor) { this.tutor = tutor; setBackground(Color.white); ImageIcon background = new ImageIcon("src/prof.png"); JLabel label = new JLabel(); label.setIcon(background); add (question); add (ok); ok.addActionListener(this); add(label); } @Override public void actionPerformed(ActionEvent e) { tutor.askQuestion(question.getText()); } } 

Javier Gonzalez-Sanchez | CSE564 | Fall 2020 | 8 jgs Classroom (version 2) import java.awt.GridLayout; import javax.swing.JFrame; public class Classroom extends JFrame { public Classroom() { Student student = new Student (); Tutor tutor = new Tutor(); tutor.addObserver(student); setLayout(new GridLayout(2,1)); add(student); add(new TutorPanel(tutor)); } public static void main(String[] args) { JFrame window = new Classroom(); window.setSize(500, 500); window.setVisible(true); } } 

Javier Gonzalez-Sanchez | CSE564 | Fall 2020 | 9 jgs Output 

jgs Singleton 

Javier Gonzalez-Sanchez | CSE564 | Fall 2020 | 11 jgs GoF Patterns 

Javier Gonzalez-Sanchez | CSE564 | Fall 2020 | 12 jgs Singleton 

Javier Gonzalez-Sanchez | CSE564 | Fall 2020 | 13 jgs Singleton class MainApp { public static void Main(String []a){ // Constructor is protected -- cannot use new Singleton s1 = Singleton.getInstance(); Singleton s2 = Singleton.getInstance(); // Test for same instance if (s1 == s2){ // true - Objects are the same instance } } } 

Javier Gonzalez-Sanchez | CSE564 | Fall 2020 | 14 jgs Singleton class Singleton{ private static Singleton _instance; // Constructor is 'protected' protected Singleton() {} public static Singleton getInstance(){ if (_instance == null){ _instance = new Singleton(); } return _instance; } } 

Javier Gonzalez-Sanchez | CSE564 | Fall 2020 | 15 jgs Scenario Quiz Tutor System Companion ControlCenter << singleton>> 

jgs Decorator To be Continued … 

jgs CSE 564 Computer Systems Fundamentals Javier Gonzalez-Sanchez [email protected] Fall 2020 Disclaimer. These slides can only be used as study material for the class CSE564 at ASU. They cannot be distributed or used for another purpose.