CSE564 Lecture 10

Software Design
Observer
(202109)

Transcript

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   CSE 564
   Software Design
   Lecture 10: Design Patterns - Observer
   Dr. Javier Gonzalez-Sanchez
   javiergs@asu.edu
   javiergs.engineering.asu.edu | javiergs.com
   PERALTA 230U
   Office Hours: By appointment
   

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   Previously
   

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   Observer
   

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   Subject
   import java.util.LinkedList;
   public abstract class Subject {
   private LinkedList observers = new LinkedList();
   public void addObserver(Observer observer) {
   observers.add(observer);
   }
   public void removeObserver(Observer observer) {
   observers.remove(observer);
   }
   public void notifying() {
   for (Observer ob : observers) {
   ob.update(this);
   }
   }
   }
   

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   Observer
   public abstract class Observer {
   public abstract void update(Subject s);
   }
   

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   ConcreteSubject
   class ConcreteSubject extends Subject {
   private String data;
   public String getData() {
   return data;
   }
   public void setData(String s) {
   data = s;
   notifying();
   }
   }
   

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   ConcreteObserver
   class ConcreteObserver extends Observer {
   @Override
   public void update(Subject s) {
   String info = ((ConcreteSubject)s).getData();
   System.out.println("Subject is doing this: "+info);
   }
   }
   

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   Main
   public class Main {
   public static void main(String[]a) {
   // Configure pattern
   ConcreteSubject s = new ConcreteSubject();
   ConcreteObserver eyeOne = new ConcreteObserver();
   ConcreteObserver eyeTwo = new ConcreteObserver();
   s.addObserver(eyeOne);
   s.addObserver(eyeTwo);
   // Change subject
   s.setData("ABC");
   s.setData("XYZ");
   }
   }
   

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   Main
   public class Main {
   public static void main(String[]a) {
   // Configure pattern
   ConcreteSubject s = new ConcreteSubject();
   ConcreteObserver eyeOne = new ConcreteObserver();
   ConcreteObserver eyeTwo = new ConcreteObserver();
   s.addObserver(eyeOne);
   s.addObserver(eyeTwo);
   // Change subject
   s.setData("ABC");
   s.setData("XYZ");
   }
   }
   

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    Java Implementation
    java.util.*
    

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    Note
    § Java 9 came out, and deprecated Observer implementation in java.util
    package.
    Deprecated their own implementation of the concept, not the concept
    itself.
    🤔
    § Problems: serialization and thread safety problems
    § Optional solution PropertyChangeListener from java.beans package.
    

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    Scenario
    Student Tutor
    Classroom
    

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

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    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);
    }
    }
    

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    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;
    }
    }
    

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    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();
    }
    }
    

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    Output
    

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    Example 2
    Observer
    

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    Output
    

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    Scenario
    JFrame
    Observable
    + addObserver()
    + notifyObservers()
    + setChanged()
    Tutor
    Student Classroom
    Observer
    + update()
    JPanel
    TutorPanel
    JPanel
    

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    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);
    }
    }
    

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    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;
    }
    }
    

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    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());
    }
    }
    

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    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);
    }
    }
    

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    Questions about Observer
    

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    CSE 564 Software Design
    Javier Gonzalez-Sanchez, Ph.D.
    javiergs@asu.edu
    Fall 2021
    Copyright. These slides can only be used as study material for the class CSE564 at ASU.
    They cannot be distributed or used for another purpose.
    

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