JAVA, JAVA, JAVA Object-Oriented Problem Solving Ralph Morelli Trinity College Hartford, CT presentation slides for published by Prentice Hall Second Edition

Java, Java, Java Object Oriented Problem Solving Chapter 4 Applets: Programming for the World Wide Web

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Objectives Be able to design and implement a Java applet. Understand Java's event handling model. Be able to handle button clicks in your programs. Have a better appreciation of inheritance and polymorphism. Know how to design a simple Graphical User Interface (GUI).

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Outline Introduction The java.applet.Applet Class Class Inheritance Applet Subclasses A Simple Applet Event-Driven Programming Case Study: The CyberPetApplet Object-Oriented Design: Inheritance and Polymorphism From the Java Library: Image In the Laboratory: CyberPetApplet

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Introduction An applet is a Java program embedded within a Web page and run by a Web browser. An applet has a graphical user interface (GUI ). Java applet programming is event-driven programming.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets The java.applet.Applet Class Applet’s public methods help define an interface between the applet and the browser. Applets can easily incorporate multimedia resources. Applet execution begins in the init() method.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Java's GUI Components GUI components that can be put into an applet. An Applet isa Panel which isa Container which isa Component By default, applets use a FlowLayout.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Objects, Assignments, and Types An instance of a class can be validly assigned to variables whose type is any of its superclasses. Panel p = new Panel(); Container cont = p; // Valid: A Panel is a Container Component comp = p; // Valid: A Panel is a Component Object o = p; // Valid: A Panel is an Object Container c = new Container(); Panel p = c; // Invalid: A container is // not necessarily a Panel

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets The Java Class Hierarchy By default, a class is a subclass of the java.lang.Object class. CyberPet is a direct subclass of Object

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Inheritance: Specializing a Class Inheritance allows us to specialize a class. Subclasses inherit getCreditCardNumber() and add their own special methods. Methods in the superclass can be shared by the subclasses.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Specialization: A Square is a Rectangle A Square is a Rectangle whose length equals width. Only protected and public elements can be inherited. To construct a Square, you specify its side.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets A Square is an Extension of a Rectangle public class Rectangle { protected double length; protected double width; public Rectangle (double l, double w) { length = l; width = w; } // Rectangle() public double calculateArea() { return length * width; } // calculateArea() } // Rectangle public class Square extends Rectangle { public Square (double side) { //Call the superconstructor super(side, side); } } // Square The super keyword refers to the superclass. The Square() constructor calls the Rectangle() constructor, passing it side as a value for both length and width.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Using the Square Class public class TestSquare { public static void main(String argv[]) { Square square = new Square (10); System.out.println("square's area is " + square.calculateArea()); } } // TestSquare The inherited calculateArea() method can be used just as if it were defined in Square. Create a new Square with a side of 10. Output Produced square’s area is 100.0

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets An Applet Example SimpleApplet contains a single button. When the button is clicked, its label is toggled from “The machine is off” to the “The machine is on”

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Applets Java applets are made by first creating a subclass of java.applet.Applet. And then instantiating the subclass. Applet a = new Applet(); // Not the way to do it! public class SimpleApplet extends Applet {... } // Define subclass SimpleApplet myApplet = new SimpleApplet(); // and instantiate it The Applet class cannot be instantiated.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets A Simple Applet These methods are inherited by SimpleApplet An interface is an abstract class with no variables. The actionPerformed() method is defined abstractly in ActionListener and implemented here. SimpleApplet inherits from Applet and implements the ActionListener interface.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Java Definition of SimpleApplet import java.applet.*; import java.awt.*; import java.awt.event.*; public class SimpleApplet extends Applet implements ActionListener { private Button toggle; // From java.awt.* public void init() { toggle = new Button ("The machine is off"); toggle.addActionListener(this); add(toggle); } // init() public void actionPerformed(ActionEvent e) { String str = toggle.getLabel(); // Get the Button's label if (str.equals("The machine is on")) // and change it toggle.setLabel("The machine is off"); else // or toggle.setLabel("The machine is on"); // change it back } // actionPerformed() } // SimpleApplet Import class names Create a subclass Execution begins here The applet handles button clicks Called when toggle is clicked.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets SimpleApplet Inherits Functionality Override: This init() overrides the init() method inherited from java.applet.Applet Implement: This actionPerformed() is implemented according to the definition inherited from ActionListener interface Use: A SimpleApplet can use the add() method inherited from java.awt.Container.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Interacting Objects A Web browser creates a SimpleApplet named applet1 and tells it to initialize itself. The applet creates a Button named toggle and adds it to itself. When the browser detects a click, it tells the applet to perform an action. The applet tells Button to toggle its label.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Overriding the Applet.init() Method The init() method initializes the applet. It is where applet execution begins. It may be overridden in the applet subclass. public void init () { toggle = new Button ("The machine is off"); toggle.addActionListener(this); add(toggle); } // init() Create a new Button. This applet will listen for button clicks. The add() method, inherited from Container, puts the button in the applet.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Implementing an Interface An interface contains abtract methods that must be implemented in the subclass. public abstract interface ActionListener extends EventListener { public abstract void actionPerformed (ActionEvent e); } public void actionPerformed (ActionEvent e) { String str = toggle.getLabel(); // Get the toggle's label if (str.equals("The machine is on")) // and change it toggle.setLabel("The machine is off"); else // or toggle.setLabel("The machine is on"); // change it back } // actionPerformed() The implementation must have the exact same signature.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Polymorphism and Extensibility Polymorphism = many (poly) forms (morph) A polymorphic method -- init() -- has different behavior for different objects. Overriding Applet.init() creates a method that performs appropriate initializations for your applet. Extensibility: Applet functionality (init()) defined in the superclass (Applet) is extended to the subclass (SimpleApplet).

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets The Polymorphic eat() Method Polymorphism: The eat() method has different behavior for CyberDogs and CyberCats. CyberPet p1 = new CyberDog(); p1.eat(); // Eats dog food p1 = new CyberCat(); p1.eat(); // Eats cat food

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets The Polymorphic init() Method Applet applet= new AppletA(); applet.init(); // Prints “AppletA” applet = new AppletB(); applet.init(); // Prints “AppletB” public class AppletA extends Applet { public void init() { System.out.println(“AppletA”); } public class AppletB extends Applet { public void init() { System.out.println(“AppletB”); } Polymorphism: The init() method has different behavior for AppletA and AppletB.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Event-Driven Programming Events are triggered by user actions (mouse, keyboard). Events are handled by methods in the applet. Java’s Event Model

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets The Java Event Hierarchy An action event occurs when an applet Button is clicked. An mouse event occurs when the mouse is moved. An key event occurs when the user types a key.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets toggle = new Button ("The machine is off"); toggle.addActionListener(this); Creating an ActionListener After this applet is designated as a ActionListener for the toggle Button... … it listens for clicks on toggle. The this keyword is self- referential. It’s like saying “I”.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Handling Action Events import java.applet.*; import java.awt.*; import java.awt.event.*; public class SimpleApplet extends Applet implements ActionListener { private Button toggle; // From java.awt.* public void init() { toggle = new Button ("The machine is off"); toggle.addActionListener(this); add(toggle); } // init() public void actionPerformed(ActionEvent e) { String str = toggle.getLabel(); // Get the toggle Button's label if (str.equals("The machine is on")) // and change it toggle.setLabel("The machine is off"); else // or toggle.setLabel("The machine is on"); // change it back } // actionPerformed() } // SimpleApplet 1. Applet subclass implements the ActionListener interface 2. Declare a Button instance. 3. Create a Button instance. 4. Designate this applet as the button’s listener. 5. Add the button to the applet. 6. Define the specific actions to be performed when the button is clicked..

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Responding to a Click Event The Java Virtual Machine is embedded in the Browser.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Running an (Embedded) Applet A summary of what happens from the browser’s (or appletviewer’s) perspective. SimpleApplet applet1 = new SimpleApplet(); // Create an applet object applet1.init() ; // and call its init()... // Handle other tasks repeat until applet_is_stopped {... // Handle other tasks if (ACTION_EVENT) // If an action occurs, applet1.actionPerformed(anActionEvent); // pass it to the applet if (user_quits_browser) // If the user quits, applet1.destroy(); // kill the applet } Create an instance of the applet. Repeatedly pass events that belong to the applet to its actionPerformed() method.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Tracing the Applet import java.applet.*; import java.awt.*; import java.awt.event.*; public class SimpleApplet extends Applet implements ActionListener { private Button toggle; // From java.awt.* public void init() { toggle = new Button ("The machine is off"); toggle.addActionListener(this); add(toggle); } // init() public void actionPerformed(ActionEvent e) { String str = toggle.getLabel(); // Get the Button's label if (str.equals("The machine is on")) // and change it toggle.setLabel("The machine is off"); else // or toggle.setLabel("The machine is on"); // change it back } // actionPerformed() } // SimpleApplet The order in which applet statements are executed when the user makes two successive clicks on its toggle button. The applet is initialized The user clicks on the button. First Click Second Click

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Case Study: The CyberPetApplet The applet will serve as an user interface between the CyberPet and a user.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets CyberPetApplet’s GUI Interface

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets CyberPetApplet: Design Specification Uses

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Defining CyberPetApplet import java.awt.*; import java.applet.*; import java.awt.event.*; public class CyberPetApplet extends Applet implements ActionListener { } // CyberPetApplet Step 1. Define a subclass of java.applet.Applet that implements the ActionListener interface.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Defining CyberPetApplet (cont.) public class CyberPetApplet extends Applet implements ActionListener { // Declare instance variables. private CyberPet pet1; // The CyberPet private Label nameLabel; // Label private TextField stateField; // TextField private Button eatButton, sleepButton; // Buttons public void init() { } } // CyberPetApplet Step 2. Declare the instance variables. Stub version of init()

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Defining CyberPetApplet (cont.) public void init() {// Instantiate the instance variables pet1 = new CyberPet("Socrates"); // The CyberPet nameLabel = new Label("Hi! My name is " + pet1.getName() + " and currently I am : "); stateField = new TextField(12); // TextField eatButton = new Button("Eat!"); // Buttons eatButton.addActionListener(this); // Assign the listeners sleepButton = new Button("Sleep!"); sleepButton.addActionListener(this); stateField.setText(pet1.getState()); // Initialize the TextField stateField.setEditable(false); add(nameLabel); // Add the components to the applet. add(stateField); add(eatButton); add(sleepButton); } // init() Step 3. Define the init() method.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Default Layout: FlowLayout add(nameLabel); // Add the components to the applet. add(stateField); add(eatButton); add(sleepButton); The applet’s default FlowLayout means components are arranged from left to right in the container, wrapping around to the next row if necessary.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Interacting Objects stateField.setText(pet1.getState()); // Initialize the TextField

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Defining CyberPetApplet (cont.) public class CyberPetApplet extends Applet implements ActionListener { // Variable declarations and init() method not shown. public void actionPerformed(ActionEvent e) { if (e.getSource() == eatButton) // If eat button clicked, pet1.eat(); // tell the pet to eat else if (e.getSource() == sleepButton) // If sleep button clicked, pet1.sleep(); // tell the pet to sleep stateField.setText(pet1.getState()); // Display the pet's state } // actionPerformed() } // CyberPetApplet Step 4. Implement the actionPerformed() method. The getSource() method returns the source of the ActionEvent.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets The CyberPetApplet Program

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Object-Oriented Design: ToggleButton A ToggleButton isa Button that toggles its own label. Whenever it is clicked, it toggles between label1 and label2.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Design Spec: The ToggleButton Class Design Specification –Class Name: ToggleButton –Role: To be a button that toggles between two labels –Information (instance variables) Label1: Primary label (initial value of label) Label2: Secondary label –Actions (public methods) ToggleButton: constructor initializes the two labels actionPerformed(): Toggles between the two labels

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Class Definition: ToggleButton import java.awt.*; import java.awt.event.*; public class ToggleButton extends Button implements ActionListener { private String label1; // Two Labels to toggle between private String label2; public ToggleButton (String l1, String l2) { // Constructor method super(l1); // Use l1 as the default label label1 = l1; label2 = l2; addActionListener(this); } public void actionPerformed (ActionEvent e) { String tempS = label1; // Swap the labels label1 = label2; label2 = tempS; setLabel (label1); } // actionPerformed() } // ToggleButton Inheritance: A ToggleButton is a Button Invoke Button’s constructor to set initial label A ToggleButton acts as its own ActionListener Each time a ToggleButton is clicked it swaps labels. Inheritance: setLabel() is inherited from Button superclass.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Using a ToggleButton import java.applet.*; import java.awt.*; import java.awt.event.*; public class ToggleTest extends Applet implements ActionListener { private ToggleButton lightSwitch; public void init() { lightSwitch = new ToggleButton ("off","on"); add(lightSwitch); lightSwitch.addActionListener(this); } // init() public void actionPerformed(ActionEvent e) { showStatus("The light is " + lightSwitch.getLabel()); } // actionPerformed() } // ToggleTest This applet uses a ToggleButton. Initialize the ToggleButton. The applet is the listener. The applet just displays the button’s label.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Interacting Objects The ToggleButton has two ActionListeners, itself and the applet. The JVM calls both ActionListeners

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Java Library: Using Images in an Applet

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Using the java.awt.Image Class import java.applet.*; import java.awt.*; public class Lights extends Applet { private Image lightOn, lightOff; // Declare two Image variables public void init() { lightOn = getImage(getCodeBase(),"lighton.gif"); lightOff = getImage(getCodeBase(),"lightoff.gif"); } // init() public void paint (Graphics g) { g.drawImage(lightOn, 10, 10, this); g.drawImage(lightOff, 70, 10, this); } // paint() } // Lights Declare a reference variable for each image. Applet.getImage() loads the images from GIF files. GIF is a data format for images. Draw the image in the applet’s paint() method drawImage(Image, x, y, ImageObserver) is part of java.awt.Graphics.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets In the Laboratory: CyberPetApplet To introduce the principles of writing a Java applet. To introduce some of the basic GUI Components. To give additional practice using the if and if-else control structures. To introduce the Image object (optional) The objectives of this lab are:

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets In the Laboratory: CyberPetApplet Problem Statement Extend the CyberPet and CyberPetApplet by adding a third state to the pet simulation -- for example, thinking. The applet GUI should continue to display the CyberPet’s name and state. An image can be used to depict the state.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Technical Terms abstract class abstract interface abstract method applet Application Programming Interface (API) event-driven programming Graphical User Interface (GUI) import declaration inheritance inheritance hierarchy interface polymorphic method

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Summary Of Important Points An applet is an embedded program that runs within the context of a Web browser. The Java Application Programming Interface (API) is a set of predefined classes that can be used to write programs. A Graphical User Interface (GUI) enables the user to interact with a program via graphical elements such as windows, button, menus and so on. Java's GUI components are defined in the Abstract Windowing Toolkit (AWT) package.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Summary of Important Points (cont) The import statement is used to import definitions of predefined classes into a Java program. The extend keyword is used to define a class's pedigree -- its place in the Java class hierarchy. A class that extends another class is said to be a subclass of that class which is its superclass. A subclass inherits the public and protected methods and fields (variables) of its superclasses. Methods defined in a class’s superclasses can be overridden in the subclass by defining a method with the same signature -- same name, return type, and same number and type of parameters.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Summary of Important Points (cont) A method that behaves differently for different objects is said to be polymorphic. The Applet.init() method is an example, since it is redefined in all of Applet subclasses. Applets are event-driven: they react to certain events. An Event object records specific information about a particular event. Clicking on a Button in an applet generates an ACTION_EVENT which should be handled by an actionPerformed() method.

Java, Java, Java, 2E by R. Morelli Copyright All rights reserved. Chapter 4: Applets Summary of Important Points (cont) A Label is a GUI component that displays a single line of uneditable text on the applet. A Button is a labeled GUI component that is used to trigger some kind of action when clicked. A TextField is a GUI component that displays a single line of editable text. The setText() and getText() methods can be used to set a Button's label or a TextField's text. The default layout pattern for Java applets is the FlowLayout.