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Object-Oriented Problem Solving
JAVA, JAVA, JAVA
Second Edition
Ralph Morelli
Trinity College
Hartford, CT
published by Prentice Hall
Java, Java, Java
Object Oriented Problem Solving
Chapter 4 Applets:
Programming for the
World Wide Web
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 2002. 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 2002. 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 2002. All rights reserved.
Chapter 4: Applets
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Applications vs. Applets
•
•
•
•
•
Java Applications
Stand-alone program
Runs independently
Has a main() method
No HTML file
Run using JDK’s java
interpreter
Java, Java, Java, 2E by R. Morelli
•
•
•
•
•
Java Applets
Embedded program.
Runs in a Web browser
No main() method.
Requires an HTML file
Run using JDK’s
appletviewer
Copyright 2002. All rights reserved.
Chapter 4: Applets
The HelloWorld Application
Multi-line
comment block
/*
* The HelloWorld application program
*/
public class HelloWorld
{
// Class header
// Start of class body
Single-line
comments
public static void main(String argv[]) // Main method
{
System.out.println("Hello world!");
} // End of main
} // End of HelloWorld
Java, Java, Java, 2E by R. Morelli
Execution starts on
the first line of main()
Copyright 2002. All rights reserved.
Chapter 4: Applets
The HelloWorld Applet
/*
* HelloWorld applet program
*/
import java.applet.Applet;
import java.awt.Graphics;
// Import the Applet class
// and the Graphics class
public class HelloWorld extends Applet // Class header
{
// Start of body
public void paint(Graphics g)
// The paint method
{
g.drawString("HelloWorld",10,10);
} // End of paint
} // End of HelloWorld
Java, Java, Java, 2E by R. Morelli
This statement displays
“HelloWorld” on the
browser window.
Copyright 2002. All rights reserved.
Chapter 4: Applets
Editing, Compiling, and Running
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
• R
u
n
n
i
Running a Java Applet
• Running an applet requires an HTML file
containing an <applet> tag:
<HTML>
...
<APPLET CODE=“HelloWorld.class”
WIDTH=200 HEIGHT=200>
</APPLET>
...
</HTML>
• JDK Command: appletviewer file.html
• Browser: Open the applet’s HTML file
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Running a HelloApplet
Try running HelloApplet
Java, Java, Java, 2E by R. Morelli
Copyright 2002. 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 2002. All rights reserved.
Chapter 4: Applets
Java's GUI Components
GUI components that
can be put into an
applet.
By default, applets use a
FlowLayout.
An Applet isa Panel which
isa Container which isa
Component
Java, Java, Java, 2E by R. Morelli
Copyright 2002. 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 2002. 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 2002. All rights reserved.
Chapter 4: Applets
Inheritance: Specializing a Class
• Inheritance allows us to specialize a class.
Methods in the superclass
can be shared by the
subclasses.
Subclasses inherit
getCreditCardNumber()
and add their own
special methods.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. 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 2002. All rights reserved.
Chapter 4: Applets
A Square is an Extension of a Rectangle
public class Rectangle
{
protected double length;
protected double width;
The Square() constructor
calls the Rectangle()
constructor, passing it side
as a value for both length
and width.
public Rectangle (double l, double w)
{
length = l;
width = w;
public class Square extends Rectangle
} // Rectangle()
{
public Square (double side)
public double calculateArea()
{
{
//Call the superconstructor
return length * width;
super(side, side);
} // calculateArea()
}
} // Rectangle
} // Square
The super keyword refers to
the superclass.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Using the Square Class
Create a new Square
with a side of 10.
public class TestSquare
{
public static void main(String argv[])
{
Square square = new Square (10);
System.out.println("square's area is " + square.calculateArea());
}
} // TestSquare
Output
Produced
The inherited calculateArea()
method can be used just as if
it were defined in Square.
square’s area is 100.0
Java, Java, Java, 2E by R. Morelli
Copyright 2002. 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 2002. 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.
public class SimpleApplet extends Applet { ... } // Define subclass
SimpleApplet myApplet = new SimpleApplet(); // and instantiate it
• The Applet class cannot be instantiated.
Applet a = new Applet();
Java, Java, Java, 2E by R. Morelli
// Not the way to do it!
Copyright 2002. All rights reserved.
Chapter 4: Applets
A Simple Applet
• SimpleApplet inherits from Applet and
implements the ActionListener interface.
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.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Java Definition of SimpleApplet
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
Import class names
Create a subclass
public class SimpleApplet extends Applet implements ActionListener
{
private Button toggle; // From java.awt.*
The applet handles
button clicks
public void init()
{
toggle = new Button ("The machine is off");
toggle.addActionListener(this);
add(toggle);
} // init()
Execution begins here
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
Called when toggle is clicked.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
SimpleApplet Inherits Functionality
Use: A SimpleApplet can use the add()
method inherited from
java.awt.Container.
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
Java, Java, Java, 2E by R. Morelli
Copyright 2002. 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 2002. 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.
Create a new Button.
public void init ()
{
toggle = new Button ("The machine is off");
toggle.addActionListener(this);
add(toggle);
} // init()
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 2002. 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);
}
• The implementation must have the exact
same signature.
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()
Java, Java, Java, 2E by R. Morelli
Copyright 2002. 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 2002. 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 2002. All rights reserved.
Chapter 4: Applets
The Polymorphic init() Method
• Polymorphism: The init() method has
different behavior for AppletA and 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”);
}
}
Applet applet= new AppletA();
applet.init();
applet = new AppletB();
applet.init();
Java, Java, Java, 2E by R. Morelli
// Prints “AppletA”
// Prints “AppletB”
Copyright 2002. All rights reserved.
Chapter 4: Applets
Event-Driven Programming
Java’s Event Model
Events are handled by
methods in the applet.
Events are triggered
by user actions
(mouse, keyboard).
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
The Java Event Hierarchy
An action event occurs
when an applet Button is
clicked.
An key event occurs when
the user types a key.
An mouse event occurs
when the mouse is moved.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Creating an ActionListener
toggle = new Button ("The machine is off");
toggle.addActionListener(this);
After this applet is
designated as a
ActionListener for the
toggle Button...
The this keyword is selfreferential. It’s like saying “I”.
… it listens for clicks on
toggle.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Handling Action Events
1. Applet subclass implements the
ActionListener interface
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
2. Declare a Button instance.
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()
{
3. Create a Button instance.
4. Designate this applet as the button’s
listener.
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");
5. Add the button to the applet.
else
// or
toggle.setLabel("The machine is on");
// change it back
} // actionPerformed()
6. Define the specific actions to
} // SimpleApplet
be performed when the button is
clicked..
Java, Java, Java, 2E by R. Morelli
Copyright 2002. 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 2002. All rights reserved.
Chapter 4: Applets
Running an (Embedded) Applet
• A summary of what happens from the
browser’s (or appletviewer’s) perspective.
Create an instance of the applet.
SimpleApplet applet1 = new SimpleApplet();
applet1.init() ;
...
// Create an applet object
// 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
}
Repeatedly pass events that belong to
the applet to its actionPerformed()
method.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Tracing the Applet
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
The order in which applet
statements are executed when the
user makes two successive clicks
on its toggle button.
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()
{
The applet is initialized.
1
2
3
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
The user clicks on the button.
// or
First Click Second Click
toggle.setLabel("The machine is on"); // change it back
4
7
} // actionPerformed()
5
8
} // SimpleApplet
6
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
9
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 2002. All rights reserved.
Chapter 4: Applets
CyberPetApplet’s GUI Interface
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
CyberPetApplet: Design Specification
Uses
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Defining CyberPetApplet
Step 1. Define a subclass of
java.applet.Applet that implements the
ActionListener interface.
import java.awt.*;
import java.applet.*;
import java.awt.event.*;
public class CyberPetApplet extends Applet implements
ActionListener
{
} // CyberPetApplet
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Defining CyberPetApplet (cont.)
Step 2. Declare the instance variables.
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
Stub version of init()
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Defining CyberPetApplet (cont.)
Step 3. Define the init() method.
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(stateField);
add(eatButton);
add(sleepButton);
} // init()
Java, Java, Java, 2E by R. Morelli
// Add the components to the applet.
Copyright 2002. All rights reserved.
Chapter 4: Applets
Default Layout: FlowLayout
add(nameLabel);
add(stateField);
add(eatButton);
add(sleepButton);
// Add the components to the applet.
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 2002. All rights reserved.
Chapter 4: Applets
Interacting Objects
stateField.setText(pet1.getState()); // Initialize the TextField
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Defining CyberPetApplet (cont.)
Step 4. Implement the
actionPerformed() method.
public class CyberPetApplet extends Applet implements ActionListener {
// Variable declarations and init() method not shown.
public void actionPerformed(ActionEvent e)
{
if (e.getSource() == eatButton)
pet1.eat();
else if (e.getSource() == sleepButton)
pet1.sleep();
stateField.setText(pet1.getState());
} // actionPerformed()
} // CyberPetApplet
Java, Java, Java, 2E by R. Morelli
// If eat button clicked,
// tell the pet to eat
// If sleep button clicked,
// tell the pet to sleep
// Display the pet's state
The getSource() method
returns the source of the
ActionEvent.
Copyright 2002. All rights reserved.
Chapter 4: Applets
The CyberPetApplet Program
Java, Java, Java, 2E by R. Morelli
Copyright 2002. 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 2002. 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 2002. All rights reserved.
Chapter 4: Applets
Class Definition: ToggleButton
import java.awt.*;
import java.awt.event.*;
Inheritance: A ToggleButton is a Button
public class ToggleButton extends Button implements ActionListener
private String label1;
// Two Labels to toggle between
private String label2;
{
Invoke Button’s constructor to set initial label
public ToggleButton (String l1, String l2) {
// Constructor method
super(l1);
// Use l1 as the default label
label1 = l1;
label2 = l2;
A ToggleButton acts as its own ActionListener
addActionListener(this);
}
Each time a ToggleButton is
public void actionPerformed (ActionEvent e) {
clicked it swaps labels.
String tempS = label1; // Swap the labels
label1 = label2;
label2 = tempS;
setLabel (label1);
Inheritance: setLabel() is inherited
} // actionPerformed()
from Button superclass.
} // ToggleButton
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Using a ToggleButton
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
This applet uses a ToggleButton.
public class ToggleTest extends Applet implements ActionListener {
private ToggleButton lightSwitch;
Initialize the ToggleButton.
public void init() {
lightSwitch = new ToggleButton ("off","on");
add(lightSwitch);
lightSwitch.addActionListener(this);
The applet
} // init()
is the listener.
public void actionPerformed(ActionEvent e)
{
showStatus("The light is " + lightSwitch.getLabel());
} // actionPerformed()
The applet just displays
} // ToggleTest
the button’s label.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. 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 2002. All rights reserved.
Chapter 4: Applets
Java Library: Using Images in an Applet
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets
Using the java.awt.Image Class
Declare a reference
variable for each image.
import java.applet.*;
import java.awt.*;
public class Lights extends Applet
{
private Image lightOn, lightOff;
Applet.getImage() loads the
images from GIF files.
// 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
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 2002. All rights reserved.
Chapter 4: Applets
In the Laboratory: CyberPetApplet
The objectives of this lab are:
• 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)
Java, Java, Java, 2E by R. Morelli
Copyright 2002. 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 2002. 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 2002. 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 2002. 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 2002. 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 2002. 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.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 4: Applets