Chapter 14 Applets, Images, and
Audio
Prerequisites for Part III
Chapter 8 Inheritance and Polymorphism
Chapter 9 Abstract Classes and Interfaces
Chapter 11 Getting Started with GUI Programming
Chapter 12 Event-Driven Programming
Chapter 13 Creating User Interfaces
Chapter 14 Applets, Images, Audio
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1
Objectives
✦
✦
✦
✦
✦
✦
✦
✦
✦
✦
✦
To explain how the Web browser controls and executes applets (§ 14.2).
To describe the init, start, stop, and destroy methods in the Applet class (§
14.2).
To know how to embed applets in Web pages (§ 14.4).
To run applets from appletviewer and from Web browsers (§ 14.4).
To pass string values to applets from HTML (§ 14.5).
To write a Java program that can run as both an application and an applet (§
14.6).
To get image files using the URL class and display images in the panel (§ 14.9
Optional).
To develop a reusable component ImageViewer to display images (§ 14.10
Optional).
To get audio files and play sound (§ 14.12 Optional).
To package and deploy Java projects using Java archive files (§ 14.13
Optional).
To use Swing pluggable look-and-feel (§ 14.14 Optional).
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2
The Applet Class
public class MyApplet extends java.applet.Applet {
...
/** The no-arg constructor is called by the browser when the Web
page containing this applet is initially loaded, or reloaded
*/
public MyApplet() {
...
}
/** Called by the browser after the applet is loaded
*/
public void init() {
...
}
/** Called by the browser after the init() method, or
every time the Web page is visited
*/
public void start() {
...
}
/** Called by the browser when the page containing this
applet becomes inactive
*/
public void stop() {
...
}
/** Called by the browser when the Web browser exits */
public void destroy() {
...
}
/** Other methods if necessary... */
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}
3
The Applet Class, cont.
When the applet is loaded, the Web browser creates
an instance of the applet by invoking the applet’s
no-arg constructor. The browser uses the init, start,
stop, and destroy methods to control the applet. By
default, these methods do nothing. To perform
specific functions, they need to be modified in the
user's applet so that the browser can call your code
properly.
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4
Browser Calling Applet Methods
Loaded
JVM loads the
applet class
Browser creates
the applet
Created
Browser
invokes init()
Initialized
Browser
invokes start()
Browser
invokes stop()
Started
Stopped
Browser
invokes stop()
Browser invokes
destroy()
Destroyed
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5
The init() Method
Invoked when the applet is first loaded and again
if the applet is reloaded.
A subclass of Applet should override this method if
the subclass has an initialization to perform. The
functions usually implemented in this method
include creating new threads, loading images,
setting up user-interface components, and getting
string parameter values from the <applet> tag in the
HTML page.
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6
The start() Method
Invoked after the init() method is executed; also called
whenever the applet becomes active again after a period of
inactivity (for example, when the user returns to the page
containing the applet after surfing other Web pages).
A subclass of Applet overrides this method if it has
any operation that needs to be performed whenever
the Web page containing the applet is visited. An
applet with animation, for example, might use the
start method to resume animation.
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7
The stop() Method
The opposite of the start() method, which is called when the user
moves back to the page containing the applet; the stop() method is
invoked when the user moves off the page.
A subclass of Applet overrides this method if it has any
operation that needs to be performed each time the Web
page containing the applet is no longer visible. When the
user leaves the page, any threads the applet has started but
not completed will continue to run. You should override
the stop method to suspend the running threads so that the
applet does not take up system resources when it is
inactive.
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8
The destroy() Method
Invoked when the browser exits normally to inform
the applet that it is no longer needed and that it
should release any resources it has allocated.
A subclass of Applet overrides this method if it has
any operation that needs to be performed before it
is destroyed. Usually, you won't need to override
this method unless you wish to release specific
resources, such as threads that the applet created.
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9
The JApplet Class
The Applet class is an AWT class and is not designed to
work with Swing components. To use Swing components in
Java applets, it is necessary to create a Java applet that
extends javax.swing.JApplet, which is a subclass of
java.applet.Applet. JApplet inherits all the methods from the
Applet class. In addition, it provides support for laying out
Swing components.
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10
First Simple Applet
// WelcomeApplet.java: Applet for displaying a message
import javax.swing.*;
public class WelcomeApplet extends JApplet {
/** Initialize the applet */
public void init() {
getContentPane().add(new JLabel("Welcome to Java",
JLabel.CENTER));
}
}
// WelcomeApplet.java: Applet for displaying a message
import javax.swing.*;
public class WelcomeApplet extends JApplet {
/** Initialize the applet */
public WelcomeApplet() {
getContentPane().add(new JLabel("Welcome to Java",
JLabel.CENTER));
}
}
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11
First Simple Applet
<html>
<head>
<title>Welcome Java Applet</title>
</head>
<body>
<applet
code = "WelcomeApplet.class"
width = 350
height = 200>
</applet>
</body>
</html>
WelcomeApplet
Run Applet Viewer
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12
Example 14.1 Using Applets
Objective: Compute
Loans. The applet
enables the user to enter
the annual interest rate,
the number of years, and
the loan amount. Click
the Compute Loan
button, and the applet
displays the monthly
payment and the total
payment.
LoanApplet
Run Applet Viewer
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13
Writing Applets
✦
Always extends the JApplet class, which is a
subclass of Applet for Swing components.
✦
Override init(), start(), stop(), and
destroy() if necessary. By default, these
methods are empty.
✦
Add your own methods and data if necessary.
✦
Applets are always embedded in an
HTML page.
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14
The <applet> HTML Tag
<applet
code=classfilename.class
width=applet_viewing_width_in_pixels
height=applet_viewing_height_in_pixels
[archive=archivefile]
[codebase=applet_url]
[vspace=vertical_margin]
[hspace=horizontal_margin]
[align=applet_alignment]
[alt=alternative_text]
>
<param name=param_name1
value=param_value1>
</applet>
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15
Passing Parameters to Applets
<applet
code = "DisplayMessage.class"
width = 200
height = 50>
<param name=MESSAGE value="Welcome
to Java">
<param name=X value=20>
<param name=Y value=20>
alt="You must have a Java-enabled
browser to view the applet"
</applet>
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16
Example 14.2 Passing Parameters
to Java Applets
Objective: Display a
message at a
specified location.
The message and the
location (x, y) are
obtained from the
HTML source.
DisplayMessage
Run Applet Viewer
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17
Applications vs. Applets
✦
Similarities
– Since they both are subclasses of the Container class, all the user
interface components, layout managers, and event-handling
features are the same for both classes.
✦
Differences
– Applications are invoked from the static main method by the
Java interpreter, and applets are run by the Web browser. The
Web browser creates an instance of the applet using the applet’s
no-arg constructor and controls and executes the applet through
the init, start, stop, and destroy methods.
– Applets have security restrictions
– Web browser creates graphical environment for applets, GUI
applications are placed in a frame.
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18
Security Restrictions on Applets
✦
Applets are not allowed to read from, or write to,
the file system of the computer viewing the
applets.
✦
Applets are not allowed to run any programs on
the browser’s computer.
✦
Applets are not allowed to establish connections
between the user’s computer and another
computer except with the server where
the applets are stored.
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19
Conversions Between
Applications and Applets
✦
Conversions between applications and applets are
simple and easy.
✦
You can always convert an applet into an
application.
✦
You can convert an application to an
applet as long as security restrictions are
not violated.
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20
Example 14.3
Running a Program as an Applet
and as an Application
✦
Objective: Modify MessageApplet to enable it
to run both as an applet and as an application.
DisplayMessageApp
Run as Application
Run as Applet
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21
Optional
Case Study: TicTacToe
MouseListener
JPanel
Cell
-token: char
Token used in the cell (default: ' ').
+getToken():char
Returns the token in the cell.
+setToken(token: char): void
Sets a new token in the cell.
#paintComponent(g: Graphics): void
Paints the token in the cell.
+mouseClicked(e: MouseEvent): void
Handles a mouse click on the cell.
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22
Case Study: TicTacToe, cont.
Cell
JApplet
9
1
TicTacToe
-whoseTurn: char
Indicates which player has the turn, initially 'X'.
-cell: Cell[][]
A 3 by 3, two dimensional array for cells.
-jlblStatus: JLabel
A label to display game status.
+TicTacToe()
Constructs the TicTacToe user interface.
+isFull(): boolean
Returns true if all cells are filled.
+isWon(token: char): boolean
Returns true if a player with the specified token has won.
TicTacToe
Run as Application
Run as Applet
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23
Optional
Case Study: Bouncing Ball
Objective: Write an
applet that displays a ball
bouncing in a panel. Use
two buttons to suspend
and resume the
movement and use a
scroll bar to control the
bouncing speed.
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24
Case Study: Bouncing Ball, cont.
JPanel
JPanel ActionListener AdjustmentListener
1
Ball
-x: int
-y: int
-dx: int
-dy: int
-radius: int
-delay: int
-timer: Timer
BallControl
1
1
-ball: Ball
-jsbDelay: int
-jbtResume: JButton
-jbtSuspend: JButton
+Ball()
+run(): void
+suspend(): void
+resume(): void
+setDelay(delay: int): void
Ball
1
JApplet
BounceBallApp
+BounceBallApplet()
+main(args: String[]): void
+BallControl()
+actionPerformed(e: ActionEvent): void
+adjustmentValueChanged
(AdjustmentEvent e): void
BallControl
BounceBallApp
Run
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25
Optional
What is Multimedia?
Multimedia is a broad term that encompasses making, storing,
retrieving, transferring, and presenting various types of information,
such as text, graphics, pictures, videos, and sound. Multimedia
involves a complex weave of communications, electronics, and
computer technologies. It is beyond the scope of this book to cover
multimedia in great detail. This chapter concentrates on the
presentation of multimedia in Java.
Whereas most programming languages have no built-in multimedia
capabilities, Java was designed with multimedia in mind. It provides
extensive built-in support that makes it easy to develop powerful
multimedia applications. Java's multimedia capabilities include
animation that uses drawings, audio, and images.
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26
Optional
The URL Class
Audio and images are stored in files. The java.net.URL class can be
used to identify the files on the Internet. In general, a URL (Uniform
Resource Locator) is a pointer to a "resource" on the World Wide
Web. A resource can be something as simple as a file or a directory.
You can create a URL object using the following constructor:
public URL(String spec) throws MalformedURLException
For example, the following statement creates a URL object for
http://www.sun.com:
try {
URL url = new URL("http://www.sun.com");
}
catch(MalformedURLException ex) {
}
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27
Creating a URL for Local Files
The following statement creates a URL object for the file
c:\book\image\us.gif.
try {
URL url = new URL("c:\\book\\image\\us.gif");
}
catch(MalformedURLException ex) {
}
The preceding statement creates a URL for the absolute file name
c:\book\beep.au on the Windows. There are two problems: (1) the
file location is fixed; (2) it is platform-dependent. To circumvent
these problems, you can create the URLs for files using through the
resource of the class file using the java.lang.Class class.
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28
Creating a URL from a Class Reference
Whenever the Java Virtual Machine loads a class or an interface, it creates an
instance of a special class named Class. The Class class provides access to useful
information about the class, such as the data fields and methods. It also contains the
getResource(filename) method, which can be used to obtain the URL of a given
file name in the same directory with the class or in its subdirectory. As discussed in
Section 8.4.5, “The getClass Method,” you can obtain the class for the object at
runtime. Thus, you can use the following code to get the URL of a file:
Class class = this.getClass();
URL url = class.getResource(filename);
where filename is a relative file name in the same directory or a subdirectory of the
class. Suppose you run a class stored in c:\book, the following statement creates a
URL object for c:\book\image\us.gif.
URL url = this.getClass().getResource(“image\us.gif");
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29
Creating ImageIcon Using Absolute File Names
You used the ImageIcon class to create an icon from an image file
and the setIcon method or the constructor to place the image in a
GUI component, such as a button and a label. For example, the
following statements create an ImageIcon and set it on an JLabel
object jlbl.
ImageIcon imageIcon = new ImageIcon("c:\\book\\image\\us.gif");
//ImageIcon imageIcon = new ImageIcon("image/us.gif");
jlbl.setIcon(imageIcon);
This approach suffers a problem. The file location is fixed since it
uses the absolute file path on Window. Thus, the program cannot
run on other platforms and cannot run as applet.
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30
Creating ImageIcon Using Class Reference
To circumvent this problem, you can create the URLs for
files through the class reference using the java.lang.Class
class. For example, suppose the class file is in c:\book, the
following statements create a URL for
c:\book\image\us.gif.
Class class = this.getClass();
URL url = class.getResource("image/us.gif");
You can now create an ImageIcon using
ImageIcon imageIcon = new ImageIcon(url);
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31
Creating Image From ImageIcon
An image icon displays a fixed-size image. To display an
image in a flexible size, you need to use the
java.awt.Image class. An image can be created from an
image icon as follows:
Image image = imageIcon.getImage();
Given an Image, you can create an ImageIcon using
ImageIcon imageIcon = new ImageIcon(image);
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32
Drawing Images in Graphics
Using a label as an area for displaying images is simple and convenient, but you
don't have much control over how the image is displayed. A more flexible way to
display images is to use the drawImage method of the Graphics class on a panel
using one of the foure overloaded drawImage method.
drawImage(Image img, int x, int
ImageObserver observer)
drawImage(Image img, int x, int
drawImage(Image img, int x, int
drawImage(Image img, int x, int
Color bgcolor, ImageObserver
y, Color bgcolor,
y, ImageObserver observer)
y, ImageObserver observer)
y, int width, int height,
observer)
ImageObserver is an asynchronous update interface that receives notifications of
image information as the image is constructed. The Component class implements
ImageObserver. Therefore, every GUI component is an instance of ImageObserver.
To draw images using the drawImage method in a Swing component, such as
JPanel, override the paintComponent method to tell the component how to display
the image in the panel.
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33
Displaying Image
Write a program that displays an image from
/image/us.gif in the class directory on a panel.
DisplayImage
Run
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34
Case Study: ImageViewer Component
javax.swing.JPanel
ImageViewer
-image: Image
Image in the image viewer.
-imageFilename: String
Filename of the image.
-stretched: boolean
True if the image is stretched in the viewer.
-xCoordinate: int
x-Coordinate of the upper left corner of the image in the viewer.
-yCoordinate: int
y-Coordinate of the upper left corner of the image in the viewer.
+ImageViewer()
Constructs an image viewer with no image.
+ImageViewer(image: Image)
Constructs an image viewer with the specified image.
+createImage(imageFilename:
String, object: Object): Image
Creates an image from the specified image file.
+createImageIcon(imageFilename:
String, object: Object): ImageIcon
Creates an image icon from the specified image file.
+createImage(urlString: String):
Image
Creates an image from the specified URL string.
+createImageIcon(urlString:
String): ImageIcon
Creates an image icon from the specified URL string.
ImageViewer
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35
Six Flag Example
Objective: Use the
ImageViewer class
to display six flags.
SixFlags
Run
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36
Case Study: Image Animation
✦
Objective: Simulate a movie by displaying a sequence
of images in a control loop.
ImageAnimation
Run as an Application
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37
Creating AudioClip from an Audio File
To play an audio file in an applet, first create an audio clip object for
the audio file. The audio clip is created once and can be played
repeatedly without reloading the file. To create an audio clip, use the
static method newAudioClip() in the java.applet.Applet class:
AudioClip audioClip = Applet.newAudioClip(url);
Audio was originally used with Java applets. For this reason, the
AudioClip interface is in the java.applet package.
The following statements, for example, create an AudioClip for the
beep.au audio file in the same directory with the class you are running.
Class class = this.getClass();
URL url = class.getResource("beep.au");
AudioClip audioClip = Applet.newAudioClip(url);
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38
Playing Audio
java.applet.AudioClip
+play()
Starts playing this audio clip. Each time this method is called, the clip is
restarted from the beginning.
+loop()
Plays the clip repeatedly.
+stop()
Stops playing the clip.
To manipulate a sound for an audio clip, use the play(), loop(), and
stop() methods in java.applet.AudioClip.
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39
Playing Anthem Example
Objective: Display
images and playing
audio in applets
and in applications.
FlagAnthem
Run as an Application
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40
Packaging and Deploying
Java Projects
Optional
What is JAR?
Java archive file can be used to group all the project files in a
compressed file for deployment.
The Java archive file format (JAR) is based on the popular ZIP file
format.
This single file can be deployed on an end-user’s machine as an
application. It also can be downloaded to a browser in a single HTTP
transaction, rather than opening a new connection for each piece. This
greatly simplifies application deployment and improves the speed
with which an applet can be loaded onto a web page and begin
functioning.
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41
Creating JAR
You can use the JDK jar command to create an
archive file. The following command creates an
archive file named TicTacToe.jar for classes
TicTacToe.class and TicTacToe$Cell.class.
jar -cf TicTacToe.jar TicTacToe.class TicTacToe$Cell.class
The -c option is for creating a new archive file, and
the -f option specifies the archive file’s name.
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42
Viewing the Contents of a JAR File
You can view the contents of a .jar file using
WinZip.
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43
Manifest File
A manifest file was created with the path name meta-inf\. The
manifest is a special file that contains information about the files
packaged in a JAR file. For instance, the manifest file in
TicTacToe.jar contains the following information:
Manifest-Version: 1.0
Name: TicTacToe.class
Java-Bean: True
Name: TioTacToe$Cell.class
Java-Bean: True
You can modify the information contained in the manifest file to
enable the JAR file to be used for a variety of purposes. For instance,
you can add information to specify a main class to run an application
using the .jar file.
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44
Running Archived Projects Standalone
The manifest file must have an entry to contain the main class. For
example, to run TicTacToe, you need to insert the following two lines
in the manifest file:
Main-Class: TicTacToe
Sealed: true
Run the .jar file using the java command from the directory that
contains TicTacToe.jar,
java -jar TicTacToe.jar
TIP: You can write an installation procedure that creates the
necessary directories and subdirectories on the end-user’s computer.
The installation can also create an icon that the end-user can doubleclick on to start the program. For information on creating Windows
desktop icon, please see www.prenhall.com/liang/intro5e.html.
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45
Running Archived Projects As Applet
To run TicTacToe as an applet, modify the <APPLET> tag
in the HTML file to include an ARCHIVE attribute. The
ARCHIVE attribute specifies the archive file in which the
applet is contained. For example, the HTML file for
running TicTacToe can be modified as shown below:
<APPLET
CODE = "TicTacToe.class"
ARCHIVE = "TicTacToe.jar"
WIDTH = 400
HEIGHT = 300
HSPACE = 0
VSPACE = 0
ALIGN = Middle
>
</APPLET>
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46
Optional
Pluggable Look-and-Feel
The pluggable look-and-feel feature lets you design a single
set of GUI components that automatically has the look-andfeel of any OS platform. The implementation of this feature
is independent of the underlying native GUI, yet it can
imitate the native behavior of the native GUI.
Currently, Java supports the following three look-and-feel
styles:
·
·
·
Metal
Motif
Windows
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47
Setting Look-And-Feel
The javax.swing.UIManager class manages the look-andfeel of the user interface. You can use one of the following
three methods to set the look-and-feel for Metal, Motif, or
Windows:
UIManager.setLookAndFeel
(UIManager.getCrossPlatformLookAndFeelClassName());
UIManager.setLookAndFeel
(new com.sun.java.swing.plaf.motif.MotifLookAndFeel());
UIManager.setLookAndFeel
(new com.sun.java.swing.plaf.windows.WindowsLookAndFeel());
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48
Setting Look-And-Feel in Static
Initialization Block
To ensure that the setting takes effect, the setLookAndFeel method
should be executed before any of the components are instantiated.
Thus, you can put the code in a static block, as shown below:
static {
try {
// Set a look-and-feel, e.g.,
//UIManager.setLookAndFeel
// (UIManager.getCrossPlatformLookAndFeelClassName());
}
catch (UnsupportedLookAndFeelException ex) {}
}
Static initialization blocks are executed when the class is loaded. For
more information on static initialization blocks, please refer to
Section 8.12, “Initialization Block.”
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6
49