Transcript Chapter 14 slides

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
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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Objectives
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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).
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
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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... */
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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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.
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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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
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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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.
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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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.
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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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.
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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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.
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
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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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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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rights reserved. 0-13-148952-6
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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
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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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
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
13
Writing Applets
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Always extends the JApplet class, which is a
subclass of Applet for Swing components.
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Override init(), start(), stop(), and
destroy() if necessary. By default, these
methods are empty.
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Add your own methods and data if necessary.

Applets are always embedded in an
HTML page.
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
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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>
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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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>
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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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
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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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.
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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.
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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Security Restrictions on Applets
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Applets are not allowed to read from, or write to,
the file system of the computer viewing the
applets.
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Applets are not allowed to run any programs on
the browser’s computer.
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Applets are not allowed to establish connections
between the user’s computer and another
computer except with the server where
the applets are stored.
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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Conversions Between
Applications and Applets
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Conversions between applications and applets are
simple and easy.

You can always convert an applet into an
application.
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You can convert an application to an
applet as long as security restrictions are
not violated.
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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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
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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Optional
Case Study: TicTacToe
JPanel
-char token
+getToken
+setToken
-token: char
+paintComponet
+mouseClicked
+getToken():char
MouseListener
-char token
Cell +getToken
+setToken
+paintComponet
+mouseClicked
Token used in the cell (default: ' ').
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.
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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Case Study: TicTacToe, cont.
Cell
JApplet
9 token
-char
-char token
1
+getToken
+setToken
+paintComp
onet
+mouseClic
ked
+getToken
TicTacToe
+setToken
+paintComp
-whoseTurn:
char
onet
-cell: Cell[][]
+mouseClic
ked
-jlblStatus:
JLabel
Indicates which player has the turn, initially 'X'.
+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
A 3 by 3, two dimensional array for cells.
A label to display game status.
Run as Application
Run as Applet
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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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.
Liang, Introduction to Java Programming, Fifth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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Case Study: Bouncing Ball, cont.
JPanel
JPanel ActionListener AdjustmentListener
-char token
1
+getToken
Ball
+setToken
+paintComponet
-x: int
+mouseClicked
-y: int
-dx: int
-dy: int
-radius: int
-delay: int
-timer: Timer
+Ball()
+run(): void
+suspend(): void
+resume(): void
+setDelay(delay: int): void
Ball
-char
token
1
-char token
BallControl
1
1
+getToken
+setToken
-ball: Ball
+paintCo
-jsbDelay: int
mponet
-jbtResume: JButton
+mouseCli
-jbtSuspend: JButton
cked
+BallControl()
+actionPerformed(e: ActionEvent): void
+adjustmentValueChanged
(AdjustmentEvent e): void
BallControl
JApplet
+getToken
BounceBallApp
+setToken
+paintComponet
+BounceBallApplet()
+mouseClicked
+main(args:
String[]): void
BounceBallApp
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rights reserved. 0-13-148952-6
Run
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