Chapter 14 Applets, Images, and Audio

1
Chapter 14 Applets, Images, and Audio
2
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).

3
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()

4
The Applet Class, cont.

• When the applet is loaded, the Web browser
  creates an instance of the applet by invoking the
  applets 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.

5
Browser Calling Applet Methods
6
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 ltappletgt tag in
  the HTML page.

7
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.

8
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.

9
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.

10
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.

11
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))
12
First Simple Applet
lthtmlgt ltheadgt lttitlegtWelcome Java
Appletlt/titlegt lt/headgt ltbodygt ltapplet code
"WelcomeApplet.class" width 350 height
200gt lt/appletgt lt/bodygt lt/htmlgt
WelcomeApplet
Run Applet Viewer
13
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
14
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 anHTML page.

15
The ltappletgt HTML Tag

• ltapplet
• codeclassfilename.class
• widthapplet_viewing_width_in_pixels
• heightapplet_viewing_height_in_pixels
• archivearchivefile
• codebaseapplet_url
• vspacevertical_margin
• hspacehorizontal_margin
• alignapplet_alignment
• altalternative_text
• gt
• ltparam nameparam_name1 valueparam_value1gt
• lt/appletgt

16
Passing Parameters to Applets

• ltapplet
• code "DisplayMessage.class"
• width 200
• height 50gt
• ltparam nameMESSAGE value"Welcome to Java"gt
  
• ltparam nameX value20gt
• ltparam nameY value20gt
• alt"You must have a Java-enabled browser to
  view the applet"
• lt/appletgt

17
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.

Run Applet Viewer
DisplayMessage
18
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 applets
  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.

19
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 browsers computer.
• Applets are not allowed to establish connections
  between the users computer and another computer
  except with the server wherethe applets are
  stored.

20
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 anapplet as
  long as security restrictions arenot violated.

21
Example 14.3Running a Program as an Appletand
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
22
Case Study TicTacToe
Optional
23
Case Study TicTacToe, cont.
Run as Applet
TicTacToe
Run as Application
24
Case Study Bouncing Ball
Optional

• 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.

25
Case Study Bouncing Ball, cont.
Ball
BallControl
BounceBallApp
Run
26
What is Multimedia?
Optional

• 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.

27
The URL Class
Optional

• 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 MalformedURLExcepti
  on
•  
• 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)
•  

28
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.

29
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.gi
  f")

30
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\\ima
  ge\\us.gif")
• //ImageIcon imageIcon new ImageIcon("image/us.gi
  f")
• 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.

31
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)

32
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)

33
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 y, Color bgcolor,
ImageObserver observer) drawImage(Image img, int
x, int y, ImageObserver observer) drawImage(Image
img, int x, int y, ImageObserver
observer) drawImage(Image img, int x, int y, int
width, int height, Color bgcolor, ImageObserver
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.
34
Displaying Image

• Write a program that displays an image from
  /image/us.gif in the class directory on a panel.

DisplayImage
Run
35
Case Study ImageViewer Component
ImageViewer
36
Six Flag Example

• Objective Use the ImageViewer class to display
  six flags.

SixFlags
Run
37
Case Study Image Animation

• Objective Simulate a movie by displaying a
  sequence of images in a control loop.

ImageAnimation
Run as an Application
38
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)

39
Playing Audio
To manipulate a sound for an audio clip, use the
play(), loop(), and stop() methods in
java.applet.AudioClip.
40
Playing Anthem Example

• Objective Display images and playing audio in
  applets and in applications.

FlagAnthem
Run as an Application
41
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-users
  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.

42
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 TicTacToeCell.class.
• jar -cf TicTacToe.jar TicTacToe.class
  TicTacToeCell.class
• The -c option is for creating a new archive file,
  and the -f option specifies the archive files
  name.

43
Viewing the Contents of a JAR File

• You can view the contents of a .jar file using
  WinZip.

44
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 TioTacToeCell.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.

45
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-users computer. The
  installation can also create an icon that the
  end-user can double-click on to start the
  program. For information on creating Windows
  desktop icon, please see www.prenhall.com/liang/in
  tro5e.html.

46
Running Archived Projects As Applet

• To run TicTacToe as an applet, modify the
  ltAPPLETgt 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
•  
• ltAPPLET
• CODE "TicTacToe.class"
• ARCHIVE "TicTacToe.jar"
• WIDTH 400
• HEIGHT 300
• HSPACE 0
• VSPACE 0
• ALIGN Middle
• gt
• lt/APPLETgt

47
Pluggable Look-and-Feel
Optional

• The pluggable look-and-feel feature lets you
  design a single set of GUI components that
  automatically has the look-and-feel 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

48
Setting Look-And-Feel

• The javax.swing.UIManager class manages the
  look-and-feel 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.MotifLookAndF
  eel())
• UIManager.setLookAndFeel
• (new com.sun.java.swing.plaf.windows.WindowsLook
  AndFeel())

49
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.getCrossPlatformLookAndFeelCl
  assName())
• 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.