liang,introduction to java programming,revised by dai-kaiyu 1 chapter 14 applets, images, and audio...

Liang,Introduction to Java Programming,revised by Dai-kaiyu1

Chapter 14 Applets, Images, and Audio

Prerequisites for Part III

Chapter 11 Getting Started with GUI Programming

Chapter 8 Inheritance and Polymorphism

Chapter 13 Creating User Interfaces

Chapter 14 Applets, Images, Audio

Chapter 9 Abstract Classes and Interfaces

Chapter 12 Event-Driven Programming

愿生如夏花之绚烂，死如秋叶之静美 ---- 泰戈尔


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).


Introduction

Applet Program that runs in Applet Container

appletviewer (test utility for applets) Web browser (IE, Communicator)

Executes when HTML (Hypertext Markup Language) document containing applet is opened and downloaded

Applications run in command windows


Launching applets Applications are

started by a call to the main method

Applets have no main method

Class file referenced in the HTML document

Browser loads and runs the Applet

JDK includes the appletviewer application


The TicTacToe Applet

You start as player "X"Fig. 3.2 Sample execution of the TicTacToe applet.


The DrawTest AppletFig. 3.4 Sample execution of applet DrawTest.

Combo Box

Radio button


The Applet Classpublic 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... */}


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.

See figure14.1 (a) in textbook


11.4.2 Applet lifecycle methods init(), start(), stop(),

destroy() init() is called when

the applet is loaded start() is called by

init, and when the user returns to the page

stop() is called when the user leaves the page

destroy() is called when the browser is closed


Browser Calling Applet Methods

Loaded

Initialized

Browser invokes init()

Started Stopped

Destroyed

Created

Browser creates the applet

JVM loads the applet class

Browser invokes stop()

Browser invokes start()

Browser invokes stop()

Browser invokes destroy()


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.


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.


The stop() MethodThe 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.


The destroy() MethodInvoked 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.


applet methods

Applet methods are called by the browser

Different browser events result in calls to the methods


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.


First Simple Applet

// WelcomeApplet.java: Applet for displaying a messageimport javax.swing.*;public class WelcomeApplet extends JApplet { /** Initialize the applet */ public void init() { getContentPane().add(new JLabel("Welcome to Java", JLabel.CENTER)); }}


First Simple Applet

<html><head><title>Welcome Java Applet</title></head><body><applet code = "WelcomeApplet.class" width = 350 height = 200></applet></body></html>

WelcomeAppletWelcomeApplet Run Applet ViewerRun Applet Viewer


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.


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>


Example 14.1 Using AppletsObjective: 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.

LoanAppletLoanApplet Run Applet ViewerRun Applet Viewer


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>


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 ViewerRun Applet ViewerDisplayMessageDisplayMessage


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.


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 wherethe applets are stored.

Excute


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.


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.

Run as ApplicationRun as Application Run as AppletRun as Applet

DisplayMessageAppDisplayMessageApp


Case Study: TicTacToe

Cell

-token: char

+getToken():char

+setToken(token: char): void

#paintComponent(g: Graphics): void

+mouseClicked(e: MouseEvent): void

JPanel -char token +getToken +setToken +paintComponet +mouseClicked

MouseListener -char token +getToken +setToken +paintComponet +mouseClicked

Token used in the cell (default: ' ').

Returns the token in the cell.

Sets a new token in the cell.

Paints the token in the cell.

Handles a mouse click on the cell.

Optional


Case Study: TicTacToe, cont.

Run as AppletRun as AppletTicTacToeTicTacToe Run as ApplicationRun as Application

1

9

TicTacToe

-whoseTurn: char

-cell: Cell[][]

-jlblStatus: JLabel

+TicTacToe()

+isFull(): boolean

+isWon(token: char): boolean

JApplet -char token +getToken +setToken +paintComponet +mouseClicked

Cell -char token +getToken +setToken +paintComponet +mouseClicked

Indicates which player has the turn, initially 'X'.

A 3 by 3, two dimensional array for cells.

A label to display game status.

Constructs the TicTacToe user interface.

Returns true if all cells are filled.

Returns true if a player with the specified token has won.


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.

OptionalCase Study: Bouncing Ball


Case Study: Bouncing Ball, cont.

BallBall BallControlBallControl BounceBallAppBounceBallApp RunRun

BounceBallApp +BounceBallApplet() +main(args: String[]): void

JApplet -char token +getToken +setToken +paintComponet +mouseClicked

BallControl -ball: Ball -jsbDelay: int -jbtResume: JButton -jbtSuspend: JButton +BallControl() +actionPerformed(e: ActionEvent): void +adjustmentValueChanged (AdjustmentEvent e): void


Ball -x: int -y: int -dx: int -dy: int -radius: int -delay: int -timer: Timer +Ball() +run(): void +suspend(): void +resume(): void +setDelay(delay: int): void


1 1

ActionListener

AdjustmentListener

1 1


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.

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) {}

Optional


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.


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");


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.

Creating ImageIcon Using Absolute File Names


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);

Creating ImageIcon Using Class Reference


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);

Creating Image From ImageIcon


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.

Drawing Images in Graphics

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, int width, int height, 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.


Displaying Image

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

DisplayImageDisplayImage RunRun


Case Study: ImageViewer Component

ImageViewerImageViewer

ImageViewer

-image: Image

-imageFilename: String

-stretched: boolean

-xCoordinate: int

-yCoordinate: int

+ImageViewer()

+ImageViewer(image: Image)

+createImage(imageFilename: String, object: Object): Image

+createImageIcon(imageFilename: String, object: Object): ImageIcon

+createImage(urlString: String): Image

+createImageIcon(urlString: String): ImageIcon

javax.swing.JPanel

Image in the image viewer.

Filename of the image.

True if the image is stretched in the viewer.

x-Coordinate of the upper left corner of the image in the viewer.

y-Coordinate of the upper left corner of the image in the viewer.

Constructs an image viewer with no image.

Constructs an image viewer with the specified image.

Creates an image from the specified image file. Creates an image icon from the specified image file.

Creates an image from the specified URL string.

Creates an image icon from the specified URL string.


Six Flag Example

Objective: Use the ImageViewer class to display six flags.

SixFlagsSixFlags RunRun


Case Study: Image Animation

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

ImageAnimationImageAnimation Run as an ApplicationRun as an Application


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);


Playing Audio

java.applet.AudioClip

+play()

+loop()

+stop()

Starts playing this audio clip. Each time this method is called, the clip is restarted from the beginning.

Plays the clip repeatedly.

Stops playing the clip.

To manipulate a sound for an audio clip, use the play(), loop(), and stop() methods in java.applet.AudioClip.


Playing Anthem Example

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

FlagAnthemFlagAnthem

Run as an ApplicationRun as an Application


Packaging and Deploying Java Projects

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.

Optional


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.


Viewing the Contents of a JAR File

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


Manifest FileA 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.classJava-Bean: True Name: TioTacToe$Cell.classJava-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.


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: TicTacToeSealed: true

Run the .jar file using the java command from the directory that contains TicTacToe.jar,

java -jar TicTacToe.jar


Create file for double click to run

Javac to compile the source file to .class file, for example, TestUI.class

Create a config file , name it , for example, conf.txt. Put the following in it:

Main-Class: TestUI Create-By: 1.5 Make a runnable jar file from the following

command:jar cmf conf.txt TestUI.jar TestUI.class


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>


Pluggable Look-and-Feel

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

Optional


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.MotifLookAndFeel());UIManager.setLookAndFeel (new com.sun.java.swing.plaf.windows.WindowsLookAndFeel()); TestUI


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) {} }

liang,introduction to java programming,revised by dai-kaiyu 1 chapter 14 applets, images, and audio...

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