Liang,Introduction to Java Programming,revised by Dai-kaiyu 1 Chapter 14 Applets, Images, and Audio 愿生如夏花之绚烂，死如秋叶之静美 ---- 泰戈尔

Liang,Introduction to Java Programming,revised by Dai-kaiyu 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 (§ Optional). To get audio files and play sound (§ Optional). To package and deploy Java projects using Java archive files (§ Optional). To use Swing pluggable look-and-feel (§ Optional).

Liang,Introduction to Java Programming,revised by Dai-kaiyu 3 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

Liang,Introduction to Java Programming,revised by Dai-kaiyu 4 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

Liang,Introduction to Java Programming,revised by Dai-kaiyu 5 The TicTacToe Applet You start as player "X" Fig. 3.2Sample execution of the TicTacToe applet.

Liang,Introduction to Java Programming,revised by Dai-kaiyu 6 The DrawTest Applet Fig. 3.4Sample execution of applet DrawTest. Combo Box Radio button

Liang,Introduction to Java Programming,revised by Dai-kaiyu 7 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,revised by Dai-kaiyu 8 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

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

Liang,Introduction to Java Programming,revised by Dai-kaiyu 10 Browser Calling Applet Methods

Liang,Introduction to Java Programming,revised by Dai-kaiyu 11 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 tag in the HTML page.

Liang,Introduction to Java Programming,revised by Dai-kaiyu 12 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,revised by Dai-kaiyu 13 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,revised by Dai-kaiyu 14 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,revised by Dai-kaiyu 15 applet methods Applet methods are called by the browser Different browser events result in calls to the methods

Liang,Introduction to Java Programming,revised by Dai-kaiyu 16 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.

Liang,Introduction to Java Programming,revised by Dai-kaiyu 17 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)); }

Liang,Introduction to Java Programming,revised by Dai-kaiyu 18 First Simple Applet Welcome Java Applet <applet code = "WelcomeApplet.class" width = 350 height = 200> WelcomeAppletRun Applet Viewer

Liang,Introduction to Java Programming,revised by Dai-kaiyu 19 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.

Liang,Introduction to Java Programming,revised by Dai-kaiyu 20 The 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] >

Liang,Introduction to Java Programming,revised by Dai-kaiyu 21 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. LoanAppletRun Applet Viewer

Liang,Introduction to Java Programming,revised by Dai-kaiyu 22 Passing Parameters to Applets <applet code = "DisplayMessage.class" width = 200 height = 50> alt="You must have a Java-enabled browser to view the applet"

Liang,Introduction to Java Programming,revised by Dai-kaiyu 23 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 ViewerDisplayMessage

Liang,Introduction to Java Programming,revised by Dai-kaiyu 24 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.

Liang,Introduction to Java Programming,revised by Dai-kaiyu 25 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. Excute

Liang,Introduction to Java Programming,revised by Dai-kaiyu 26 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.

Liang,Introduction to Java Programming,revised by Dai-kaiyu 27 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. Run as ApplicationRun as Applet DisplayMessageApp

Liang,Introduction to Java Programming,revised by Dai-kaiyu 28 Case Study: TicTacToe Optional

Liang,Introduction to Java Programming,revised by Dai-kaiyu 29 Case Study: TicTacToe, cont. Run as Applet TicTacToeRun as Application

Liang,Introduction to Java Programming,revised by Dai-kaiyu 30 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. Optional Case Study: Bouncing Ball

Liang,Introduction to Java Programming,revised by Dai-kaiyu 31 Case Study: Bouncing Ball, cont. BallBallControlBounceBallAppRun

Liang,Introduction to Java Programming,revised by Dai-kaiyu 32 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

Liang,Introduction to Java Programming,revised by Dai-kaiyu 33 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 try { URL url = new URL(" } catch(MalformedURLException ex) { } Optional

Liang,Introduction to Java Programming,revised by Dai-kaiyu 34 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.

Liang,Introduction to Java Programming,revised by Dai-kaiyu 35 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");

Liang,Introduction to Java Programming,revised by Dai-kaiyu 36 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

Liang,Introduction to Java Programming,revised by Dai-kaiyu 37 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

Liang,Introduction to Java Programming,revised by Dai-kaiyu 38 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

Liang,Introduction to Java Programming,revised by Dai-kaiyu 39 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.

Liang,Introduction to Java Programming,revised by Dai-kaiyu 40 Displaying Image Write a program that displays an image from /image/us.gif in the class directory on a panel. DisplayImage Run

Liang,Introduction to Java Programming,revised by Dai-kaiyu 41 Case Study: ImageViewer Component ImageViewer

Liang,Introduction to Java Programming,revised by Dai-kaiyu 42 Six Flag Example Objective: Use the ImageViewer class to display six flags. SixFlags Run

Liang,Introduction to Java Programming,revised by Dai-kaiyu 43 Case Study: Image Animation Objective: Simulate a movie by displaying a sequence of images in a control loop. ImageAnimationRun as an Application

Liang,Introduction to Java Programming,revised by Dai-kaiyu 44 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);

Liang,Introduction to Java Programming,revised by Dai-kaiyu 45 Playing Audio To manipulate a sound for an audio clip, use the play(), loop(), and stop() methods in java.applet.AudioClip.

Liang,Introduction to Java Programming,revised by Dai-kaiyu 46 Playing Anthem Example Objective: Display images and playing audio in applets and in applications. FlagAnthem Run as an Application

Liang,Introduction to Java Programming,revised by Dai-kaiyu 47 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

Liang,Introduction to Java Programming,revised by Dai-kaiyu 48 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.

Liang,Introduction to Java Programming,revised by Dai-kaiyu 49 Viewing the Contents of a JAR File You can view the contents of a.jar file using WinZip.

Liang,Introduction to Java Programming,revised by Dai-kaiyu 50 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.

Liang,Introduction to Java Programming,revised by Dai-kaiyu 51 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

Liang,Introduction to Java Programming,revised by Dai-kaiyu 52 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

Liang,Introduction to Java Programming,revised by Dai-kaiyu 53 Running Archived Projects As Applet To run TicTacToe as an applet, modify the 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 >

Liang,Introduction to Java Programming,revised by Dai-kaiyu 54 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

Liang,Introduction to Java Programming,revised by Dai-kaiyu 55 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

Liang,Introduction to Java Programming,revised by Dai-kaiyu 56 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) {} }