Lecture 22


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Lecture 22

  1. 1. Core Java Debasish Pratihari Applet:  Applets are small applications that are accessed on an internet server, transposed over the internet, automatically installed, and run as part of a web component.  Applets are Java programs that are embedded within a Web page. Therefore, unlike applications, applets require a Java-enabled browser, such as Microsoft Internet Explorer 4.0 or later, Netscape Navigator 4.0 or later, or HotJava. These browsers are said to be Javaenabled because they have a built-in Java platform (JVM and Java API).  An applet is loaded and executed when you load a Web page by using a Web browser. When a Web page containing an applet is displayed, you can interact with the applet. You can use applets to add dynamic features, such as animation and sound, to a Web page. You can also use them to make a Web page interactive.  After an applet arrives on the client, it has limited access to resources, so that it can produce a reliable user interface without introducing the risk of viruses. More About Applet :         Are sub-classes of Applet Class <applet> tag of html is used to load an applet in a web page. Are executed remotely by java-enabled browser. Do not have a main( ) method. Execution begins at init( ) method Are not independent objects like application Are purely GUI. It can access the resources of only the host computer; it cannot access the files on the computer on which it is downloaded. Lecture/core/applet/22 Page #1 feel the Technology…
  2. 2. Core Java Debasish Pratihari The life cycle of an Applet :  init() is executed only once  start() Executed multiple times every time the focus comes backs to the page  stop() Executed multiple times every time The focus is lost from the web Page.  destroy() Is called only once when the page is terminated or closed.  An applet writes to its window only when its update() or paint() method is called by the AWT.  The fundamental architecture constraints imposed on an applet is that it must quickly return control to the AWT run-time system. So you shouldn’t create a loop in side paint().  The repaint() method is defined by the AWT. It causes the AWT run-time system to execute a call to your applet’s update( ) method, which in its default implementation, calls paint ( ).  The paint() method Called every time the window is resized or restored or Called forcibly by the user by calling repaint( ) method Lecture/core/applet/22 Page #2 feel the Technology…
  3. 3. Core Java Debasish Pratihari Creating an Applet: import java.awt.* import java.appet.* sub-class the applet class over-ride methods as per need. Save the file with extension .java Compile to create .class file Create a HTML file Use applet tag to position the applet inside the page  <applet code = x width =300 height=200> </applet>  save the file with extension .html  open a browser and load the .html file         Attributes of Applet Tag: code : specifies the applet class file [codebase] : is an optional attribute that specifies the base URL of the Applet. height : specifies the height of applet width : specifies width of applet [align] : specifies alignment, the align must constant are LEFT, RIGHT, TEXTTOP, MIDDLE, ABSMIDDLE, BOTTOM, ABSBOTTOM, BASELINE [vspace] : specifies vertical space [Hspace] : specifies horizontal space [archieve] : specifies the jar file [Name] : specifies the alias name for the applet used for inter applet communication. Example : import java.awt.*; import java.applet.*; /*<applet code = myapplet width=300 height=400></applet>*/ public class Myapplet extends Applet{ public void paint(Graphics g){ g.drawString(“ hello every body”,10,20); } } Lecture/core/applet/22 Page #3 Note : You can test Applets using the Java Tool appletviewer. feel the Technology…
  4. 4. Core Java Debasish Pratihari A summary of Methods in the Applet Package: Method Function public String getAppletInfo() public URL getDocumentBase() public String getParameter(String name) public String [][] getParameterInfo() public AudioClip getAudioClip(URL) Returns information about the applet, such as author Returns the URL of the HTML document Returns the parameters for an applet Returns a summary of what the parameters control Used to load an audio clip public Image getImage(URL) Used to load an image file Used to play a previously loaded audio clip Lets you know whether an applet is active public void play(URL) public boolean isActive() public void resize(int, int) Used to resize the applet public void showStatus(String msg) Displays a status string in the applet's browser public void init() Initializes the applet Starts the applet when it's finished initializing Stops the applet when you leave the applet's page Destroys the applet when you leave the browser public void start() public void stop() public void destroy() Difference between Application & Applet : Application Applet Are independent Dependent Are executed under the local o/s. Executed remotely by a browser. CUI or GUI GUI Execution starts with main( ) Starts with init( Terminates with main() Terminates only when the page is closed Lecture/core/applet/22 Page #4 ) method. feel the Technology…
  5. 5. Core Java Debasish Pratihari Notes The Graphics Class The Graphics class is an abstract class that represents the display area of an applet. This class is a part of the java.awt package and you use it to draw images within the display area of the applet. The object of the Graphics class is used for painting the applet. The init() Method The init() method is called when an applet is loaded into the memory of a computer for the first time. The init() method works like a constructor, which means that it is executed automatically by the system. By using the init() method, you can initialize variables and add components such as buttons and check boxes to an applet. The start() Method The start() method is called immediately after the init() method is called and is executed each time you visit other pages and return to the page containing the applet. You can use this method when you want to restart a process each time a user visits a page. For example, you can use the start() method in situations where you want to restart an animation sequence or a thread for your applet. If your applet does not execute any statements when a user exits from the current Web page, you need not implement this method. The stop() Method The stop() method is called each time an applet loses its focus. For example, when a user exits out of the page on which the applet is loaded, the stop() method is called. You can use this method to reset variables and stop the threads that are running. This method gives you a chance to stop activities that slow down the computer. The destroy() Method The destroy() method is called when you start viewing another Web page. You can use this method to perform clean-up operations, such as closing a file. Java calls the stop() method before calling the destroy() method. The update() Method The update() method takes the Graphics class object as a parameter. When the applet area needs to be redrawn, the Windows system starts the painting process. The update() method is called to clear the screen and it in turn calls the paint() method. The system then updates the screen. The paint() Method The paint() method draws the graphics of an applet in the drawing area. The method is automatically called when an applet is displayed on the screen for the first time and each time the applet receives focus. The paint() method can be triggered by invoking the repaint() method. The paint() method of an applet takes an object of the Graphics class as a parameter. The repaint() Method You can call the repaint() method when you want to redraw an applet area. The repaint() method calls the update() method to signal that an applet has to be updated. The default action of the update() method is to clear the applet area and call the paint() method. You can override the update() method if you do not want the applet area to be cleared. Lecture/core/applet/22 Page #5 feel the Technology…
  6. 6. Core Java Debasish Pratihari getDocumentBase() Although the getDocumentBase() method simply returns the URL of the document your applet is embedded in, this URL comes in very handy with other methods. For example, the methods discussed in the next two sections take a URL as one of their arguments. Instead of hard coding a URL, you can use the getDocumentBase() method to pass the URL to any methods that require a URL, such as getAudioClip() and getImage(), as in the following example: graphic = getParameter("graphic"); clip = getParameter("clip"); image = getImage(getDocumentBase(), graphic); sound = getAudioClip(getDocumentBase(), clip); getAudioClip(URL, String) The getAudioClip() method accepts a URL, which specifies the location of sound files on the server, and a string to represent the name of the file. Keep in mind that you can use the getDocumentBase() method to provide the URL, so if you move your applet, you don't have to recode the getAudioClip() method. If you wanted to load an audio file called soundfile.au, you could use the following code: AudioClip clip; clip = getAudioClip(getDocumentBase(), "soundfile.au"); This code just defines a variable called clip for the audio file and then makes clip equal to the result of the getAudioClip() method. The getAudioClip() method uses the getDocumentBase() method to supply the URL, and then you give the getAudioClip() method the name of the sound file directly. You could also use a variable for the name of the sound file, which would make the filename a little more flexible. Audio methods are contained with the Applet Package within the AudioClip interface. An interface is a specification the ensures that certain methods will be defined for a class. For example, the AudioClip interface ensures that the getAudioClip(), play(), and loop() methods will be defined. Table 10.2 summarizes the available audio methods. Audio methods. Method Function getAudioClip() Loads an audio file from the server play() Plays the audio file once through loop() Plays the audio file in a continuous loop stop() Stops a play() or loop() method that is in progress Lecture/core/applet/22 Page #6 feel the Technology…