Chapter 20 Introductionto Java Applets

Chapter 20: Introduction to Java Applets • • Applets – Java programs that can be embedded in HyperText Markup Language (HTML) documents – The browser that executes an applet is generically known as the applet container (appletviewer or Internet Explorer) Creating the applet class – Import JApplet class form the package javax.swing – Create an applet class that extends the JApplet class. – An applet container can create only objects of classes that are public and extend JApplet class. – An applet container expects every Java applet class to have methods named init, start, paint, stop and destroy Method • These methods are inherited from class JApplet and can be overridden. • When an applet container loads an applet class, the container creates an object of the class then calls methods init, start and paint in sequence. When the method is called and its purpose public void init() Called once by the applet container when an applet is loaded for execution. This method initializes an applet. Typical actions performed here are initializing fields, creating GUI components, loading sounds to play, loading images to display (see Chapter 20, Multimedia: Applets and Applications) and creating threads (see Chapter 23, Multithreading). public void start() Called by the applet container after method init completes execution. In addition, if the user browses to another Web site and later returns to the applet’s HTML page, method start is called again. The method performs any tasks that must be completed when the applet is loaded for the first time and that must be performed every time the applet’s HTML page is revisited. Actions performed here might include starting an animation (see Chapter 21) or starting other threads of execution 80 (see Chapter 23). Method When the method is called and its purpose public void paint( Graphics g ) Called by the applet container after methods init and start. Method paint is also called when the applet needs to be repainted. For example, if the user covers the applet with another open window on the screen and later uncovers the applet, the paint method is called. Typical actions performed here involve drawing with the Graphics object g that is passed to the paint method by the applet container. public void stop() This method is called by the applet container when the user leaves the applet’s Web page by browsing to another Web page. Since it is possible that the user might return to the Web page containing the applet, method stop performs tasks that might be required to suspend the applet’s execution, so that the applet does not use computer processing time when it is not displayed on the screen. Typical actions performed here would stop the execution of animations and threads. Method When the method is called and its purpose public void destroy() This method is called by the applet container when the applet is being removed from memory. This occurs when the user exits the browsing session by closing all the browser windows and may also occur at the browser’s discretion when the user has browsed to other Web pages. The method performs any tasks that are required to clean up resources allocated to the applet.  Declaring methods init, start, paint, stop or destroy with method headers that differ from those shown previously, results in methods that will not be called by the applet container. The code specified in your versions of the methods will not execute. • Overriding method paint for drawing – • The applet container calls method paint with a Graphics object as an argument to tell the applet when to draw. Applets are embedded in Web pages for execution in an applet container. 81 – Before executing the applet, you must create an HTML document that specifies which applet to execute. • HTML documents typically end with an “.html” or “.htm” file-name extension. • Most HTML elements are delimited by pairs of tags • – – • All HTML tags begin with a left angle bracket, <, and end with a right angle bracket, > Execute the Applet in the appletviewer • From TextPad  open both of the applet and the HTML document  Tools menu  Run Java Applet • The appletviewer understands only the and HTML tags and ignores all other tags Execute the Applet using internet explorer: In the directory containing your applet and HTML document, double click in the HTML document, the applet will execute using internet explorer. Java coordinate system – Upper-left corner of a GUI component has the coordinates (0, 0) – Contains x-coordinate (horizontal coordinate) - horizontal distance moving right from the left of the screen. – Contains y-coordinate (vertical coordinate) - vertical distance moving down from the top of the screen • Coordinate units are measured in pixels. A pixel is a display monitor’s smallest unit of resolution. • A Java graphics context enables drawing on the screen 82 • Graphics Class – Manages a graphics context and draws pixels on the screen • Graphics methods for drawing lines, rectangles and ovals  Example: 83  The Output: 84 – Notes: • To ensure that it can be viewed properly on most computer screens, an applet should generally be less than 1024 pixels wide and 768 pixels tall—dimensions supported by most computer screens. • Forgetting the ending tag prevents the applet from executing in some applet containers. The appletviewer terminates without indicating an error. Some Web browsers simply ignore the incomplete applet element. • The only statements that should be placed in an applet’s init method are those that should execute only once when the applet is initialized. • Example: Applet AdditionApplet – computes the sum of two values input by the user and displays the result by drawing a String inside a rectangle on the applet • The sum is stored in an instance variable of class AdditionApplet – So it can be used in both method init and method paint 85 1 2 3 4 86