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The Java Library
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Method void showStatus(String str)
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Description Displays str in the status window of the browser or applet viewer If the browser does not support a status window, then no action takes place Called by the browser when an applet should start (or resume) execution It is automatically called after init( ) when an applet first begins Called by the browser to suspend execution of the applet Once stopped, an applet is restarted when the browser calls start( )
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The Methods Defined by Applet (continued)
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An applet is a window-based program As such, its architecture is different from the console-based programs shown in the first part of this book If you are familiar with Windows programming, you will be right at home writing applets If not, then there are a few key concepts you must understand First, applets are event driven Although we won t examine event handling until the following chapter, it is important to understand in a general way how the event-driven architecture impacts the design of an applet An applet resembles a set of interrupt service routines Here is how the process works An applet waits until an event occurs The run-time system notifies the applet about an event by calling an event handler that has been provided by the applet Once this happens, the applet must take appropriate action and then quickly return This is a crucial point For the most part, your applet should not enter a mode of operation in which it maintains control for an extended period Instead, it must perform specific actions in response to events and then return control to the run-time system In those situations in which your applet needs to perform a repetitive task on its own (for example, displaying a scrolling message across its window), you must start an additional thread of execution (You will see an example later in this chapter) Second, the user initiates interaction with an applet not the other way around As you know, in a nonwindowed program, when the program needs input, it will prompt the user and then call some input method, such as readLine( ) This is not the way it works in an applet Instead, the user interacts with the applet as he or she wants, when he or she wants These interactions are sent to the applet as events to which the applet must respond For example, when the user clicks the mouse inside the applet s window, a mouse-clicked event is generated If the user presses a key while the applet s window has input focus, a keypress event is generated As you will see in later chapters, applets can contain various controls, such as push buttons and check boxes When the user interacts with one of these controls, an event is generated While the architecture of an applet is not as easy to understand as that of a console-based program, Java makes it as simple as possible If you have written programs for Windows, you know how intimidating that environment can be Fortunately, Java provides a much cleaner approach that is more quickly mastered
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21:
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The Applet Class
An Applet Skeleton
All but the most trivial applets override a set of methods that provides the basic mechanism by which the browser or applet viewer interfaces to the applet and controls its execution Four of these methods, init( ), start( ), stop( ), and destroy( ), apply to all applets and are defined by Applet Default implementations for all of these methods are provided Applets do not need to override those methods they do not use However, only very simple applets will not need to define all of them AWT-based applets (such as those discussed in this chapter) will also override the paint( ) method, which is defined by the AWT Component class This method is called when the applet s output must be redisplayed (Swing-based applets use a different mechanism to accomplish this task) These five methods can be assembled into the skeleton shown here:
// An Applet skeleton import javaawt*; import javaapplet*; /* <applet code="AppletSkel" width=300 height=100> </applet> */ public class AppletSkel extends Applet { // Called first public void init() { // initialization } /* Called second, after init() the applet is restarted */ public void start() { // start or resume execution } Also called whenever
// Called when the applet is stopped public void stop() { // suspends execution } /* Called when applet is terminated method executed */ public void destroy() { // perform shutdown activities } This is the last
// Called when an applet's window must be restored public void paint(Graphics g) { // redisplay contents of window } }
Part II:
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