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pplications written for the Mobile Internet Device Profile (MIDP) are called MIDlets. This chapter shows you how to develop a MIDlet and places a special emphasis on how to use the graphical user interface (GUI) elements. Even if you ve never written a Java GUI before, you don t have to worry about anything. It s simple. In fact, the javax.microedition.lcdui package is a good place to start learning GUI programming because the package is so limited. Yet it contains the same core elements as Standard Edition Java GUI packages such as the java.awt and javax.swing packages, so once you ve written a few micro-edition GUIs, you can apply the same ideas and switch to writing Standard Edition GUIs pretty easily. This chapter s example MIDlet will be a simple maze game that has a size preferences screen that the user can access to modify the maze s size. Figure 2-1 shows this project open in KToolbar and running on the emulator. This example game will illustrate simple MIDlet concepts such as buttons, menus, changing screens, forms, and simple graphics, as well as describe the MIDlet lifecycle and highlight some differences between Java Micro Edition (Java ME) programming and programming for the other two main Java editions (Standard and Enterprise).
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Figure 2-1. A MIDlet running on the WTK emulator with KToolbar
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CHAPTER 2 USING MIDLETS
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Your starting point when developing a MIDlet will be to extend the MIDlet class. The MIDlet class is primarily concerned with controlling the application s lifecycle: starting, stopping, and pausing. It also provides the developer with a handle to the device s display so that you can control what appears on the screen.
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Understanding the MIDlet Lifecycle
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Just as Standard Edition Java GUI packages help you understand Java ME GUI writing, your knowledge of Applets will help you grasp MIDlets easily. (Or vice versa once you ve learned how MIDlets work, you ll discover that you understand Applets, too.) The parallels are striking. Applets and MIDlets are both run by application management software that acts as a layer of protection between the (possibly untrusted) Applet or MIDlet and the target Java Virtual Machine (JVM). For Applets, the application management software is generally a browser. In both cases the application management software performs the same role: it controls the Applet or MIDlet s lifecycle, and it provides a sandbox for the Applet or MIDlet to run in that limits access to resources. And in both cases, the program s starting point isn t the public static void main(String[] args) method. An Applet starts with the start() method, and a MIDlet starts with the startApp() method. After the application management software calls startApp(), it has a couple more methods it uses to control the MIDlet s lifecycle: pauseApp() and destroyApp(). Both of these are signals that the MIDlet needs to free up shared resources, such as input/output (IO) connections, so other applications can use them. You should think of pauseApp() as the method the application management software will call when the user temporarily switches to using another function on the device but wants to go back to your game later. destroyApp() is, of course, the signal that the user is quitting your program. If you disagree with the application management software s decision to start or destroy your MIDlet, you can throw a MIDletStateChangeException to stop it. Bear in mind that if destroyApp() is called with the unconditional argument set to true, your MIDletStateChangeException will be ignored. Since the MIDletStateChangeException is something thrown by the MIDlet developer s code, you can safely ignore any MIDletStateChangeException in a try/catch block (as I ve done in Listing 2-1) if you re certain that no such Exception will be thrown. Listing 2-1. Maze.java package net.frog_parrot.maze; import javax.microedition.midlet.*; import javax.microedition.lcdui.*; /** * This is the main class of the maze game. * * @author Carol Hamer */ public class Maze extends MIDlet implements CommandListener { //----------------------------------------------------------------
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