MobileJava MobileMIDP Programming with J2ME

MIDP Programming with J2ME

This is Chapter 3: MIDP Programming from the book J2ME Application Development (ISBN:0-672-323095-9) written by Michael Kroll and Stefan Haustein , published by Sams Publishing.

Chapter 3: MIDP Programming

In This Chapter

  • MIDlets

  • High-Level API

  • Low-Level API

This chapter handles the life cycle and user interface of Mobile Information Device Profile (MIDP) applications. First, the general design of MIDP applications will be discussed. Then, the high-level user interface API will be explained. Finally, the low-level user interface API for free graphics and games will be described.


All applications for the MID Profile must be derived from a special class, MIDlet. The MIDlet class manages the life cycle of the application. It is located in the package javax. microedition.midlet.

MIDlets can be compared to J2SE applets, except that their state is more independent from the display state. A MIDlet can exist in four different states: loaded, active, paused, and destroyed. Figure 3.1 gives an overview of the MIDlet lifecycle. When a MIDlet is loaded into the device and the constructor is called, it is in the loaded state. This can happen at any time before the program manager starts the application by calling the startApp() method. After startApp() is called, the MIDlet is in the active state until the program manager calls pauseApp() or destroyApp(); pauseApp() pauses the MIDlet, and desroyApp() terminates the MIDlet. All state change callback methods should terminate quickly, because the state is not changed completely before the method returns.

Figure 3.1 The life cycle of a MIDlet.

In the pauseApp() method, applications should stop animations and release resources that are not needed while the application is paused. This behavior avoids resource conflicts with the application running in the foreground and unnecessary battery consumption. The destroyApp() method provides an unconditional parameter; if it is set to false, the MIDlet is allowed to refuse its termination by throwing a MIDletStateChangeException. MIDlets can request to resume activity by calling resumeRequest(). If a MIDlet decides to go to the paused state, it should notify the application manager by calling notifyPaused(). In order to terminate, a MIDlet can call notifyDestroyed(). Note that System.exit() is not supported in MIDP and will throw an exception instead of terminating the application.

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