MIDlet

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A MIDlet, a Java program for embedded devices, is an application written for MIDP (Mobile Information Device Profile). MIDlet applications are subclasses of the javax.microedition.midlet.MIDlet class that is defined by MIDP. A MIDlet has to fulfill the following requirements in order to run on a mobile phone:

• The main class is a subclass of javax.microedition.midlet.MIDlet .


• If an application consists of multiple classes, a .jar file is used to group all the classes together so that the application is easy to distribute and deploy. The jar tool could be used.


• The .jar file needs to be pre-verified by using a preverifier. It preprocesses the program for use by the K virtual machine (KVM) without changing the name of a class.

^†Note that the MIDlet development environment KToolbar of J2ME Wireless Toolkit 2.3 has simplified the above steps.

The following list gives hints for writing MIDlets:

• Heap sizes can be very small, around 200KB.


• Programs can not expect more than 20KB persistent storage.


• A device can have very different screen dimensions, orientations, and color depths. In other words, do not use absolute position.


• Not all devices have keypads or pointers. One should be able to use the program with both. Key layouts may vary too, so use Game Actions for up-down-left-right-action style events.


• Do not white list devices. If the program has not been tested on a specific device, assume it works. Never attempt to block the user from trying it.


• Use an obfuscator such as Proguard to compress the MIDlet size.