Beginning J2ME: Building MIDlets

Get a taste of MIDlet development

By Jonathan Knudsen and Sing Li, JavaWorld.com, 05/02/05

MIDP (Mobile Information Device Profile) applications are piquantly called MIDlets, a continuation of the naming theme begun by applets and servlets. Writing MIDlets is relatively easy for a moderately experienced Java programmer. After all, the programming language is still Java. Furthermore, many of the fundamental APIs from java.lang and java.io are basically the same in the MIDP as they are in J2SE. Learning the new APIs (in the javax.microedition hierarchy) is not terribly difficult.

The actual development process, however, is a little more complicated for MIDlets than it is for J2SE applications. Beyond a basic compile-and-run cycle, MIDlets require some additional tweaking and packaging. The complete build cycle goes like this: edit, source code, compile, preverify, package, test or deploy.

To show how things work, and to give you a taste of MIDlet development, this article is dedicated to building and running a simple MIDlet. In this article, you'll get a feel for the big picture of MIDlet development.

Tooling up

MIDlets are developed on regular desktop computers, although the MIDlet itself is designed to run on a small device. To develop MIDlets, you'll need some kind of development kit, either from Sun Microsystems or another vendor. Remember, MIDP is only a specification; vendors are free to develop their own implementations.

The world is full of MIDlet development tools if you know where to look. Furthermore, many of these tools are freely available.

The bare bones set of tools is Sun's MIDP reference implementation. This includes the preverify tool (more on this later), a MIDP device emulator, source code, and documentation. You can download the MIDP reference implementation from Sun. However, we don't recommend using the reference implementation unless you really enjoy being in the middle of the gritty details of building and packaging MIDlets. (You should also investigate the reference implementation if you are interested in porting the MIDP runtime to a new device or platform.)

A much better tool for beginners is Sun's J2ME Wireless Toolkit. The J2ME Wireless Toolkit (or J2MEWTK, as it's affectionately known) includes a GUI tool that automates some of the tedious details of building and packaging MIDlets, providing a simple path from source code to running MIDlets. At the same time, the J2ME Wireless Toolkit is a relatively lightweight solution, almost a miniature IDE, not something that will choke your machine.

Larger IDEs are available in abundance from device manufacturers, wireless carriers, IDE vendors, and open source communities, including the following (links to tools are available in Resources):

Borland JBuilder X Mobile Edition
IBM WebSphere Studio Device Developer
Research In Motion BlackBerry Java Development Environment
Sun Java Studio Mobility
NetBeans IDE 4.x
Eclipse J2ME plug-in
Nokia Developer's Suite for J2ME

You can use whatever development kit you wish. We suggest you start with the J2ME Wireless Toolkit, which is easy to use and authoritative. We'll be using the J2ME Wireless Toolkit (version 2.2, or WTK 2.2). Other development environments generally use the J2ME Wireless Toolkit as a plug-in anyhow, so your experiences are likely to be similar no matter what tool you use. You'll notice details about the development environment most in this article, where we'll go into detail about the build tools and the emulators.

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