There are few things worse for a programmer than the delay between altering some lines of code and seeing those changes working in game. Developing J2ME games is especially prone to build process delay, with many steps required between compilation and final jar file output. Add to this the complication of many different device emulators with their slow start up times, poor keyboard response and limited debugging support. Mobile Java programming can be sped up and even made a pleasurable experience with the help of some J2SE code which simulates the J2ME APIs. This article will lead programmers through the creation of a deceptively simple J2ME simulator which speeds up programming, improves debugging, facilitates rapid prototyping, aids the creation of device independent code and frees you from build processes that may take minutes to complete.
What's wrong with emulators?
I'm not going to list point by point all the things I personally dislike about using device manufacturer's emulators for J2ME development. I'm sure if you've spent any time developing J2ME applications you've felt there must be something better. Firstly, there are just so many SDKs and emulators available. Installing, maintaining, learning and integrating them into your development process is a time consuming and cumbersome task. Secondly, actually using them is all too often frustrating and slow, when they should be saving you from the even slower task of testing your game on a real device. They are slow to invoke, offer little or no feedback when errors occur, and debugging support is usually unreliable or functionally limited.
Figure 1. What you usually do.
The one area where a good emulator is essential is gauging the memory usage of your game. You can be pretty much assured that if your game runs in the Nokia 7210 MIDP SDK v1.0, it will work on a real 7210. However, you can't say the same for all emulators, and quite often you're left in the dark regarding memory errors.
If you are an experienced J2ME developer who knows how to write memory efficient, jar size and performance conscious code, I would suggest that your primary need for the majority of the programming work on a project is simply a J2ME environment that supports arbitrary screen sizes and all the relevant APIs your code utilises. You can get this in a tool that can be invoked in the blink of an eye both for normal execution and source level debugging, much more simply than you might think.
Figure 2. What you could do.
Compare the steps in figures 1 and 2, which you would potentially perform thousands of times during the programming of a J2ME game. I'd take the three steps over five any day.
Simulating J2ME with Desktop Java
Implementing the necessary J2ME APIs using pure Java turns out to be quite simple. The classes in CLDC 1.0 and 1.1 (java.io, java.lang and java.util) are all derived from their J2SE counterparts, so literally no wrapping or re-writing is required for the base standard library. The javax.microedition.midlet package is simple, well defined and easy to implement. The javax.microedition.lcdui package contains classes that (for the main part) have AWT counterparts, and those that don't, you probably never use anyway, so there's no need to implement them! You can probably do without the rest too, although javax.microedition.rms comes in handy, it's just file storage and again, easy to implement.
Figure 3. Simulator Class Overview
Figure 3 shows all but one class (MIDletStateChangeException) that needs to be implemented to create a functioning J2ME environment. The boxes in red are the main framework of the application and provide the entry point, window to display the midlet upon and keyboard input. The boxes in grey are the J2ME API classes that midlets use. These hook into each other and the simulator classes. Image, Graphics and Font are implemented using their AWT equivalent classes, shown in blue.
