In J2ME, the Java runtime environment is adapted for constrained devices - devices that have limitations on what they can do when compared to standard desktop or server computers. For low-end devices, the constraints are fairly obvious: extremely limited memory, small screen sizes, alternative input methods, and slow processors. High-end devices have few, if any, of these constraints, but they can still benefit from the optimized environments and new programming interfaces that J2ME defines.
Learning about J2ME is not hard: Once you understand the new terminology, it's mostly about learning new APIs (application programming interfaces) and learning how to work in constrained environments. (If you think writing an applet is challenging, wait until you try to fit an application into the 30K of memory some cellphones provide!) You can use most of the same tools you already use in your code development, and with careful coding you can develop libraries of classes that are portable to any device or computer with a Java virtual machine.
