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ARTICLE ON J2ME( Java 2 Micro Edition)
Posted Date: 18 Dec 2007 Resource Type: Articles/Knowledge Sharing Category: Computer & Technology
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Posted By: BHARATH Member Level: Gold
Rating:  Points: 5
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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 A.P.I's (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.
Personal Java and Embedded Java
J2ME is not the first attempt at adapting Java for constrained environments. Two other technologies, Personal Java and Embedded Java, made it possible to run Java 1.1.x applications on high-end devices.
Personal Java uses the basic Java 1.1 runtime classes and throws in a few features from Java 2. Support for some of the runtime classes is optional, but a Personal Java implementation still requires a couple of of megabytes of memory and a fast processor to run, so it's not a practical solution for truly constrained devices like cellphones and many personal digital assistants.
Embedded Java makes every behavior of both the Java VM and the runtime classes optional - the implementor can choose exactly which classes and methods are required. There is one limitation, however: The Java runtime environment can only be used by the implementor and cannot be exposed to third parties. In other words, you can use it to write Java code that runs inside a device, usually as part of the software to control the device, but no one else can write applications for the device. This is done to preserve the "write once, run anywhere" nature of Java, since an Embedded Java environment can do away with fundamental things like runtime class verification and change the public interfaces of core classes. Embedded Java is really a way to build a "private" Java runtime environment.
Both Personal Java and Embedded Java are being phased out. There is a migration path from Personal Java to J2ME, as we'll see later in this series, though the current version of Personal Java continues to be supported. Embedded Java is no longer supported because J2ME defines suitable small-footprint runtime environments.
What's Next
In the next article, we'll examine the core concepts of J2ME - configurations, profiles, and optional packages - and some other terms like KVM and CVM. You can't write a J2ME application without knowing which configuration and profile you're targeting, or knowing which optional packages are required, so it's important to understand the terminology before you attempt your first application.
If you can't wait to get started with J2ME, download Sun's J2ME Wireless Toolkit and read through some of the extensive J2ME documentation on the Sun Web site.
Next: J2ME Core Concepts
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