Java 2 Micro Edition - Conclusion

The Good

J2ME is small, actually very small; writing programs for devices with very limited memory capabilities has been made into a reality by J2ME. J2ME allows the programmer to customize his program to the utmost possibility, right from the configuration he wants to work with to the very profile he is using.

Further, the future versions of the KVM will have the capabilities of integrating the present programs with further memory hungry operations.

The Bad

The very strength of J2ME is its biggest weakness. In order to make the KVM small, a lot of compromises had to be made in the Java language itself. Writing complex programs for J2ME requires higher skill and technique than writing one for computers and servers.

Further because of the configurations and profiles, the basic fundamental principle of portability had to be compromised. That is, a program written for a particular profile is portable with other devices following the same profile.

Future Of J2ME

J2ME is still in its blooming stages, and definitely the platform will be evolving. Many more configurations and profiles will be introduced. Further several technologies, such as Jini (network architecture for distributed systems), will arise with the help of J2ME, improving the capabilities of the mobile devices. Needless to say there will be many more devices following the J2ME technology in future.

Java 2 Micro Edition - Case Study

Extending one’s knowledge of Java to J2ME is very easy. Every program in J2ME is called a midlet and extends from the midlet class. Here is a sample program in NetBeans using the CLDC configuration and the MIDP 2.0 profile.

As evident from the screen-shots, the constructors in both the programs are very much similar. A few differences exist in the both such as the import statements in J2ME where two packages - midlet and lcdui are imported. Further an alert object is constructed to display the message.

Three important functions are present in every J2ME program. They are -

startApp	Called whenever the application is started
pauseApp	Called whenever the application is paused
destroyApp	Called whenever the application is terminated

Java 2 Micro Edition - Profiles

A Profile primarily defines the type of device supported. Profiles are built on top of configurations, since they are specific to the memory available. They add an additional layer on the top of the configuration layer providing APIs for a specific class of devices.

Profiles are specific to the configurations selected. For different configurations, different profiles are available.

CDC Configuration

For CDC configuration, a foundation profile is available, which is primarily a skeleton profile over which we can create our own profile.

CLDC Configuration - MIDP, KJava And Doja

For the CLDC configuration, predefined profiles such as MIDP, KJava and Doja are available. The KJava profile is one of the popular profiles for the Palm OS. It consists of Sun specific APIs. Further it's not a standard J2ME profile.

The Doja profile is another popular profile, though not widely used in our mobiles; it is a rapidly growing profile, because of its simple user interface and as it easy to understand. This profile is popularly used in Japan for their local cellular phone companies. However its influence on the entire mobile network is still a long way ahead.

The most popular profile used these days in the Mobile Information Device Profile (MIDP). It is mainly based on the CLDC configuration and is widely used in cellular phones and pagers. Currently there are two versions of MIDP that are available for programming - MIDP 1.0 and MIDP 2.0. Mainly the differences between these two profiles are the fact that the MIDP 2.0 has extended features when compared to MIDP 1.0 such as audio and 2D gaming. On the other hand MIDP 1.0 has greater portability when compared to MIDP 2.0. Many of the cellular phone brands such as Nokia, LG, Samsung, and Motorola use these profiles.

Java 2 Micro Edition - Configurations

Configurations define the basic run-time environment as a set of core classes. They add the classes required for the program to be compatible with the virtual machine.

J2ME has two main configurations – Connected Limited Device Configuration (CLDC) and Connected Device Configuration (CDC). These configurations actually specify the main type of the device, which defines how much memory is available for the operations to be performed.

CLDC requires less memory whereas CDC requires at least 2MB of memory to perform its computations. The CLDC configuration consists of classes and a set of libraries more specific to the mobile devices. The CDC configuration is basically a stripped down version of J2SE with the CLDC classes added to this.

The CLDC configuration is supported by KVM and the CDC configuration is supported by CVM. Configurations decide whether optional features such as multi dimensional arrays, threads, JNI and others have to be included or not.

CLDC configuration is used when the memory available is low, such as the case of PDAs and cell phones. CDC configuration is used when the memory available is more than 2MB but less than that available on our computers, such as the case of set-top boxes.

Java 2 Micro Edition - Virtual Machines

A virtual machine is the immediate layer overlying the operating system and is mainly responsible for running any program written in Java. This is the main reason that codes written in Java are highly portable. The virtual machine interprets the Java byte-code and converts into native system calls. Further as every program runs in the confinement of the virtual machine, no penetration is possible into the operation system, reducing the risk of viruses.

J2ME offers two different virtual machines, namely CVM (for higher end mobile devices) and KVM (for lower end mobile devices). CVM is used for devices which have higher memory capabilities and which are closer to computers whereas KVM is used for devices which have lower memory capabilities, such as mobile phones. KVM and CVM are primarily nothing but the subset of JVM. They can be thought as just shrunken versions of the JVM and are more specific to J2ME.

Java 2 Micro Edition - An Overview

Mobile devices have become a part and parcel of almost everybody’s life. By mobile devices, we don’t mean just our cell phones, but also devices such as pagers, personal digital assistants (PDA’s) and small retail terminals (devices such as set-top boxes).

To use these devices more effectively and more efficiently, programming was introduced into these devices. Every mobile device has a basic operating system which co-ordinates the various tasks to be performed by the device itself. Examples of these operating systems are the Symbian operating system, Windows operating system, Linux operating systems and many others. Many of the primitive models had a PROM built in them, instead of an operating system, so as to start the mobile device without any hassles.

Different file formats have been developed for these mobile devices. Many of these are basic extensions of a high level language so as to perform properly even at the availability of less memory. Examples of this are the *.sis and *.sisx file formats which are basic extensions of C, *.jar and *.jad which are basic extensions of the Java 2 Standard Edition.

Bringing J2ME Into The Picture

One of the popular languages used in the development of these mobile programs is Java, more specifically the Micro Edition of Java called J2ME. As known, Java is divided into three segments J2SE, J2EE and J2ME, while the former two are used for servers and higher end computers, J2ME is used for mobile devices.

At first J2ME was intended as a platform for game development, but now-a-days J2ME is used for developing almost every kind of application. Following the fundamental principle of Java, J2ME also follows the principle of "Write Once Run Anywhere (WORA)" to a large extent. Also the security of these programs is high as it runs in the confinement of the virtual machine itself. Further the possible users of this technology are estimated to be very high, making it popular as a whole.

J2ME Architecture

J2ME runs mainly on virtual machines, which actually runs the Java byte code on the device. Before the implementation of any byte code, every program written in J2ME should specify its configuration and its profile. They are layers immediately above the virtual machine layer, as shown in the figure. These layers form the very bone structure of the J2ME program and selecting these layers properly in the key of writing a suitable program for a particular device. The first and foremost layer is the Operating System layer which co-ordinates the tasks of the entire device itself.