Tips to use Java ME for mobile apps

Modern mobile devices come in more flavors than maybe spiders on earth and are increasingly creating webs of their own by connecting with each other and to the Internet through technologies such as WLAN, GPRS/EDGE and 3G/UMTS. This diversity is a huge challenge for application developers, who have to modify or port their applications to make them run on various mobile platforms.

In the desktop world, Java and various other languages have been able to make applications independent of the platform, but what about mobile platforms? The same technologies are also available in the embedded world of mobile devices, but with modifications, as mobile devices have some inherent limitations:

1. They have slower processors than their present day desktop cousins, however, compared to PCs of the mid-90s, they are still way ahead.

2. They have limited amount of primary memory (RAM) and secondary storage (Flash), but are lucky they dont need to deal with memory hogs such as Windows Vista.

Java Platform, Micro Edition

One of the leading technologies in this area, which is widely supported, is the Java Platform--Micro Edition (Java ME). Formerly, this was known as J2ME. The advantage of using this platform is the same as that associated with the standard Java technology – platform independent applications. The programming language remains Java, but there are limitations and modifications. The Java ME platform is layered and modularized as follows (see Figure 1):

1. The Virtual Machine (VM): This is the core of the Java runtime environment and is responsible for executing the Java code, independent of the underlying OS and processor. For the Micro Edition, the VM is modified to run in resource constraint devices and is often called the KVM or kilo-virtual-machine due to its smaller size. For less resource constrained devices such as Digital TVs or Smartphones, the standard Java Virtual Machine (JVM) is used.

2. The Configuration: This is the most basic set of libraries for broad range of devices. There are two configurations defined by the Java ME platform:

a. Connected Limited Device Configuration (CLDC): This is a set of libraries for the most resource constrained devices such as low and mid range mobile phones.

b. Connected Device Configuration (CDC): This is a set of libraries for devices with more processing capabilities such as high end mobile phones (smartphones) and digital TVs.

3. The Profile: The profile is a set of APIs specific to devices such as a Mobile Phone or a Digital TV. For example, the common profile for the mobile device is called the Mobile Information Device Profile (MIDP) and runs on CLDC. On the other hand, the profiles available for the CDC are the Foundation Profile, the Personal Basis Profile and the Personal Profile.

4. Optional Packages: These are additional libraries for richer user experience such as multimedia and 3D.

The layered architecture allows the device manufacturers an easy way to configure the capabilities provided by Java ME on their mobile platforms.

Developing a Java ME application

In this article we will go through a typical installation of the Micro Edition tools and develop a sample application which can be tested on PC and deployed on a mobile device. We will use the Windows XP/Vista PC as our development platform and write an application using the J2ME tools with CLDC and MIDP. These applications are called MIDlets.

Requirements and Downloads

1. Minimum Hardware Requirements:

a. 100 MB HDD space, 128 MB RAM

b. 800 MHz Pentium III CPU

2. Software Requirements:

a. Microsoft Windows XP or Microsoft Windows Vista (I tested on Vista Ultimate)

b. Java 2 Platform, Standard Edition (Java SE SDK), version 1.5.0l URL: http://java.sun.com/ javase/downloads/ index_jdk5.jsp

l Download the item named “JDK 5.0 Update 16”

c. Sun Java Wireless Toolkit 2.5.2 for CLDC URL: http://java.sun.com/ products/ sjwtoolkit/ download.html

l Scroll down for the 'Download' button

Installation

Install the JDK first (the sequence is important) followed by the Wireless Toolkit and you are all set to create your first MIDlet. Your installation (by default) would have created the folders in C:\WTK2.5.2 as shown in Figure 2.

The Toolkit

You can start the toolkit from

Windows Start Button

Programs-> Sun Java Wireless Toolkit 2.5.2 for CLDC->Wireless Toolkit 2.5.2

The WTK Window will come up as shown in Figure 3

Creating a New Project for Hello World MIDlet

Click on the 'New Project' button. Now, enter the following details and click on 'Create Project'.

Project Name: HelloWorldApp

MIDlet Class Name: com.j2me.part1. HelloWorldApp

This will bring up the API Selection Window shown in Figure 5. Make the following changes and click OK:

Target Platform: JTWI

Configurations: CLDC 1.1

Optional: None (deselect Mobile Media API)

(Only the relevant part of the window is shown here).

The toolkit will create the project and show messages as in Figure 6. This shows the paths where you have to create the source files.

Your source file path will be different from the one shown in Figure 6.

You have to use the path as shown in your messages. You can mark and select the first path shown (source file path) and copy it using Ctrl+C. This will be required in the next step.

Creating the source file for Hello World Application

Open Notepad and enter the following source code:

package com.j2me.part1;

import javax.microedition.lcdui.Alert;

import javax.microedition.lcdui.Display;

import javax.microedition.midlet.MIDlet;

public class HelloWorldApp extends MIDlet {

Alert myAlert;

public HelloWorldApp() {

myAlert = new Alert("Alert!");

myAlert.setString("Hello World");

}

public void startApp() {

Display.getDisplay(this).setCurrent(myAlert);

}

public void pauseApp() {

}

public void destroyApp(boolean unconditional) {

}

}

Save this file as HelloWorldApp.java in the fully qualified source path, which is the path you copied in the previous step plus 'com\j2me\part1'. In my case the path isD:\Users\Sudipto Chanda\j2mewtk\ 2.5.2\apps\ HelloWorldApp\src\com\ j2me\part1

Important: Make sure that you select 'All Files', while saving from Notepad, otherwise, Notepad may append a '.txt' extension. It is a good idea to verify the file name to be HelloWorldApp.java.

Running the Hello World Application

Now come back to the WTK window and from the Menu select Project->Run. If there are no errors, then this will Compile, Preverify, Package, and run the HelloWorldApp in the Emulator. The Emulator Window is shown in Figure 7.

To run the Hello World Application, you have to click on the button below the 'Launch' item (marked with a red box).

This will execute the application and you will see the message “Hello World”.

Figure 5: Messages

Deploying the Hello World App on a Mobile Device

Before continuing, please refer to the Manual of your Mobile Device for details of support of J2ME, method for connecting your device to your PC to copy files and also check the method for installing J2ME Applications on your device.

You can create the package for deploying to the Mobile Device by selecting from the Menu Project->Package->Create Package.

This will create the JAR file and the JAD file in a bin folder (at the same level as the src folder). In my case this is

D:\Users\Sudipto Chanda\j2mewtk\2.5.2\apps\HelloWorldApp\bin

Now copy the files (HelloWorldApp.jar and HelloWorldApp.jad) to your mobile device using the USB/IrDA/WLAN or whichever connectivity method given for your mobile. On the device, locate the folder where you copied the files and click on the JAD file to install the Hello World MIDlet on your Mobile. Now you can run the MIDlet on your Mobile by clicking on it or as per the procedure given in your manual.

Conclusion

As you can see, it is really easy to create MIDlets and test them on the PC and deploy on mobile devices. The applications will run on most modern mobile phones. The Java ME platform also provides multimedia and 3D Graphics support for rich applications likes Games. There are many demo applications you can play around with to get the hang of the tools. You'll find them in C:\WTK2.5.2\apps.