Building MIDlets - Beginning J2ME: From Novice to Professional - page 20

Tour of MIDP Features
Now that you have run your first MIDlet, take a moment to admire it. There are several salient
features, even in such a small example.
It's Java
First of all, Jargoneer is written in the Java language, the same language you'd use to code servlets,
Enterprise JavaBeans, or J2SE client applications. If you're already a J2SE developer, you should
be quite comfortable developing MIDlets.
Not only is the Java language familiar, but also many core APIs are very similar to J2SE.
Notice, for example, that multithreading in Jargoneer is just the same as it might be in any
other Java code. The MIDlet class Jargoneer implements java.lang . Runnable , and the technique
for kicking off a new thread is the same as it always was:
Thread t = new Thread(this);
t.start();
Significant parts of java.lang are essentially unchanged from J2SE, as are parts of java.io
and java.util . The code that reads the result from the server in lookUp() is familiar stream
handling, just like what you might see in J2SE.
MIDlet Life Cycle
Jargoneer also demonstrates the basic structure of MIDlets. Like all MIDlets, it extends
javax.microedition.midlet.MIDlet , the base class for all MIDP applications. Special software
on the device, called the Java Application Manager (JAM), Application Management Software
(AMS), or MIDlet management software, allows the user to control the process of installing,
running, and removing MIDlets. When a user chooses to run your MIDlet, it is the JAM that
creates an instance of the MIDlet class and runs methods on it.
The sequence of methods that will be called in your MIDlet subclass is defined by the MIDlet
life cycle. MIDlets, like applets and servlets, have a small set of well-defined states. The JAM will
call methods in the MIDlet to signify transitions from one state to another. You can see these
methods in Jargoneer — startApp() , pauseApp() , destroyApp() , and Jargoneer 's constructor
are all part of the MIDlet life cycle.
Generalized User Interface
Jargoneer 's user-interface code may take you by surprise. Later on, we'll spend several chapters
on user interface. For now, the important thing to notice is how Jargoneer 's user interface is
flexible enough to run on devices with different screen sizes and different input capabilities.
A big part of MIDP's appeal, after all, is the concept of writing one set of source code that runs
on multiple devices.
One example of MIDP's generalized user interface is the TextBox that is initially shown
when Jargoneer is launched. Figure 2-3 shows this TextBox .