Java continues to make inroads at all levels of computing. Most
recently, Java has returned to its roots and found a home in the small
devices for which it was originally designed. Java 2 Micro Edition
and its K Virtual Machine (KVM) allow you to write Java applications
for handheld devices, including ones that run Palm Computing's
Palm OS. You can download an early version of the KVM from
java.sun.com/products/kvm.
The KVM includes a reduced set of APIs from the Java 2 platform's
java.io, java.lang, java.net, and java.util packages. It also
includes a package called com.sun.kjava, which includes user interface
and event handling classes for writing applications. At the moment,
it only appears to support the Palm OS.
You can write a Palm application by subclassing com.sun.kjava.Spotlet,
which provides callbacks for handling events. The KVM manages the
event loop and forwards events to the Spotlet, which are handled by
methods such as keyDown() and penDown(). A Spotlet first needs to
register its event handlers with the register() method before it is
able to receive events. This is very different from the traditional
AWT/Swing event model. In fact, none of your AWT or Swing code will
port to the current incarnation of the KVM. You unregister your
event handlers with unregister().
You can compile your KVM apps for the Palm using your regular Java
compiler, setting the classpath to use the KVM classes. But you'll
need to use the utilities included with the KVM distribution in order
to convert your programs into a format you can download to your Palm.
These are the palm.database.MakePalmApp program and the
palm.database.MakePalmDB program. MakePalmApp will convert a Java
program into a Palm .prc file that will automatically load the KVM
to run the program when invoked on the Palm. MakePalmDB allows you to
add classes to the KVM's class database so that multiple KVM
applications may share classes.
The downloadable code demonstrates a simple HelloWorld Spotlet which displays some text and an exit button.
This 10-Minute Solution originally appeared in Ask the Java Pro.
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