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Fallacies of J2ME
This list is meant to clear some misunderstandings you might
have about Java 2 Micro Edition (J2ME). A fallacy, in
case you're wondering, is a false notion. Fallacies abound in
this world, this is my attempt to dispel a few of them.
The list of fallacies is:
- Java programs "will just work" on J2ME
- KVM == J2ME
- KVM == CLDC and CVM == CDC
- J2ME replaces WAP
- If you're learning Java, start with J2ME
If you think I need to add to the list, please contact me
using the link at the bottom of this page.
Fallacy 1: Java programs "will just work" on J2ME
Realistically, don't expect to be able to run your existing
Java applications unchanged on any J2ME platform. Apart from
some technical issues (such as the CLDC 1.0 not supporting floating
point), there will usually be two barriers: missing classes
and resource problems. Not only do J2ME platforms subset the J2SE core
classes, but they add some of their own unique classes
as well. But even if you write code that only uses classes common to
all the platforms you're interested in,
chances are that you'll
still have some work to do to optimize your application so that
is uses less memory and has acceptable performance. Good design
and modular programming will help a lot in this area.
This isn't to say that you can't reuse any code, just don't expect
to reuse it all.
Return to the list of fallacies.
Fallacy 2: KVM == J2ME
Many people use KVM and J2ME synonomously. They're different
things. The KVM is a Java virtual machine. J2ME is a set of specifications.
Not every J2ME platform uses the KVM. The CDC, for example, uses the
"classic" VM, not the KVM.
Return to the list of fallacies.
Fallacy 3: KVM == CLDC and CVM == CDC
In a similar vein, others use KVM and CLDC synonomously,
or the CVM and CDC synonomously.
Again, they're different. The KVM and CVM are Java virtual machines, the
CLDC and CDC are J2ME configurations.
The CLDC has specific requirements for
its underlying VM, and the KVM can be configured to be that VM. The
KVM in fact forms part of the reference implementation of the CLDC.
However, there's nothing preventing a specific CLDC implementation
from substituting its own VM not based on the KVM source code.
IBM's J9 VM, used in recent Palm devices, is one such example.
So is Research In Motion's VM for the BlackBerry handhelds.
Similarly, the CDC has specific requirements for its underlying
VM, and the CVM can be configured to be that VM. And yes, it too
forms part of the reference implementation of the CDC.
But CDC implementations can use their own VM, just like IBM uses
its J9 VM in its CDC-based J2ME implementations.
Don't get all worked up about which VM is used. As long as it
passes the compatibility tests and meets the appropriate specifications,
it generally doesn't matter to you, the developer. The only time it
matters is when performance is an issue and you'd like to switch to
a faster VM. That said, remember that most handheld devices are
closed platforms and only the platform vendor can actually
install a different VM.
Return to the list of fallacies.
Fallacy 4: J2ME replaces WAP
WAP is a set of technologies for wireless communication. It includes
a markup language called WML for writing browser-based applications.
You can find out more about these technologies at
WirelessDevNet.com.
WAP and J2ME are different things, and each has its place when it
comes to application creation and deployment. WAP (or really,
WML and WMLScript) lets you build browser-based, thin-client
applications optimized for wireless delivery. J2ME lets you
build "thicker" client applications with more sophisticated logic
and the ability to run in an "offline" mode. It's really like
trying to say: "which is better, a page of HTML or a Java applet?"
Both are different, and both have their uses. Hopefully, though,
J2ME won't suffer the same problems that caused applets to fall
out of grace so quickly. This is why the standardization process,
industry support, and compatible implementations are so important
for the success of J2ME.
Return to the list of fallacies.
Fallacy 5: If you're learning Java, start with J2ME
If you're new to Java, skip over J2ME and go straight to
J2SE (Java 2 Standard Edition). You'll find lots of material
there about Java programming, you'll have lots of working
examples to play with, and you won't have to worry about the
sometimes arcane configuration issues and crippled tools
you run into with the handheld devices that are J2ME's
primary target. Don't start with J2ME until you're comfortable
with J2SE on the desktop.
Return to the list of fallacies.
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