Java's back: after spending seven years wandering around the wilderness of Web browsers (where it gave us applets) and servers (where it's had a little more success, with servlets and JavaServer Pages entrenched as a serious platform for dynamic content), it's returned to its roots as a language for programming consumer electronic devices.

J2ME, Java 2 Micro Edition, is a cut-down version of Java, suitable for running on restricted hardware such as mobile phones, that may have less storage or processing power than regular PCs. It allows small applications, called "midlets," to be stored and run on the phone. This lets developers build more sophisticated services than browser-based platforms like WAP will allow.

The key problem J2ME addresses is the disparity of mobile phone hardware and operating systems. Rather than forcing developers to produce a version of their applications for each individual phone handset, J2ME lets them write applications once and deploy them to any handset that supports Java, saving a great deal of development effort in the process. In practice, this "write once, run anywhere" approach isn't always practical, but Java takes at least some of the pain out of the process.

As with many innovations in mobile telephony, Japan has gotten there first: NTT DoCoMo launched their first Java phones (branded "i-appli") in January 2001, and has seen excellent commercial results from these devices. i-appli handset owners typically spend twice as much per month on data services as regular i-mode users.

Mobile operators across the world are understandably keen to emulate this success, which explains the enthusiasm with which they are launching Java handsets onto their networks. In many ways it's a logical extension of the logos and ringtones market which has exploded in Europe over the last few years. Personalization has always been one of the most popular mobile data services, and mobile subscribers in Japan and elsewhere are using Java applications to personalize their handsets.

In the UK, we've recently seen mobile operator O2 launch their Java gaming service, offering both classic arcade games (Asteroids, Pong, etc.) and new content (for example, the "Men in Black 2" game from Digital Bridges) for £1.50 ($2.40 USD) per game per month. New operator Hutchison, who is due to launch their 3G service in the UK in the next few months, similarly sees Java content as an important part of their service offering. They recently ran a competition to create compelling applications for their first wave of subscribers.

Overwhelmingly, the emphasis so far has been on gaming, but we expect to see a range of other applications once developers get a feel for the potential (and limitations) of J2ME.

These limitations are significant; Java doesn't yet offer the same performance as native applications, and the difficulties of designing mass-market services for mobile phones are many. A restrictive interface (tiny screens and keyboards), the need for users to be able to move between applications quickly (say, to answer an incoming call during a game session), and networks that offer low bandwidth in comparison to desktop PCs, all create problems.

So, what's next for Java? A new version of the J2ME spec, dubbed "MIDP 2.0," is in preparation, and will allow for more sophisticated applications - use of sound, and full-screen graphics, both of which are currently impossible without using proprietary APIs provided by vendors like Nokia and Motorola. I'd also expect to see more imagination from developers. Much J2ME content at the moment consists of conversions of classic arcade games which, while familiar to many consumers, are hardly groundbreaking stuff. And, given that Java in phones opens up the development of mobile applications to a massive worldwide developer community, there should be no shortage of innovation over the next few years.