Actually, the OpenJDK is available via the extras repository:
http://maemo.org/downloads/product/Maemo5/icedtea6/

Don't be confused by the name icedtea. This is the full OpenJDK. Admittedly, this version is quite old but it gives you all the things you need to develop and run Java applications.

On the other hand, the main intent of packaging the OpenJDK was to allow running exisiting Java applications, not to develop new ones. I wouldn't actually suggest to start with Java for Maemo development but rather advice to go with a language that also supports the mobile UI or at least offers bindings for these.


Personally, for the beginning I would suggest Python + the PySide Qt bindings. This should give a pretty flexible programming environment. Also you will be set for easily porting your work to, e.g., Harmattan.
Edit: And you don't need to compile anything which rapidly speeds up the develop and test cycle.

When talking about Qt, I would strongly recommend to implement the UI in QML. First of all, QML is also interpreted and can be easily edited, even in place, on the device itself. Secondly, by choosing QML you also have the easiest migration path to other Maemo or Maemo-based platforms like Harmattan or Nemo/Mer.

Edit2: There is also a nice guide and introduction to Pyside and QML written by thp: http://thpmaemo.blogspot.de/2011/01/...-on-meego.html
While this guide was actually written for MeeGo (even for the Netbook UX as far as I see) very much should still apply also for Fremantle. This should give you a pretty quick start.