Sounds like what I have been planning for the last year or so.
I too like the idea of using Java, one because I am familiar with it and two because of the flexibility.
Hardware wise this is what I am planning on using;
Pico-ITX EPIA PX10000G (I like the size & 7.1 audio out pin headers...)
PW-200 dc-dc PS (from mp3car.com)
SATA HD probably 80-120GB
1GB+ Compact Flash (for OS)
1GB DDR2 DODIMM
Panasonic UJ-85J-B slit loading DVD RW
Lilliput 619GL-70P touchscreen
Mechatroniks dual-din in-dash chassis for the Lilliput 619GL (also available on mp3car.com)
Micro Shutdown Controller
Class-T 100x4ch amp
(like the AMP9 from 41Hz but I may design a custom board with only DC, size would then be about 50x100 mm (2"x4"))
probably more but this is a good start.
Custom Linux distro (Ubuntu based, possibly a modified LinuxICE with java)
Custom Java frontend using the following projects;
-Sun's JRE 1.5 or 1.6
-javazoom (Customized jlGui music player)
-OpenMap (Components for GPS Nav using shape files)
-Custom Skin and/or LF manager
-OBDII Elmscan Java library (not sure if one exists)
-I would like to be able to load .jar files on the fly to add more modules.
-A lot of the other basic functions would use the standard java libraries; javax.media, javax.speech, javax.usb, etc...
here is a link to another thread that is a bit old but on the same topic:
Also, here is site that is working on something similar;
Let me know if you would be interested in collaborating on a project.