I just upgraded from Ubuntu 9.10 Karmic Koala to 10.04 Lucid and my Oracle SQL Developer stopped working entirely. I didn't bother trying to look in to it as my SQL Developer was rather old anyway so I downloaded 126.96.36.199 and it didn't work either. Seeing the warnings on the SQL Developer page about supported JDKs I first checked what I had installed.
It appeared I had:
The first thing I remembered is that my ~/.sqldeveloper dir contained a jdk file pointing to the JDK. Once that was changed to "/usr/lib/jvm/java-1.6.0-openjdk" SQL Developer started and appeared to work but mysteriously some menu items were missing (most notably, compile on the right click on a package and the compile/debug buttons when editing a package). After a lot of digging about I discovered I needed to install openjdk-6-jdk because otherwise you are missing some jar files (possibly like tools.jar). However, after having got this far I realised I already had all I needed installed in /usr/lib/jvm/java-6-sun-188.8.131.52. Removing openjdk-6-jdk and changing my jdk file in ~/.sqldeveloper to /usr/lib/jvm/java-6-sun-184.108.40.206 also got me my menu items back.