I can get an empty box to appear by pressing Ctrl Space, but content assist won't pop up at all otherwise. I've read lots of answers here on this topic but none have helped.
I'm using...
Ubuntu 12.10 Eclipse Version: 3.8.0 Build id: I20120502-2000 Java openjdk-7-jre 7u15-2.3.7-0ubuntu1~12.10 Execution Environment JavaSE-1.7 (java-6-openjdk)
I've checked the Preferences settings and they seem reasonable. I've checked the project build settings and they look ok. I don't have any compile errors (and only a few lines of code.)
Any suggestions as to what my next move is?
Windows → Preference → General → Keys → Restore Default
Another thing you can check:
Windows → Preference → Java → Editor → Content Assist → Advanced → Java Proposals should be checked.
This should fix the problem.
Note that you don't have to restore the default, make sure that CTRL + SPACE is not overridden by another operation.
As mentioned on the comments, sometime creating a new project workspace fixes this problem.. for some reason.