Who hasn’t ever happened that Eclipse magically stops autocompleting Java source codes? Without having touched anything and without any apparent cause, you start Eclipse quietly and where yesterday it completed a normal .java, today it does not. The solution is simpler than you think:
Window -> Preferences -> Java -> Editor -> Content Assist -> Advanced
When you get here you probably have unchecked all the options. Then do a “Restore Defaults” and you will see that several options are checked. Now it should autocomplete.
The credits for this contribution go to Mr. Rubén Quintero.