This is an old problem. But sometimes it cannot be solved. After finding many ways on the Internet, we finally solved the problem.
1. First of all, the default eclipse4.2 version cannot be automatically prompted, and the display is flickering (at least my ubuntu12.04 is like this), so that you do not know what is going on. Then, I felt that the shortcut key may have the same two functions as the shortcut key. Go to 'window --> General --> keys 'and find that two function keys are 'alt +/': work completion and content assist. if you are in a habit, modify the work completion shortcut key.
2. Then, add ". abcdefghijklmnopqrstuvwxyz" to the regular 'window --> JAVA --> edit --> content assist' selected in auto activation triggers for Java.
3. Check 'window --> JAVA --> edit --> content assist --> advanced 'and tick all the options in the default proposal kinds (except word proposals ).