Automatically opening completion window in Eclipse
Solution 1:
It is possible. You can simply write: .abcdefghijklmnopqrstuvwxyz in Auto activation triggers for Java field, under Preferences > Java > Editor > Content Assist.
While writing Java code, after the dot . the window opens automatically after a configurable amount of time (default is 200 ms.)
In general, what you can do is add an auto activation trigger, for example _, so when you want the windows to popup, you type and delete that character (althought I don't think that's better than CTRL + SPACE).