If you would like to use autocomplete all the time without having to worry about hitting Ctrl + Spacebar or your own keyboard shortcut, you can make the following adjustment in the Eclipse preferences to trigger autocomplete simply by typing several different characters:
Eclipse > Preferences > Java > Editor > Content Assist
Auto Activation > Auto activation triggers for Java
Enter all the characters you want to trigger autocomplete, such as the following:
abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ._
Now any time that you type any of these characters, Eclipse will trigger autocomplete suggestions based on the context.