I have shifted by Dev Env recently to Eclipse Helios on Windows 7. After that the Ctrl + Space Content Assist feature is not working. I found out that that key option is utilize
Configure Eclipse’s content assist, go “Preferences>Java>Editor>Content Assist>Advanced“. Make sure “Other Java Proposals” is ticked.
http://www.mkyong.com/java/content-assist-ctrl-space-is-not-working-eclipse/