Eclipse/Java code completion not working

Try restoring the default options in 'Windows > Preferences > Java > Editor > Content Assist > Advanced'

An example of the kind of data you see in this preference screen, however not necessarily what you currently have.

(From Vadim in this blog post " Content Assist Duplicates in Eclipse (Mylyn)":
if have duplicate Mylyn entries, uncheck the duplicate entries that do not contain "(Mylyn)" in their name)

The Eclipse help page defines the default list to restore:

Select the proposal kinds contained in the 'default' content assist list:

• Other Java Proposals,
• SWT Template Proposals,
• Template Proposals,
• Type Proposals

I'm adding an answer here in case someone else finds this on Google. Same symptoms; different problem. For me, the type caches had become corrupt.

From http://mschrag.blogspot.co.nz/2009/01/open-type-cant-find-your-class.html

• Quit Eclipse
• Go to workspace/.metadata/.plugins/org.eclipse.jdt.core
• Remove *.index and savedIndexNames.txt
• Restart Eclipse and search Ctrl+T for the offending type. The indexes will be rebuilt.

In case someone comes here and want to activate the autocomplete function, go to

Preferences -> Java -> Editor -> Content Assist.

Then in the Auto Activation section fill in Auto activation triggers for Java:

abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ._