IntelliJ Code Completion not working for new Java classes

Question

IntelliJ IDEA 13 has started exhibiting a very weird behavior in my local setup.

Namely, in any new Java class added to an existing project

Answer 1

I had a groovy project where somehow the jdk was not set as a dependency, so jdk classes would not autocomplete.

Fix:

1. project structure
2. modules
3. (your module)
4. tab Dependencies.
5. In the field Module SDK, add a jdk.