IntelliJ IDEA “cannot resolve symbol” and “cannot resolve method”

Question

What are common causes for IntelliJ IDEA not being able to resolve built-in JVM types and methods? For example, when I mouse over String the tooltip says \"Cann

Answer 1

For me, IntelliJ could autocomplete packages, but never seemed to admit there were actual classes at any level of the hierarchy. Neither re-choosing the SDK nor re-creating the project seemed to fix it.

What did fix it was to delete the per-user IDEA directory ( in my case ~/.IntelliJIdea2017.1/) which meant losing all my other customizations... But at least it made the issue go away.