Eclipse could not delete error

Question

I\'m working on a project with Eclipse and by now everything was fine, but last time I\'ve tried building it, it returned the error,

\"The project was

Answer 1

Just hit myself with same problem: could not clean / rebuild Eclipse project, because my classes output folder could not be deleted.

I closed Eclipse and tried to delete the folder. Windows complained it is used by other application.

Being on Windows, I run resmon.exe from Windows / Search bar. Then going in CPU tab / Associated Handles I search for that folder path and I found handle for it from OUTLOOK. I sent a mail with a file from that path and it seems OUTLOOK kept it. After deleting the handle entry, everything worked again.