IntelliJ IDEA caches a great number of files, therefore the system cache may one day become overloaded. In certain situations the caches will never be needed again, for example, if you work with frequent short-term projects. Also, the only way to solve some conflicts is to clean out the cache.
Cleaning out the system caches
To clean out the system caches:
On the main menu, choose File | Invalidate Caches/Restart. The Invalidate Caches message appears informing you that the caches will be invalidated and rebuilt on the next start. Use buttons in the dialog to invalidate caches, restart IntelliJ IDEA or both.
It is important to note the following:
The files are not actually deleted until IntelliJ IDEA restarts.
Opening and closing a project does not result in deleting any files.