On some intelliJ installation the java sourcecode can not be found. You can add this by adding the sourcepath. Right click the project. Project settings
On the community edition I could not find the Project Settings. So: Right click the project. Select "Open Module Settings" in the newly opened window click: "SKD's" under Project Settings.
On archlinux you can install the java source files with:
pacman -S openjdk8-src