If you are using dwm and wanted to run Java programs, you will see, that's actually not going to happen... unless you know this solution. This I found in the Arch wiki:

Create this file: (you need sudo for that) /etc/profile.d/jre.sh

And put this in it: export _JAVA_AWT_WM_NONREPARENTING=1

I also put this: export AWT_TOOLKIT=MToolkit in ~/.xinitrc file, before the exec line, that starts your GUI.

You need to restart your system or source the jre.sh file.

I needed this for Jdownloader to work.

source: Arch wiki