cannot connect to X11 window server

[Nezar]

hello,
I'm running The Ubuntu server without a GUI using the Oracle VM virtual box and i'm trying to open a java based graphic window but i get the following error:

Exception in thread "main" java.lang.InternalError: Can't connect to X11 window
server using 'localhost:0" as the value of the DISPLAY variable. 

I don't know what DISPLAY variable i should set.
I have installed the X11 server.
help me please.

[ne555]

https://wiki.archlinux.org/index.php/xorg#Running

[Nezar]

thanks. solved :)