Oracle 12c Installation Error: "Exception ... Could not initialize class sun.awt.X11.XToolkit"
Solution 1:
Check if some JDK is installed in your system:
$ java -version
Installing JDK 7 solved my problem:
$ sudo apt-get install openjdk-7-jre-headless
If the problem persists, log as root, then execute:
$ xhost +
Switch back to another user and run installation again.
Solution 2:
I had this as well. As root, I executed xhost + first and then it worked.