Applet error

Exception in thread "main" java.lang.InternalError: Can't connect to X11 window server using ':0' as the value of the DISPLAY variable.
        at sun.awt.X11GraphicsEnvironment.initDisplay(Native Method)
        at sun.awt.X11GraphicsEnvironment.access$200(Unknown Source)
        at sun.awt.X11GraphicsEnvironment$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.awt.X11GraphicsEnvironment.<clinit>(Unknown Source)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Unknown Source)
        at java.awt.GraphicsEnvironment.createGE(Unknown Source)
        at java.awt.GraphicsEnvironment.getLocalGraphicsEnvironment(Unknown Source)
        at sun.awt.X11.XToolkit.<clinit>(Unknown Source)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Unknown Source)
        at java.awt.Toolkit$2.run(Unknown Source)
        at java.awt.Toolkit$2.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.awt.Toolkit.getDefaultToolkit(Unknown Source)
        at tectonicus.TectonicusApp.main(TectonicusApp.java:1202)