JFrame invisible border issue

I´ve a little issue I would like to get solved.

On my JFrame there seems to be like an invisible border around the frame. So when something should be invisible or stop at ykoord = 0 e.g., it becomes invisible/stops like 20 pixels from the borders you see. Although the background is painted correctly...

Maybe I need to add something here:

 public Game()
    {
        add(new Board());
        setTitle("Game");
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setSize(BOARD_WIDTH, BOARD_HEIGTH);
        setLocationRelativeTo(null);
        setVisible(true);
开发者_开发技巧        setResizable(false);
    }

Please help!