Do you mean with the JFrame maximized or FullScreen Mode? 2 different things with 2 entirely different solutions.
Fullscreen Mode Maximize Screen - Kind of depends on what JDK version you are running. You can try:
setExtendedState(JFrame.MAXIMIZED_BOTH);
after you setVisible(true) on the frame. If you just set the size as the dimensions of the screen, the frame will fill the screen but won't trully be maximized.