Answers for "swing java gui fullscreen"

jframe full screen

frame.setExtendedState(JFrame.MAXIMIZED_BOTH); 
frame.setUndecorated(true);
frame.setVisible(true);

set jframe fullscreen

GraphicsEnvironment graphics = 
		GraphicsEnvironment.getLocalGraphicsEnvironment();
        
GraphicsDevice device = graphics.getDefaultScreenDevice();
JFrame frame = new JFrame("Fullscreen");
device.setFullScreenWindow(frame);