Answers for "is there any function that makes it that the jframe must always be in ful screen"

full screen in jframe

// Getting fullscreen **effect** by extending the frame either direction.
frame.setExtendedState(JFrame.MAXIMIZED_BOTH); 
frame.setUndecorated(true);

jframe full screen

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