Comodeixar a janela ocupar toda a tela

JFrame frame = new JFrame("Meu frame");

// visível
frame.setVisible(true);

// tela cheia
frame.setExtendedState(JFrame.MAXIMIZED_BOTH);

// com a barra e sem opção de redimensionar
frame.setResizable(false);

// se você realmente não quiser a barra
frame.setUndecorated(true);

ps: o interessante é encapsular tudo dentro do construtor do seu JFrame.

Quando eu coloco esse código no construtor, ele me retorna isso:

Exception in thread “AWT-EventQueue-0” java.awt.IllegalComponentStateException: The frame is displayable.

at java.awt.Frame.setUndecorated(Frame.java:923)

at view.InicialScreen.(InicialScreen.java:21)

at view.InicialScreen$5.run(InicialScreen.java:201)

at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)

at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)

at java.awt.EventQueue.access$500(EventQueue.java:97)

at java.awt.EventQueue$3.run(EventQueue.java:709)

at java.awt.EventQueue$3.run(EventQueue.java:703)

at java.security.AccessController.doPrivileged(Native Method)

at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)

at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)

at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)

at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)

at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)

at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)