Java - Pantalla completa JFrame

public void extender(){

    GraphicsEnvironment env =

    GraphicsEnvironment.getLocalGraphicsEnvironment();

    this.setMaximizedBounds(env.getMaximumWindowBounds());

    this.setExtendedState(this.getExtendedState() | this.MAXIMIZED_BOTH);

}

// Deberás importar

//import java.awt.GraphicsEnvironment;

y llamas al metodo justo despues del initComponents();

sample

public NewJFrame() {

    this.setUndecorated(true);

    initComponents();

    extender();

}

int xx;

int yy;

private void formMousePressed(java.awt.event.MouseEvent evt) {

    xx=evt.getX();

    yy=evt.getY();

}

private void formMouseDragged(java.awt.event.MouseEvent evt) {

    int x =evt.getXOnScreen();

    int y=evt.getYOnScreen();

    this.setLocation(x-xx, y-yy);

}