Hab mal in alten Swing Sourcen geschaut und folgenden Code Schnippel gefunden
GraphicsEnvironment ge = GraphicsEnvironment.
getLocalGraphicsEnvironment();
GraphicsDevice gd = ge.getDefaultScreenDevice();
GraphicsConfiguration gc = gd.getDefaultConfiguration();
Insets insets = Toolkit.getDefaultToolkit().getScreenInsets(gc);
Dimension size = Toolkit.getDefaultToolkit().getScreenSize();
JFrame f = new JFrame("Maximieren",gc);
f.setBounds(insets.left, insets.top,
size.width-(insets.left+insets.right),
size.height-(insets.top+insets.bottom));