Schlagwort: MAXIMIZED_BOTH

import java.awt.*;

public static void main(String[] args) {
    
    GraphicsEnvironment env = GraphicsEnvironment.getLocalGraphicsEnvironment();
    
    NewJFrame j = new NewJFrame();
    j.setMaximizedBounds(env.getMaximumWindowBounds());
    j.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    j.setExtendedState(j.getExtendedState() | j.MAXIMIZED_BOTH);
    j.setVisible(true);
}