public class Main extends JPanel {

...
static JFrame displayFrame, inputFrame;

...
...

// Generator node list
ArrayList<Node> nodes = new ArrayList<Node>();

public static void main(String[] args) {
    // Set up the frame
    screenSize = Toolkit.getDefaultToolkit().getScreenSize();
    displayFrame = new JFrame("City generator");
    displayFrame.setSize(screenSize.width / 3, screenSize.width / 3);
    displayFrame.setLocation(screenSize.width / 2 - displayFrame.getWidth()
            / 2, screenSize.height / 2 - displayFrame.getHeight() / 2);
    displayFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    displayFrame.add(new Main());
    // displayFrame.setUndecorated(true);

    displayFrame.setBackground(Color.lightGray);

    displayFrame.setVisible(true);

    displayFrame.addMouseMotionListener(new MouseAdapter() {
        public void mouseMoved(MouseEvent e) {
            // Mouse movement events here

        }
    });
}

// Override function
public void paintComponent(Graphics g) {
    super.paintComponent(g);
    // Calls multiple times
    generate(g);
}

private void generate(Graphics g) {
............................