Drawing a gridline in the same part where I created the JPanel?

I need to build a gridline for a crossword puzzle. I wanted to know if I could do it in the same part where I created the JPanel and it's properties instead of doing it in methods?

class CrosswordWindow extends JFrame {


public JPanel crossPanel;

public CrosswordWindow() {
    super("Crossword");  

    setLayout(null);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setResizable(false);

    /*
    bunch of buttons, labels, JLists etc.
    */

    int size;
    crossPanel = new JPanel();
    crossPanel.setBounds(240, 40, 680, 360);
    crossPanel.setBackground(Color.white);
    crossPanel.setBorder(BorderFactory.createEtchedBorder(1, Color.lightGray, Color.lightGray));
    add(crossPanel);

I'm having a tough time trying to draw the grid lines for the crossword.