Why setBackground works only the first time? (JPanel)

In an outer class I call these methods many times, but setBackground() works only the first time I call any of these methods.

For example, I have objects CellPane cellpane1, cellpane2 and when I call cellpane1.drawPlayer() for the first time it works as expected and when I call cellpane2.drawWall() for the first time it works as expected too. But when I call drawWall() for cellpane1, cellpane1.drawWall() after I called cellpane1.drawPlayer(), it doesn't work. The background color is not changing.

Do I need to repaint or reinvalidate, or what ? I'm new to these stuff. :( What I'm doing wrong?

private static class CellPane extends JPanel {

    private Color defaultBackground;

    private CellPane() {
        addMouseListener(new MouseAdapter() {
            @Override
            public void mouseEntered(MouseEvent e) {
                defaultBackground = getBackground();
                setBackground(Color.BLUE);
            }

            @Override
            public void mouseExited(MouseEvent e) {
                setBackground(defaultBackground);
            }
        });
    }

    private void drawDefault() {
        setBackground(defaultBackground);
    }

    private void drawBoxOnGoal() {
        defaultBackground = getBackground();
        setBackground(Color.decode("#4a3d32"));
    }

    private void drawPlayerOnGoal() {
        defaultBackground = getBackground();
        setBackground(Color.decode("#f9cda8"));
    }

    private void drawGoal() {
        defaultBackground = getBackground();
        setBackground(Color.decode("#f9cda8"));
    }

    private void drawBox() {
        defaultBackground = getBackground();
        setBackground(Color.decode("#7a5230"));
    }

    private void drawPlayer() {
        defaultBackground = getBackground();
        setBackground(Color.YELLOW);

    }

    private void drawWall() {
        defaultBackground = getBackground();
        setBackground(Color.RED);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(50, 50);
    }

}

[EDIT]

Example:

public class MainClass {



public static void main(String[] args) {
    new MainClass();
}

private MainClass() {
    EventQueue.invokeLater(new Runnable() {
        @Override
        public void run() {
            try {
                UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
            } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) { }

            JFrame frame = new JFrame("Test");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setLayout(new BorderLayout());

            frame.add(new Map());

            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        }
    });
}

private static class Map  extends JPanel implements ActionListener{

    CellPane cellPane;
    boolean flag = true;

    JButton button = new JButton("button");

    public Map() {
        setLayout(new GridBagLayout());

        button.addActionListener(this);

        add(button);

        create(flag);
    }

    public void create(boolean flag) {
        GridBagConstraints gbc = new GridBagConstraints();

        cellPane = new CellPane();
        gbc.gridx = 0;
        gbc.gridy = 0;
        if(flag)
            cellPane.drawPlayer();
        else cellPane.drawWall();
        add(cellPane, gbc);
    }

    @Override
    public void actionPerformed(ActionEvent arg0) {
        flag = false;
        create(flag);
    }

}


@SuppressWarnings("serial")
private static class CellPane extends JPanel {

    private Color defaultBackground;


    private void drawPlayer() {
        defaultBackground = getBackground();
        setBackground(Color.YELLOW);

    }

    private void drawWall() {
        defaultBackground = getBackground();
        setBackground(Color.RED);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(50, 50);
    }

}}

I know that it's not the perfect example, but it's runnable. So I want to change the background of the cellPane when I click the button. The constructor calls create(flag) when flag is true and the background is set to YELLOW. Then I click the button, the flag changes to false and call create(flag) where flag=false, so it calls the cellPane.setWall() instead which sets the background RED, but it doesn't.

What I'm doing wrong? Maybe something easy, but I'm new to these stuff.

You're not changing the color of the cellPane when clicking the button. You're creating another cellPane with a different color, and you're adding it in the same gridbag cell as the existing one.

Just change the color, and it will work fine:

    public Map() {
        setLayout(new GridBagLayout());

        button.addActionListener(this);

        add(button);

        create();
        draw();
    }

    private void draw() {
        if (flag) {
            cellPane.drawPlayer();
        }
        else {
            cellPane.drawWall();
        }
    }

    private void create() {
        GridBagConstraints gbc = new GridBagConstraints();

        cellPane = new CellPane();
        gbc.gridx = 0;
        gbc.gridy = 0;
        add(cellPane, gbc);
    }

    @Override
    public void actionPerformed(ActionEvent arg0) {
        flag = false;
        draw();
    }