Java paintComponent not executed

[namdaemun]

hi, I'm currently building a game and stuck at the paintComponent method. I'm trying to add 3 graphic buttons to the JPanel
but the paintComponent does not execute (all I get is gray window)

here's my code :

public class TwinMoonTowers extends JPanel implements ActionListener{

        private class TMTMenu extends MouseAdapter{
            private Icon start, shop, quit;
            private Icon main_bg;

            public TMTMenu (){
                System.out.println("starting menu...");

                start = new Icon(TMTConstant.START_SOURCE,
                            TMTConstant.START_COORD_X, TMTConstant.START_COORD_Y);

                shop = new Icon (TMTConstant.SHOP_SOURCE,
                            TMTConstant.SHOP_COORD_X, TMTConstant.SHOP_COORD_Y);

                quit = new Icon (TMTConstant.EXIT_SOURCE,
                            TMTConstant.EXIT_COORD_X, TMTConstant.EXIT_COORD_Y);

                main_bg = new Icon (TMTConstant.MAIN_MENU_BG, 0, 0);
            }

            @Override
            public void mousePressed(MouseEvent me){

                int x = me.getX();
                int y = me.getY();

                //System.out.println("coord: "+x+","+y);

                if (me.getButton() == MouseEvent.BUTTON1){
                    if (x > start.x && x < start.x + start.getIcon().getWidth(null)
                            && y > start.y && y < start.y + start.getIcon().getHeight(null)){
                        //System.out.println("1");
                        curState = TMTConstant.MODE_STATE;
                    }
                    else if (x > quit.x && x < quit.x + quit.getIcon().getWidth(null)
                            && y > quit.y && y < quit.y + quit.getIcon().getHeight(null)){
                        //System.out.println("2");
                        curState = TMTConstant.EXIT_STATE;
                    }
                    else if (x > shop.x && x < shop.x + shop.getIcon().getWidth(null)
                            && y > shop.y && y < shop.y + shop.getIcon().getHeight(null)){
                        //System.out.println("3");
                        curState = TMTConstant.SHOP_STATE;
                    }
                }

            }
        }

        private class TMTModeSelector extends MouseAdapter{
            private Icon TwinTower, TripleTower, back;

            public TMTModeSelector(){

            }

            @Override
            public void mousePressed(MouseEvent e){

            }
        }

        private String testDebug;
        private int curState;
        private TMTMenu menu;
        private TMTModeSelector selector;

        public TwinMoonTowers (){
            menu = new TMTMenu();
            selector = new TMTModeSelector();
            curState = TMTConstant.MAIN_MENU_STATE;

            this.setLayout(null);

            System.out.println("creating panel..");

            this.setVisible(true);
            this.setDoubleBuffered(true);
            this.setSize(TMTConstant.WINDOW_WIDTH, TMTConstant.WINDOW_HEIGHT);
            this.addMouseListener(menu);
        }

        @Override
        public void actionPerformed(ActionEvent arg0) {
            // TODO Auto-generated method stub
            this.repaint();

        }
        @Override
        public void paintComponent (Graphics g){
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g;

            System.out.println("preparing canvas..");

            switch(curState){
                case TMTConstant.MAIN_MENU_STATE :
                    g2d.drawImage(menu.main_bg.getIcon(), menu.main_bg.x, menu.main_bg.y, null);
                    g2d.drawImage(menu.start.getIcon(), menu.start.x, menu.start.y, null);
                    g2d.drawImage(menu.quit.getIcon(), menu.quit.x, menu.quit.y, null);
                    g2d.drawImage(menu.shop.getIcon(), menu.shop.x, menu.shop.y, null);
                    break;
                case TMTConstant.MODE_STATE :
                    break;
                case TMTConstant.GAME_STATE :
                    break;
                case TMTConstant.SHOP_STATE :
                    break;
            }

            System.out.print("render image...");

            //Toolkit.getDefaultToolkit().sync();
            //g.dispose();
            //this.validate();
            this.repaint();

        }
}

this is my icon class :

public class Icon{

    private Image pic;
    public int x, y;

    public Icon (String path, int x, int y){

        ImageIcon ii = new ImageIcon (path);
        pic = ii.getImage();

        this.x = x;
        this.y = y;
    }

    public Image getIcon() {return pic;}

}

and this is the main.java run test

public class Main extends JFrame{

    /**
     * @param args
     */
    public Main(){
        this.setLayout(null);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setSize(TMTConstant.WINDOW_WIDTH, TMTConstant.WINDOW_HEIGHT);
        this.setVisible(true);
        this.setResizable(false);
        this.setLocation(20, 20);

        TwinMoonTowers tmt = new TwinMoonTowers();
        add(tmt);
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        new Main();
    }

}

help plsss... THX