JButton isselected method doesnt work

问题

The code below is to change the colour of the background on selecting any of the 3 buttons: red, green or blue. When I select either of them, nothing actually happens. However, changing from JButtons to JRadioButtons or JToggleButtons does work. Anyone knows why? Is it because JButton.isselected() method is bugged and it always returns false? I appreciate any help...thank you.

public class bgcolor2 extends JFrame
{
private static final int FRAME_WIDTH = 300;
private static final int FRAME_HEIGHT= 400;
private ActionListener listener;
private JButton greenbutton;
private JButton redbutton;
private JButton bluebutton;
private JPanel colorpanel;
private JPanel buttonpanel;

public bgcolor2()
{
    colorpanel = new JPanel();
    add(colorpanel,BorderLayout.CENTER);

    class bgcolorlistener implements ActionListener
    {
        public void actionPerformed(ActionEvent event)
        {
            changebgcolor();    
        }
    }
    listener=new bgcolorlistener();
    createbuttons();
    setSize(FRAME_WIDTH,FRAME_HEIGHT);
}

public void createbuttons()
{
    greenbutton = new JButton("Green");
    greenbutton.addActionListener(listener);
    bluebutton = new JButton("Blue");
    bluebutton.addActionListener(listener);
    redbutton = new JButton("Red");
    redbutton.addActionListener(listener);
    buttonpanel = new JPanel();

    buttonpanel.add(greenbutton);
    buttonpanel.add(redbutton);
    buttonpanel.add(bluebutton);
    add(buttonpanel,BorderLayout.SOUTH);
}

public void changebgcolor()
{
    if (greenbutton.isSelected()) 
        {
            colorpanel.setBackground(new Color(0,255,0));
        }
    if (bluebutton.isSelected()) 
        {
            colorpanel.setBackground(new Color(0,0,255));
        }
    if (redbutton.isSelected()) 
        {
            colorpanel.setBackground(new Color(255,0,0));
        }
}
}