Using a JSlider to change JLabel background

Question

I have set up a JSlider to change the opacity of a JLabel's background. I have found that the only way to make the change take effect, is to toggle the visibility of the JLabel in the stateChanged method. Why is this? Shouldn't just setting the JLabel background to a new color work? Try running this with and without the last couple lines in stateChanged:
package slidertest;
import java.awt.BorderLayout;
import java.awt.Color;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JSlider;
import javax.swing.SwingConstants;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;
public class Main extends JFrame {
    private static Main instance = new Main();
    private JLabel colorLabel = new JLabel("  ");
    public Main(){
        colorLabel.setOpaque(true);
        colorLabel.setBackground(Color.green);
        JPanel mainPanel = new JPanel(new BorderLayout());
        mainPanel.add(colorLabel, BorderLayout.NORTH);
        mainPanel.add(createOpacityColorSlider(colorLabel), BorderLayout.CENTER);
        this.add(mainPanel);
    public static void main(String[] args) {
       instance.setSize(200,200);
       instance.setVisible(true);
    static final int ALPHA_MIN = 0;
    static final int ALPHA_MAX = 255;
     * @param style
     * @param colorLabel
     * @return
    private JSlider createOpacityColorSlider( final JLabel colorLabel ){
        JSlider opacitySlider = new JSlider(SwingConstants.HORIZONTAL,
                ALPHA_MIN, ALPHA_MAX, colorLabel.getBackground().getAlpha() );
//      Turn on labels at major tick marks.
        opacitySlider.setMajorTickSpacing(20);
        opacitySlider.setMinorTickSpacing(10);
        opacitySlider.addChangeListener(new ChangeListener(){
          public void stateChanged(ChangeEvent e) {
                            JSlider source = (JSlider)e.getSource();
                                int alpha = source.getValue();
                                Color newColor = new Color(colorLabel.getBackground().getRed(), colorLabel.getBackground().getGreen(), colorLabel.getBackground().getBlue(), alpha);
                                colorLabel.setBackground(newColor);
                                //will not work without doing this:
                                colorLabel.setVisible(false);
                                colorLabel.setVisible(true);
        return opacitySlider;
}-Eric

Answer 1

   //will not work without doing this:
                                colorLabel.setVisible(false);
                                colorLabel.setVisible(true);have you tried in stead of those lines putting in this
colorLabel.repaint();
that line tells the button that somthing has changed and needs to re draw its self