Semi-transparent JToolBar = strange behaviour?

Question

I need to have semi-transparent toolbar in the application I am making. I am using toolbar.setBackground(new Color(0, 0, 0, 50)); to do so.
At start the icon looks like this:
[Screenshot1|http://i29.tinypic.com/j66cky.jpg]
After a few mouse transition, the transparent part of png icon becomes totally black:
[Screenshot2|http://i26.tinypic.com/dmwjo9.jpg]
I�ve tried using JButton (with ActionListener), instead of JLabel component, but the behaviour remains the same.
Here�s the test code:
package toolbartest;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class MainFrame extends JFrame {
    private MainFrame(String title) {
        setSize(new Dimension(800, 600));
        setTitle(title);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
        JToolBar toolbar = new JToolBar();
        toolbar.setBackground(new Color(0, 0, 0, 50)); // <-- when remove this
        toolbar.add(new ToolBarButton());              //     line then is OK
        add(toolbar, BorderLayout.NORTH);
    public static void main(String[] args) {
        new MainFrame("ToolBar Test").setVisible(true);
class ToolBarButton extends JLabel {
    public ToolBarButton() {
        super(new ImageIcon(MainFrame.class.getResource("exit.png"))); // <-- transparent png
        setBorder(BorderFactory.createLineBorder(Color.BLACK));        //     icon 32px x 32px
        addMouseListener(new MouseAdapter() {
            @Override
            public void mouseEntered(MouseEvent e) {
                setBorder(BorderFactory.createLineBorder(Color.RED));
            @Override
            public void mouseExited(MouseEvent e) {
                setBorder(BorderFactory.createLineBorder(Color.BLACK));
            @Override
            public void mouseClicked(MouseEvent e) {
                System.exit(0);
}

Answer 1

Thanks camickr, very helpful explanation.
Here's (good) working code:
package toolbartest;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class MainFrame extends JFrame {
    private MainFrame() {
        setSize(800, 600);
        setTitle("ToolBar Test");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
        JToolBar toolbar = new JToolBar() {
            @Override //<-- Paint background
            protected void paintComponent(Graphics g) {
                g.setColor(getBackground());
                g.fillRect(0, 0, getSize().width, getSize().height);
                super.paintComponent(g);
        toolbar.setOpaque(false);   //<-- Toolbar is non-opaque
        toolbar.setBackground(new Color(0, 0, 0, 50)); //<-- Background color
        toolbar.add(new ToolBarButton());              //that has an alpha value
        add(toolbar, BorderLayout.NORTH);
        setVisible(true);
    public static void main(String[] args) {
        new MainFrame();
class ToolBarButton extends JLabel {
    public ToolBarButton() {
        super(new ImageIcon(MainFrame.class.getResource("exit.png"))); // <-- transparent png
        setBorder(BorderFactory.createLineBorder(Color.BLACK));        //     icon 32px x 32px
        addMouseListener(new MouseAdapter() {
            @Override
            public void mouseEntered(MouseEvent e) {
                setBorder(BorderFactory.createLineBorder(Color.RED));
            @Override
            public void mouseExited(MouseEvent e) {
                setBorder(BorderFactory.createLineBorder(Color.BLACK));
            @Override
            public void mouseClicked(MouseEvent e) {
                System.exit(0);
    }