java - How contents are made translucent in a JPanel?

Question

I am confused regarding the display of components in a JPanel.

Suppose if I create a custom JPanel with translucency of 0.8f as follows :-

JPanel panel=new JPanel(){
        @Override
        public void paint(Graphics g)
        {
            super.paint(g);
            BufferedImage img=(BufferedImage)createImage(getWidth(),getHeight());
            Graphics2D g2=(Graphics2D) g.create();
            g2.setComposite(AlphaComposite.SrcOver.derive(0.8f));
            g2.drawImage(img,0,0,null);
        }
        @Override
        public Dimension getPreferredSize()
        {
            return new Dimension(300,300);
        }
    };

Now I set it as the contentPane of the frame.

frame.setContentPane(panel);

Now I add some buttons to it.

    frame.add(new JButton("Click Here"));
    frame.add(new JButton("Click Here"));
    frame.add(new JButton("Click Here"));
    frame.add(new JButton("Click Here"));

1)Then in the output why I get translucent buttons?As JPanel is single layered and I painted the translucent image first when I overrided its paintmethod and then buttons were added,the buttons must not be translucent as they should come over it.

2)Also out of those 4 buttons only 2 are translucent .Why is there such partiality?

3)If I also add a table before adding these 4 buttons then everything becomes translucent.Why?

Object[] names = new Object[] {
             "Title", "Artist", "Album"
         };
         String[][] data = new String[][] {
             { "Los Angeles", "Sugarcult", "Lights Out" },
             { "Do It Alone", "Sugarcult", "Lights Out" },
             { "Made a Mistake", "Sugarcult", "Lights Out" },
             { "Kiss You Better", "Maximo Park", "A Certain Trigger" },
             { "All Over the Shop", "Maximo Park", "A Certain Trigger" },
             { "Going Missing", "Maximo Park", "A Certain Trigger" }
         };
         JTable table = new JTable(data, names);
         frame.add(table); 

4)If I use paintComponent(Graphics g) for JPanel then nothing is translucent,whether I add table or not or as many buttons are added.Why?

(I am asking about the immediate output when application is run.I know that when mouse is rolled over those buttons or if any row in table is clicked it will become opaque,which is due to Swing's painting mechanism.)

Answer 1

1) Then in the output why I get translucent buttons? As JPanel is single layered and I painted the translucent image first when I overrided its paint method and then buttons were added, the buttons must not be translucent as they should come over it.

Actually, you painted the translucent effect OVER the buttons. paint calls

• paintComponent
• paintBorder
• paintChildren

Then you painted your translucent effect OVER the top of what has already been painted (the children). It will make no difference when you add the components, Swing will paint the component under a number of circumstances, the first been when the component is first realised (made visible on the screen) and in response to changes to the dirty regions (and lots of others)...don't fool your self, you have no control over this...

Think of the paint process as a layering approach. First you paint the background, then you paint the mid ground and then finally, the fore ground, which then you went and splashed over...

public class TestTranslucentControls {

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

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

                JFrame frame = new JFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private BufferedImage background;

        public TestPane() {
            setLayout(new GridBagLayout());
            try {
                background = ImageIO.read(new File("C:/Users/shane/Dropbox/MegaTokyo/poster.jpg"));
            } catch (IOException ex) {
                ex.printStackTrace();
            }
            add(new JButton("1"));
            add(new JButton("2"));
            add(new JButton("3"));
            add(new JButton("4"));
            add(new JButton("5"));
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            if (background != null) {
                Graphics2D g2 = (Graphics2D) g.create();
                g2.setComposite(AlphaComposite.SrcOver.derive(0.25f));
                int x = (getWidth() - background.getWidth()) / 2;
                int y = (getHeight() - background.getHeight()) / 2;
                g2.drawImage(background, x, y, this);
                g2.dispose();
            }
        }

        @Override
        public Dimension getPreferredSize() {
            return background == null ? new Dimension(300, 300) : new Dimension(background.getWidth(), background.getHeight());
        }
    }
}

I think you may find...

• Performing Custom Painting
• Painting in AWT and Swing

useful ;)