How to make a translucent JPanel within the region Jpanel

Question

I want to make a translucent JPanel for which I can choose the x-coordinate, y-coordinate, width and height. I have found some material that made a translucent JPanel that filled its parent container, but I don't need this effect. I need a translucent JPanel whose position can be specified in the parent container, such that only this area is translucent, while others in the parent container are opaque. I tried this code but it is incorrect. The resultant area is smaller than the area I intend. What is wrong, or how I can get this effect?

public class TranslucentJPanel extends JPanel {

    private float transparency;

    public TranslucentJPanel(){
    }



    /**set the transparency
     * 
     * @param transparency:the transparency you want to set
     * 
     * @return void
     */
    public void setTransparent(float transparency) {
        this.transparency = transparency;
    }

    @Override
    protected void paintComponent(Graphics g){
        super.paintComponent(g);

        Graphics2D graphics2d = (Graphics2D) g.create();
        graphics2d.setComposite(AlphaComposite.SrcOver.derive(transparency));

        graphics2d.fill(getBounds());
        graphics2d.dispose();
    }
}

Answer 1

So, two things come to mind immediately...

First, don't use getBounds, the Grapghics context is already translated to the components x/y position, so you're doubling that up. Instead, simply use 0x0 and provide the width and height, something like...

graphics2d.fillRect(0, 0, getWidth(), getHeight());

Also, remember, most Swing components are opaque by default, this includes the JList, JScrollPane and it's JViewport, you need to set these to be transparent if you want them to be see through at all.

Also, the "default" renderer that the JList uses also renderers it self as opaque, so you need to be able to supply your own if you want the items to be transparent...

import java.awt.AlphaComposite;
import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagLayout;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;
import javax.swing.DefaultListCellRenderer;
import javax.swing.DefaultListModel;
import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.border.EmptyBorder;

public class Test {

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

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

                DefaultListModel<String> model = new DefaultListModel<>();
                model.addElement("Bananas");
                model.addElement("Apples");
                model.addElement("Pears");
                model.addElement("Grapes");
                model.addElement("Tim Tams");

                JList list = new JList(model);
                list.setCellRenderer(new DefaultListCellRenderer() {

                    @Override
                    public Component getListCellRendererComponent(JList<?> list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
                        super.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
                        setOpaque(isSelected);
                        return this;
                    }

                });
                list.setOpaque(false);
                JScrollPane sp = new JScrollPane(list);
                sp.setOpaque(false);
                sp.getViewport().setOpaque(false);

                TranslucentPane tp = new TranslucentPane();
                tp.setLayout(new GridBagLayout());
                tp.setLayout(new BorderLayout());
                tp.add(sp);

                JFrame frame = new JFrame("Testing");
                frame.setContentPane(new BackgroundPane());
                frame.setLayout(new GridBagLayout());
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(tp);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TranslucentPane extends JPanel {

        private float alpha = 0.75f;

        public TranslucentPane() {
            setOpaque(false);
            setBorder(new EmptyBorder(5, 5, 5, 5));
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.setComposite(AlphaComposite.SrcOver.derive(alpha));

            g2d.setColor(getBackground());
            g2d.fillRect(0, 0, getWidth(), getHeight());
            g2d.dispose();
        }

    }

    public class BackgroundPane extends JPanel {

        private BufferedImage background;

        public BackgroundPane() {
            try {
                background = ImageIO.read(new File("C:\\Users\\shane\\Dropbox\\MegaTokyo\\thumnails\\megatokyo_omnibus_1_3_cover_by_fredrin-d4oupef.jpg"));
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(background.getWidth(), background.getHeight());
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            int x = (getWidth() - background.getWidth()) / 2;
            int y = (getHeight() - background.getHeight()) / 2;
            g.drawImage(background, x, y, this);
        }

    }

}