Bug of Transparent JCheckBox in a JPanel?

Question

Here are my codes for adding the new component:

addButton.addActionListener(new ActionListener() {          
        @Override
        public void actionPerformed(ActionEvent e) {
            // TODO Auto-generated method stub
            if (!todoListInput.getText().equals("")) {
                JPanel panel = new JPanel(new FlowLayout(FlowLayout.LEFT));
                panel.setBackground(new Color(213, 134, 145, 55));
                JCheckBox checkBox = new JCheckBox("");
                checkBox.setOpaque(false);
                checkBox.setForeground(Color.WHITE);
                //checkBox.setBorder(line);

                panel.add(checkBox);
                Border border = new LineBorder(Color.GRAY, 1, true);
                Border margin = new EmptyBorder(10,10,10,10);
                panel.setBorder(new CompoundBorder(margin, border));

                GridBagConstraints gbc = new GridBagConstraints();
                gbc.gridwidth = GridBagConstraints.REMAINDER;
                gbc.weightx = 1;
                gbc.fill = GridBagConstraints.HORIZONTAL;
                mainList.add(panel, gbc, 0);

                validate();
                repaint();

                todoListInput.setText("");
            }                
        }           
    });

My problem is that when I do an "onmouseover" action to the checkbox, part of the the whole jFrame will be appeared behind the checkbox

I found that this only appear when I do checkBox.setOpaque(false) or checkBox.setBackground(new Color(122,122,122,55)).

May I know what is the problem of my code?

Your problem is here `panel.setBackground(new Color(213, 134, 145, 55));`. Swing doesn't know how to deal with alpha based background colors, it only deals with full opaque or transparent components. In order to make it work, you need to fake it, by making the component fully transparent and painting the translucent color in the components `paintComponent` method — MadProgrammer, Jan 22 '16 at 04:34
Something like [this](http://stackoverflow.com/questions/32216625/how-to-make-a-translucent-jpanel-within-the-region-jpanel/32217554#32217554) for example — MadProgrammer, Jan 22 '16 at 04:36

score 3 · Accepted Answer · answered Jan 22 '16 at 04:37

panel.setBackground(new Color(213, 134, 145, 55));

The problem is your panel is using a transparent background and you are breaking the painting contract between a component. An opaque component needs to repaint the entire background (with an opaque color), however because of the transparency you are getting painting artifacts.

Check out Background With Transparency for more information and a couple of solutions.

The basic solution is:

JPanel panel = new JPanel()
{
    protected void paintComponent(Graphics g)
    {
        g.setColor( getBackground() );
        g.fillRect(0, 0, getWidth(), getHeight());
        super.paintComponent(g);
    }
};
panel.setOpaque(false);
panel.setBackground( new Color(255, 0, 0, 20) );
frame.add(panel);

However, the link also provides a reusable solution so you don't need to use custom painting on every component that has a transparent background.

I understand the problem now. Thanks a lot!! – ykn121 Jan 22 '16 at 04:47 — ykn121, Jan 22 '16 at 04:47

Bug of Transparent JCheckBox in a JPanel?

1 Answers1