How can I resolve these sizing issues with JToggleButtons, JPanels, and JScrollPanes?

Question

I'm trying to develop a form of an accordion menu. There are a small number (2-12) options that can be toggled on/off. When toggled on, there will be a JPanel with additional settings that become visible. When toggled off, the additional settings will not be visible.

I've created a SelectableExpandablePanel class that extends JPanel and implements ActionListener and ComponentListener. The panel holds two things - a JToggleButton and a child Component (which will typically be a JPanel, but I don't want to limit myself for future reuse of this concept) in a BoxLayout to enforce one column. When the toggle button is selected, the child becomes visible. When the toggle is deselected, the child is hidden.

When I use this component, I intend to put it on a JPanel inside of a JScrollPane, as demonstrated in the sample main method.

There appear to be two problems that I'm having trouble overcoming:

1. If I don't specify a JFrame size, it's only large enough for the width of each child and tall enough for three buttons. When I click on the button, I would expect the JScrollPane to do its thing and generate a vertical scroll bar. This isn't happening.

2. I'd like the toggle buttons to be the full width of the JPanel that contains them. I thought what I did in the constructor plus the Component Listener would handle that, but it doesn't.

What is provided below compiles and has a main method. If compiled and executed, it drives the component I'm building to provide a test frame and the ability to reproduce the issues I'm talking about.

import javax.swing.BoxLayout;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JToggleButton;

import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.ComponentEvent;
import java.awt.event.ComponentListener;

public class SelectableExpandablePanel extends JPanel implements
        ActionListener, ComponentListener {
    private JToggleButton titleButton;
    private JComponent childComponent;

    public SelectableExpandablePanel(JComponent child) {
        this(child, null, null);
    }

    public SelectableExpandablePanel(JComponent child, String title) {
        this(child, title, null);
    }

    public SelectableExpandablePanel(JComponent child, String title,
            String tooltip) {
        super();

        if (child == null) {
            throw new IllegalArgumentException("Child component cannot be null");
        }

        childComponent = child;

        titleButton = new JToggleButton();
        titleButton.setText(title);
        titleButton.addActionListener(this);
        titleButton.setPreferredSize(new Dimension(getSize().width, titleButton
                .getPreferredSize().height));
        titleButton.setToolTipText(tooltip);

        setLayout(new BoxLayout(this, BoxLayout.Y_AXIS));
        setPreferredSize(new Dimension(childComponent.getPreferredSize().width,
                titleButton.getPreferredSize().height));
        setSize(new Dimension(childComponent.getPreferredSize().width,
                titleButton.getPreferredSize().height));

        add(titleButton);

        this.addComponentListener(this);
    }

    public void actionPerformed(ActionEvent e) {
        if (titleButton.isSelected()) {
            add(childComponent);
            setSize(new Dimension(childComponent.getPreferredSize().width,
                    titleButton.getPreferredSize().height
                            + childComponent.getPreferredSize().height));

        } else {
            remove(childComponent);
            setSize(new Dimension(childComponent.getPreferredSize().width,
                    titleButton.getPreferredSize().height));
        }

        invalidate();
        revalidate();
    }

    public void componentHidden(ComponentEvent arg0) {
        // Do nothing
    }

    public void componentMoved(ComponentEvent arg0) {
        // Do nothing
    }

    public void componentResized(ComponentEvent arg0) {
        titleButton.setSize(this.getWidth(),
                titleButton.getPreferredSize().height);
    }

    public void componentShown(ComponentEvent arg0) {
        // Do nothing
    }

    public static void main(String[] args) {
        JScrollPane scrollPane = new JScrollPane();

        // These panels simulates a complex, multi-line configuration panel.
        JPanel testPanel = new JPanel();
        testPanel.setLayout(new BoxLayout(testPanel, BoxLayout.Y_AXIS));
        testPanel.add(new JLabel("Test JLabel"));
        testPanel.add(new JLabel("Test JLabel 2"));
        testPanel.add(new JLabel("Test JLabel 3"));

        JPanel testPanel2 = new JPanel();
        testPanel2.setLayout(new BoxLayout(testPanel2, BoxLayout.Y_AXIS));
        testPanel2.add(new JLabel("Test JLabel"));
        testPanel2.add(new JLabel("Test JLabel 2"));
        testPanel2.add(new JLabel("Test JLabel 3"));

        JPanel testPanel3 = new JPanel();
        testPanel3.setLayout(new BoxLayout(testPanel3, BoxLayout.Y_AXIS));
        testPanel3.add(new JLabel("Test JLabel"));
        testPanel3.add(new JLabel("Test JLabel 2"));
        testPanel3.add(new JLabel("Test JLabel 3"));

        // This panel simulates the panel that will contain each of the
        // SelectableExpandablePanels.
        JPanel testHolder = new JPanel();
        testHolder.setLayout(new BoxLayout(testHolder, BoxLayout.Y_AXIS));
        testHolder.add(new SelectableExpandablePanel(testPanel, "Test"));
        testHolder.add(new SelectableExpandablePanel(testPanel2, "Test 2"));
        testHolder.add(new SelectableExpandablePanel(testPanel3, "Test 3"));

        // We add the test holder to the scroll pane. The intention is that if
        // the expansion is too big to fit, the holding JFrame won't expand, but
        // the scroll pane will get scroll bars to let the user scroll up and
        // down through the toggle buttons and any enabled items.
        scrollPane.setViewportView(testHolder);

        JFrame testFrame = new JFrame("Expandable Panel Test");
        testFrame.getContentPane().add(scrollPane);
        testFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        testFrame.pack();
        testFrame.setVisible(true);
    }
}

Answer 1

Don't try to manage the sizes yourself:

//titleButton.setPreferredSize(new Dimension(getSize().width, titleButton.getPreferredSize().height));
 titleButton.setToolTipText(tooltip);

 setLayout(new BoxLayout(this, BoxLayout.Y_AXIS));
//setPreferredSize(new Dimension(childComponent.getPreferredSize().width, titleButton.getPreferredSize().height));
//setSize(new Dimension(childComponent.getPreferredSize().width, titleButton.getPreferredSize().height));

Also, get rid of the setSize() code in the ActionListener. This will be ignored anyway as the layout manager will determine the size.

The scrollbars will appear when the preferred size of the panel is greater than the size of the scrollpane. If you hardcode the preferred size then you default the purpose of the layout manager and the preferred size won't change as you add/remove components.

Note for something like this I generally use a BorderLayout. Put the button in the PAGE_START and the other panel in the CENTER. The components will automatically fill the space available.

Answer 2

1. Remove all the setSize/setPreferredSize calls and let the LayoutManager do its thing.
2. To allow the JButtons to fill the width of the panel, you can use a BorderLayout (for instance, add the button to CENTER, and the child container to SOUTH and remove all those setSize values to let the LayoutManager handle it).