OK, with a lot of help I discovered a solution, along with some more
crazy stuff. To recap, the problem is illustrated by the following
snippet:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class BuggySwing extends JPanel {
private JLabel messageBar;
private GridBagConstraints gbc;
private JButton button;
private JPanel panel;
private DefaultListModel listModel;
private JList list;
private JScrollPane scrollPane;
private boolean emptyList = true;
public BuggySwing() {
super();
setLayout(new GridBagLayout());
gbc = getBaseConstraints();
gbc.gridx = 0; gbc.gridy = 0;
gbc.gridheight = GridBagConstraints.REMAINDER;
add(new JTextArea("Some text"), gbc);
button = new JButton("Toggle");
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
listModel.clear();
if(emptyList) {
emptyList = false;
listModel.addElement("one list item");
messageBar.setText(null);
}
else {
emptyList = true;
messageBar.setText("The list is empty");
}
}
});
panel = new JPanel();
panel.add(button);
gbc = getBaseConstraints();
gbc.gridx = 1; gbc.gridy = 0;
gbc.weighty = 0.;
add(panel, gbc);
listModel = new DefaultListModel();
list = new JList(listModel);
emptyList = true;
scrollPane = new JScrollPane(list);
gbc = getBaseConstraints();
gbc.gridx = 1; gbc.gridy = 1;
gbc.weighty = 1.;
add(scrollPane, gbc);
}
private GridBagConstraints getBaseConstraints() {
GridBagConstraints baseGbc = new GridBagConstraints();
baseGbc.weightx = baseGbc.weighty = 1.;
baseGbc.fill = GridBagConstraints.BOTH;
return baseGbc;
}
private static JLabel makeMessageBar() {
JLabel label = new JLabel();
int width = (int) (label.getPreferredSize().getWidth());
int height = label.getFontMetrics(label.getFont()).getHeight();
label.setPreferredSize(new Dimension(width, height));
return label;
}
public static void main(String[] args) {
JFrame frame = new JFrame();
BuggySwing panel = new BuggySwing();
panel.messageBar = makeMessageBar();
panel.messageBar.setText("The list is empty");
frame.getContentPane().add(panel.messageBar, "South");
frame.getContentPane().add(panel, "Center");
frame.pack();
frame.show();
}
}
First, as an illustration that there's at least one bug afoot,
after I posted my last post, I serendipitously discovered that getting
rid of the lines that cause the change in the contents of the *message
bar* also gets rid of the unwanted resizing of components:
...
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
listModel.clear();
if(emptyList) {
emptyList = false;
listModel.addElement("one list item");
// messageBar.setText(null);
}
else {
emptyList = true;
// messageBar.setText("The list is empty");
}
}
});
(Unfortunately, this is no solution, since an unchanging message bar is
useless--and I need a message bar).
The message bar is not even within the purview of the GridBagLayout.
If this is not a bug, I don't know what is.
Next, I had been told in earlier posts that the important parameter
when using a ScrollPane within a GridBagLayout was minimumSize, not
preferredSize, so before posting my last post I had tried:
scrollPane = new JScrollPane(list);
scrollPane.setMinimumSize(scrollPane.getPreferredSize());
This appeared to work, but only if the user did not resize the window
in such a way that its *height* increased (there's another bit of
GridBagLayout crazy magic for ya!).
Later, after posting, in desperation I finally replaced the above line
with the crazy-looking
scrollPane = new JScrollPane(list);
scrollPane.setPreferredSize(scrollPane.getPreferredSize());
and this seems to work, even after resizing, and with a functional
message bar.
In other words, explicitly setting the preferredSize (only once) to
the value it supposedly *already has* causes the behavior to change.
This gem is also completely undocumented, as far as I can tell.
It's really a shame. It took me 2 1/2 workdays (not including the
time of posters who offered their analysis and suggestions) to sort
this out. (Kind of: many mysteries remain.)
Anyway, *many many* thanks for your help.
KJ
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