GridBag help

[Mousa A. Rafi]

Finally I'm feeling comfortable with GridBagLayout and the constraints used by GridBagConstraints. However I'm still confused with weightx and weighty. I'm not sure how they work and what good using them. The only visible effect I got is, suppose when using weighty, extra space is entered between two components when the window is enlarged vertically. I'm also confused about the values (0.0 - 1.0) used for these.

Despite its flexibility, I'm unable to do the following using GridBagLayout: I have three buttons. two below the other. I want to make the top one as long as the total length of the bottom two. That is the top button spans two columns.

[Craig Wood]

Using the weight constraints is more subtle and is a key to fluid layouts that resize gracefully. The nominal range is 1 to 1.0 but some prefer to use larger, integer values.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class GridBagTest
{
    JCheckBox check;
    JPanel panel;
    GridBagLayout gridbag;
    GridBagConstraints gbc;
 
    public GridBagTest()
    {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(getUIPanel(), "North");
        f.add(getPanel());
        f.add(getButtonPanel(), "South");
        f.setSize(500,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
 
    private JPanel getUIPanel()
    {
        String[] ids = { "0", "0.1", "1.0" };
        JRadioButton[] radios = new JRadioButton[ids.length];
        ButtonGroup group = new ButtonGroup();
        JPanel panel = new JPanel();
        panel.setBorder(BorderFactory.createTitledBorder("weightx and fill"));
        ActionListener l = new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                JRadioButton radio = (JRadioButton)e.getSource();
                double value = Double.parseDouble(radio.getActionCommand());
                adjustWeights(value);
            }
        };
        for(int j = 0; j < radios.length; j++)
        {
            radios[j] = new JRadioButton(ids[j], ids[j].equals("0"));
            radios[j].setActionCommand(ids[j]);
            radios[j].addActionListener(l);
            group.add(radios[j]);
            panel.add(radios[j]);
        }
        check = new JCheckBox("fill");
        check.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                adjustWeights(-1.0);
            }
        });
        panel.add(check);
        return panel;
    }
 
    private void adjustWeights(double weightx)
    {
        Component[] c = panel.getComponents();
        int fill = check.isSelected() ? GridBagConstraints.HORIZONTAL :
                                        GridBagConstraints.NONE;
        for(int j = 0; j < c.length; j++)
        {
            GridBagConstraints gbc = gridbag.getConstraints(c[j]);
            if(weightx > -1.0)
                gbc.weightx = weightx;
            gbc.fill = fill;
            gridbag.setConstraints(c[j], gbc);
        }
        panel.revalidate();
        panel.repaint();
    }
 
    private JPanel getPanel()
    {
        gridbag = new GridBagLayout();
        panel = new JPanel(gridbag);
        gbc = new GridBagConstraints();
        gbc.insets = new Insets(2,2,2,2);
        gbc.weighty = 1.0;
        panel.add(new JTextField(8), gbc);
        gbc.gridx = 2;
        panel.add(new JTextField(9), gbc);
        gbc.gridy = 1;
        gbc.gridx = gbc.RELATIVE;
        addComponents(getLabel("label 1"), 1, new JTextField(8), 1, panel, gbc);
        addComponents(getLabel("label 2"), GridBagConstraints.RELATIVE,
                      new JTextField(8), GridBagConstraints.REMAINDER, panel, gbc);
        gbc.gridy = gbc.RELATIVE;
        gbc.anchor = gbc.CENTER;
        gbc.gridwidth = 1;
        panel.add(getLabel("label 3"), gbc);
        panel.add(getLabel("label 4"), gbc);
        gbc.gridwidth = gbc.RELATIVE;
        panel.add(getLabel("label 5"), gbc);
        gbc.gridwidth = gbc.REMAINDER;
        panel.add(getLabel("label 6"), gbc);
        return panel;
    }
 
    private JLabel getLabel(String s)
    {
        JLabel label = new JLabel(s, JLabel.CENTER);
        label.setBorder(BorderFactory.createEtchedBorder());
        return label;
    }
 
    private void addComponents(JLabel label, int width1, JTextField tf, int width2,
                               JPanel panel, GridBagConstraints gbc)
    {
        gbc.anchor = gbc.EAST;
        gbc.gridwidth = width1;
        panel.add(label, gbc);
        gbc.anchor = gbc.WEST;
        gbc.gridwidth = width2;
        panel.add(tf, gbc);
    }
 
    private JPanel getButtonPanel()
    {
        JPanel panel = new JPanel(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.insets = new Insets(2,2,2,2);
        gbc.weightx = 1.0;
        gbc.fill = gbc.HORIZONTAL;
        gbc.gridwidth = 2;
        panel.add(new JButton("top"), gbc);
        gbc.gridy = 1;
        gbc.gridwidth = gbc.RELATIVE;
        panel.add(new JButton("left"), gbc);
        gbc.gridwidth = gbc.REMAINDER;
        panel.add(new JButton("right"), gbc);
        return panel;
    }
 
    public static void main(String[] args)
    {
        new GridBagTest();
    }
}