- Introducing GridBagLayout
- GridBagLayout Demonstration Application 1: Ten Dialog Box Scenarios
- Whats Next?
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GridBagLayout Demonstration Application 1: Ten Dialog Box Scenarios
Getting the constraints just right is the hard part of working with GridBagLayout. To help you with this task, I’ve created a GBLDemo application (contained in the source.zip for this article) that demonstrates how various constraints combinations affect GUI layouts.
The first combination presents six buttons arranged in a two-row by three-column grid. Listing 1 shows the source code for this combination’s public JPanel buildGUI1() method (excerpted from GBLDemo.java).
Listing 1 buildGUI1().
public JPanel buildGUI1 ()
{
JPanel panel = new JPanel ();
panel.setLayout (new GridBagLayout ());
GridBagConstraints gbc = new GridBagConstraints ();
// Place six buttons in a 2-row by 3-column grid. The first assignment
// places "Button 1" in the grid’s origin cell.
JButton button = new JButton ("Button 1");
gbc.gridx = 0;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 2");
gbc.gridx = 1;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 3");
gbc.gridx = 2;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 4");
gbc.gridx = 0;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("?");
gbc.gridx = 1;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("Button 6");
gbc.gridx = 2;
gbc.gridy = 1;
panel.add (button, gbc);
return panel;
}
For each button, this method assigns cell column and row numbers to the button’s gridx and gridy constraints variables, respectively. It also invokes panel.add (button, gbc); to associate these constraints with the button. Figure 1 reveals the resulting layout.
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Figure 1 The two-row by three-column button layout.
The layout in Figure 1 is similar to a two-row by three-column GridLayout-based layout. As with GridLayout, all display areas have the same dimensions. Unlike with GridLayout, however, not all components have the same size: GridBagLayout sizes each component to its preferred size.
Following a resize, GridLayout displays components with equivalent sizes stretched over their container. In contrast, GridBagLayout displays components with their preferred sizes in the container’s center, unless they’re weighted. (I demonstrate weights later.) Check out Figure 2.
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Figure 2 Buttons clump together in a container’s center area after a resize operation.
Combination 2 demonstrates internal padding (which increases the size of a component) on all components in a two-row by three-column grid. Listing 2 shows the source code for this combination’s public JPanel buildGUI2() method.
Listing 2 buildGUI2().
public JPanel buildGUI2 ()
{
JPanel panel = new JPanel ();
panel.setLayout (new GridBagLayout ());
GridBagConstraints gbc = new GridBagConstraints ();
// Set global constraints.
gbc.ipadx = 10;
gbc.ipady = 10;
JButton button = new JButton ("Button 1");
gbc.gridx = 0;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 2");
gbc.gridx = 1;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 3");
gbc.gridx = 2;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 4");
gbc.gridx = 0;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("?");
gbc.gridx = 1;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("Button 6");
gbc.gridx = 2;
gbc.gridy = 1;
panel.add (button, gbc);
return panel;
}
A component is padded internally by assigning values to the ipadx and ipady constraints fields, which increases the component’s minimum size. The buildGUI2() method globally assigns values to both fields, resulting in the six larger buttons shown in Figure 3.
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Figure 3 Internal padding increases button sizes.
Combination 3 demonstrates external padding (which places empty space between components) on all components in a two-row by three-column grid. Listing 3 shows the source code for this combination’s public JPanel buildGUI3() method.
Listing 3 buildGUI3().
public JPanel buildGUI3 ()
{
JPanel panel = new JPanel ();
panel.setLayout (new GridBagLayout ());
GridBagConstraints gbc = new GridBagConstraints ();
// Set global constraints.
gbc.insets = new Insets (5, 5, 0, 0);
JButton button = new JButton ("Button 1");
gbc.gridx = 0;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 2");
gbc.gridx = 1;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 3");
gbc.gridx = 2;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 4");
gbc.gridx = 0;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("?");
gbc.gridx = 1;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("Button 6");
gbc.gridx = 2;
gbc.gridy = 1;
panel.add (button, gbc);
return panel;
}
A component is padded externally by assigning an Insets object to the insets constraints field. The buildGUI3() method globally assigns this object to insets, resulting in five pixels of empty space to the left of and above each button in Figure 4.
)
Figure 4 External padding separates buttons.
Combination 4 demonstrates spanning a component, starting with the component’s cell, across all cells except for the last cell in the component’s row. Listing 4 shows the source code for this combination’s public JPanel buildGUI4() method.
Listing 4 buildGUI4().
public JPanel buildGUI4 ()
{
JPanel panel = new JPanel ();
panel.setLayout (new GridBagLayout ());
GridBagConstraints gbc = new GridBagConstraints ();
// Set global constraints.
gbc.insets = new Insets (5, 5, 0, 0);
JButton button = new JButton ("Button 1");
gbc.gridx = 0;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 2");
gbc.gridx = 1;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 3");
gbc.gridx = 2;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 4");
gbc.gridx = 0;
gbc.gridy = 1;
gbc.gridwidth = GridBagConstraints.RELATIVE;
gbc.fill = GridBagConstraints.HORIZONTAL;
panel.add (button, gbc);
// Reset to defaults.
gbc.gridwidth = 1;
gbc.fill = GridBagConstraints.NONE;
button = new JButton ("?");
gbc.gridx = GridBagConstraints.RELATIVE;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("Button 6");
gbc.gridy = 1;
panel.add (button, gbc);
return panel;
}
To span a component from its current cell to the next-to-last cell in its row, assign GridBagConstraints.RELATIVE to the gridwidth field and GridBagConstraints.HORIZONTAL to the fill field. The result is shown in Figure 5.
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Figure 5 Button 4 fills its entire display area up to (but not including) the last cell in its row.
Combination 5 demonstrates spanning a component, starting with the component’s cell, across all cells in the component’s row. Listing 5 shows the source code for this combination’s public JPanel buildGUI5() method.
Listing 5 buildGUI5().
public JPanel buildGUI5 ()
{
JPanel panel = new JPanel ();
panel.setLayout (new GridBagLayout ());
GridBagConstraints gbc = new GridBagConstraints ();
// Set global constraints.
gbc.insets = new Insets (5, 5, 0, 0);
JButton button = new JButton ("Button 1");
gbc.gridx = 0;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 2");
gbc.gridx = 1;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 3");
gbc.gridx = 2;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 4");
gbc.gridx = 0;
gbc.gridy = 1;
gbc.gridwidth = GridBagConstraints.REMAINDER;
gbc.fill = GridBagConstraints.HORIZONTAL;
panel.add (button, gbc);
// Reset to defaults.
gbc.gridwidth = 1;
gbc.fill = GridBagConstraints.NONE;
button = new JButton ("?");
gbc.gridx = GridBagConstraints.RELATIVE;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("Button 6");
gbc.gridy = 1;
panel.add (button, gbc);
return panel;
}
To span a component from its current cell to the last cell in its row, assign GridBagConstraints.REMAINDER to the gridwidth field and GridBagConstraints.HORIZONTAL to the fill field. The result is shown in Figure 6.
)
Figure 6 Button 4 fills its entire display area, up to and including the last cell in its row.
Buttons ? and 6 are not shown in Figure 6 because in its size calculations GridBagLayout ignores components added to a row after a component has been added whose gridwidth constraint is set to GridBagConstraints.REMAINDER. You must enlarge the GUI to see these buttons.
Combination 6 increases the display area of button 1 from one cell to two cells. Although button 2 is absent, the grid remains set to two rows by three columns. Listing 6 shows the source code for this combination’s public JPanel buildGUI6() method.
Listing 6 buildGUI6().
public JPanel buildGUI6 ()
{
JPanel panel = new JPanel ();
panel.setLayout (new GridBagLayout ());
GridBagConstraints gbc = new GridBagConstraints ();
// Set global constraints.
gbc.insets = new Insets (5, 5, 0, 0);
JButton button = new JButton ("Button 1");
gbc.gridx = 0;
gbc.gridy = 0;
gbc.gridwidth = 2;
panel.add (button, gbc);
// Reset to defaults.
gbc.gridwidth = 1;
button = new JButton ("Button 3");
gbc.gridx = 2;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 4");
gbc.gridx = 0;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("?");
gbc.gridx = 1;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("Button 6");
gbc.gridx = 2;
gbc.gridy = 1;
panel.add (button, gbc);
return panel;
}
The display area of button 1 is increased to two cells by assigning 2 to its gridwidth constraint variable. (This variable is later reset to its default of 1 so that subsequent buttons are not affected.) Figure 7 shows this button centered in its display area.
)
Figure 7 The display area for button 1 occupies two cells.
Combination 7 is nearly identical to combination 6. The only difference is the presence of button 2, which occupies part of the expanded display area of button 1. The source code for this combination’s public JPanel buildGUI7() method is shown in Listing 7.
Listing 7 buildGUI7().
public JPanel buildGUI7 ()
{
JPanel panel = new JPanel ();
panel.setLayout (new GridBagLayout ());
GridBagConstraints gbc = new GridBagConstraints ();
// Set global constraints.
gbc.insets = new Insets (5, 5, 0, 0);
JButton button = new JButton ("Button 1");
gbc.gridx = 0;
gbc.gridy = 0;
gbc.gridwidth = 2;
panel.add (button, gbc);
// Reset to defaults.
gbc.gridwidth = 1;
button = new JButton ("Button 2");
gbc.gridx = 1;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 3");
gbc.gridx = 2;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 4");
gbc.gridx = 0;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("?");
gbc.gridx = 1;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("Button 6");
gbc.gridx = 2;
gbc.gridy = 1;
panel.add (button, gbc);
return panel;
}
When a component is added to a previously added component’s display area, GridBagLayout overlaps the subsequently added component with the previously added component. In Figure 8, the previously added button 1 overlaps the subsequently added button 2.
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Figure 8 Button 1 overlaps button 2.
Combination 8 demonstrates that assigning a positive integer to gridwidth (and also to gridheight) to increase the display area can be problematic. Listing 8 shows the source code for this combination’s public JPanel buildGUI8() method.
Listing 8 buildGUI8().
public JPanel buildGUI8 ()
{
JPanel panel = new JPanel ();
panel.setLayout (new GridBagLayout ());
GridBagConstraints gbc = new GridBagConstraints ();
// Set global constraints.
gbc.insets = new Insets (5, 5, 0, 0);
JButton button = new JButton ("Button 1");
gbc.gridx = 0;
gbc.gridy = 0;
gbc.gridwidth = 2;
gbc.gridheight = 2;
gbc.fill = GridBagConstraints.BOTH;
panel.add (button, gbc);
return panel;
}
Unless components surround a component whose display area is to be increased, GridBagLayout doesn’t increase the size of the display area. As Figure 9illustrates, GridBagLayout hasn’t increased the display area of button 1 to two cells by two cells.
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Figure 9 Button 1 did not resize as expected.
Combination 9 corrects the problem with combination 8 by surrounding button 1 (on the right and below) with four button components. Listing 9 shows the source code for this combination’s public JPanel buildGUI9() method.
Listing 9 buildGUI9().
public JPanel buildGUI9 ()
{
JPanel panel = new JPanel ();
panel.setLayout (new GridBagLayout ());
GridBagConstraints gbc = new GridBagConstraints ();
// Set global constraints.
gbc.insets = new Insets (5, 5, 0, 0);
JButton button = new JButton ("Button 1");
gbc.gridx = 0;
gbc.gridy = 0;
gbc.gridwidth = 2;
gbc.gridheight = 2;
gbc.fill = GridBagConstraints.BOTH;
panel.add (button, gbc);
// Reset to defaults.
gbc.gridwidth = 1;
gbc.gridheight = 1;
gbc.fill = GridBagConstraints.NONE;
button = new JButton ("Button 2");
gbc.gridx = 2;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 3");
gbc.gridx = 2;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("Button 4");
gbc.gridx = 0;
gbc.gridy = 2;
panel.add (button, gbc);
button = new JButton ("Button 5");
gbc.gridx = 1;
gbc.gridy = 2;
panel.add (button, gbc);
return panel;
}
GridBaglayout checks for a button immediately to the right of, on each row of, immediately below, and on each column of the display area for button 1. The presence of these buttons results in button 1 obtaining (and sizing to) its two-row by two-column display area, as shown in Figure 10.
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Figure 10 After being surrounded by other buttons, button 1 resizes appropriately.
The final combination demonstrates changing the anchor value of button 1 and using weights, which results in a layout that’s very similar to the layout shown earlier in Figure 7. The source code for this combination’s public JPanel buildGUI10() method is shown in Listing 10.
Listing 10 buildGUI10().
public JPanel buildGUI10 ()
{
JPanel panel = new JPanel ();
panel.setLayout (new GridBagLayout ());
GridBagConstraints gbc = new GridBagConstraints ();
// Set global constraints.
gbc.insets = new Insets (5, 5, 0, 0);
gbc.weightx = 1.0;
gbc.weighty = 1.0;
JButton button = new JButton ("Button 1");
gbc.gridx = 0;
gbc.gridy = 0;
gbc.gridwidth = 2;
gbc.anchor = GridBagConstraints.LINE_END;
panel.add (button, gbc);
// Reset to defaults.
gbc.gridwidth = 1;
gbc.anchor = GridBagConstraints.CENTER;
button = new JButton ("Button 3");
gbc.gridx = 2;
gbc.gridy = 0;
panel.add (button, gbc);
button = new JButton ("Button 4");
gbc.gridx = 0;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("?");
gbc.gridx = 1;
gbc.gridy = 1;
panel.add (button, gbc);
button = new JButton ("Button 6");
gbc.gridx = 2;
gbc.gridy = 1;
panel.add (button, gbc);
return panel;
}
Figures 1–5, 7, and 8 reveal the default center anchor constraint. For variety, I’ve repositioned button 1 in its display area via the following line (see Figure 11):
gbc.anchor = GridBagConstraints.LINE_END;
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Figure 11 Button 1 appears on the right side of its display area.
Figure 2 revealed buttons clumped together in their container’s center because the weight constraints were ignored. Because buildGUI10() assigns equal values to these constraints for all buttons, extra space is distributed equally among the buttons, as Figure 12 shows.
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Figure 12 Extra space is distributed equally among all buttons when the GUI in Figure 11 is resized.