Altho GridBagLayout produces the best general results when components must be aligned, it's not a happy story.
GridBagLayout can do a very good job of layout for individual panels, but windows are often built from several nested panels, each with its own layout manager.
The underlying idea is rows and columns, similar to HTML tables.
GridBagLayout performs three functions using values from the
GridBagConstraints parameter in the add() method.
The names [note the case violations of the standard Java naming convention]
are the following.
gridx, gridy, gridwidth, and gridheight.
fill,
anchor,
insets,
ipadx, and
ipady.
weightx, and
weighty.
JPanel p = new JPanel(new GridBagLayout());
or
JPanel p = new JPanel(); p.setLayout(new GridBagLayout());
Do not give the number of rows and columns when you create a new GridBagLayout. The are computed by the layout manager.
Components are added to JPanel p like this:
p.add(component, constraintObj);
Constraint parameters are instance fields in a GridBagConstraints object. The add() method clones (copies) the constraint object, so typically the same GridBagConstraints object is reused to pass constraints for the next component. It is common to use only one GridBagConstraints object to add all components, changing fields as necessary between adds, but this practice also leads to many errors and code which is difficult to change.
The fields can divided into three groups:
gridx, gridy, gridwidth, and gridheight.
weightx and weighty to 1.0; you will rarely want to use other values.
Use fill depending on what the type of component is.
For example, a JTextField can usually grow horizontally, so you would use
gbc.fill = GridBagConstraints.HORIZONTAL;to allow it to stretch.
Use anchor to tell which edges of the display area a
component should be attached to.
ipadx and ipady.
To control the amount of unused space around a component, use insets.
gridx |
The int column (gridx) and row (gridy)
of the component. If requires more than
one cell (gridwidth or gridheight > 1), this is the
coordinate of the top-left cell. The row and columns start
at zero. The value GridBagConstraints.RELATIVE
places the component in the next position.
|
gridwidth |
Number of columns or rows the component occupies.
GridBagConstraints.REMAINDER indicates that
this component should fill out all rows or columns to the end.
Default 1. Note these names violates naming standards (second
word not capitalized).
|
weightx |
These double variables (default value 0) are used in calculating where
space is allocated in rows and columns when a window is resized.
extra space should be allocated in a column horizontally (weightx) and
row vertically (weighty).
A column width is originally calculated as the maximum width of
the preferred sizes of the components in that column. If a component
is narrower than the column width, the value of the Extra horizontal space is allocated to a column
in proportion to the maximum |
fill |
Fill specifies how the component should expand within its display area
if the area width/height is larger than its preferred size.
GridBagConstraints.NONE // Can't expand (Default) GridBagConstraints.VERTICAL // Expand vertically GridBagConstraints.HORIZONTAL // Expand horizontally GridBagConstraints.BOTH // Expand vertically and horizontally For example, a text field typically expands horizontally
( |
anchor |
If the component doesn't occupy its entire display area, anchor
specifies where it should be placed.
The location is usually given as a compass direction.
(A relative system which works for both right-to-left
and left-to-right languages is also available).
GridBagConstraints.CENTER (the default), GridBagConstraints.NORTH GridBagConstraints.SOUTH GridBagConstraints.NORTHEAST GridBagConstraints.SOUTHWEST GridBagConstraints.EAST GridBagConstraints.WEST GridBagConstraints.SOUTHEAST GridBagConstraints.NORTHWEST |
insets |
A java.awt.Insets object adds padding space to the component.
Insets should be rarely used because they often produce bad results,
with the exception of JPanels and JLabels.
The constructor parameters
(in pixels) specify the top, left, bottom, right.
For example,
gbc.insets = new Insets(10, 5, 10, 4);Default value: Insets(0,0,0,0). |
ipadx |
These int fields specify an increase or decrease in the horizontal and/or vertical preferred size of the component. Default value 0. Negative values can be used to tighten the spacing. These values are rarely useful, altho they can be used for fine-tuning spacing. |
It's common to write a utility method which sets the fields of a GridBagConstraints object. For example,
GridBagConstraints gbc = new GridBagConstraints();
. . .
private void set_gbc(int row, int column, int width, int height, int fill) {
gbc.gridy = row;
gbc.gridx = column;
gbc.gridwidth = width;
gbc.gridheight = height;
gbc.fill = fill; // GridBagConstraints.NONE .HORIZONTAL .VERTICAL .BOTH
// leave other fields (eg, anchor) unchanged.
}
. . .
set_gbc(3, 2, 2, 1, GridBagConstraints.HORIZONTAL);
gbc.insets = new Insets(3,3,3,3); // put spacing around this field.
p.add(myTextField, gbc);
To add myField to the third row (2) and first column (0), where the button will be one grid cell wide and one high, and it will be able to expand horizontally, do this:
set_gbc(2, 0, 1, 1, GridBagConstraints.HORIZONTAL); gridbag.setConstraints(myField, gbc); panel.add(myField);
GridBagLayout is not a nice piece of software. The GridBagConstraints object is awkward to use. It allows you to make careless mistakes in the layout without warnings or error messages. There is little good documentation on how the parameters are used. A better version of it would be very welcome.
Consider using a better layout manager. See Alternate Layout Managers.