The
GridBagLayout
class is a flexible layout
manager that aligns components vertically, horizontally or along their
baseline without requiring that the components be of the same size.
Each
GridBagLayout
object maintains a dynamic,
rectangular grid of cells, with each component occupying
one or more cells, called its
display area.
Each component managed by a GridBagLayout
is associated with
an instance of GridBagConstraints
. The constraints object
specifies where a component's display area should be located on the grid
and how the component should be positioned within its display area. In
addition to its constraints object, the GridBagLayout
also
considers each component's minimum and preferred sizes in order to
determine a component's size.
The overall orientation of the grid depends on the container's
ComponentOrientation
property. For horizontal left-to-right
orientations, grid coordinate (0,0) is in the upper left corner of the
container with x increasing to the right and y increasing downward. For
horizontal right-to-left orientations, grid coordinate (0,0) is in the upper
right corner of the container with x increasing to the left and y
increasing downward.
To use a grid bag layout effectively, you must customize one or more
of the GridBagConstraints
objects that are associated
with its components. You customize a GridBagConstraints
object by setting one or more of its instance variables:
GridBagConstraints.gridx
,
GridBagConstraints.gridy
- Specifies the cell containing the leading corner of the component's
display area, where the cell at the origin of the grid has address
gridx = 0
,
gridy = 0
. For horizontal left-to-right layout,
a component's leading corner is its upper left. For horizontal
right-to-left layout, a component's leading corner is its upper right.
Use GridBagConstraints.RELATIVE
(the default value)
to specify that the component be placed immediately following
(along the x axis for gridx
or the y axis for
gridy
) the component that was added to the container
just before this component was added.
GridBagConstraints.gridwidth
,
GridBagConstraints.gridheight
- Specifies the number of cells in a row (for
gridwidth
)
or column (for gridheight
)
in the component's display area.
The default value is 1.
Use GridBagConstraints.REMAINDER
to specify
that the component's display area will be from gridx
to the last cell in the row (for gridwidth
)
or from gridy
to the last cell in the column
(for gridheight
).
Use GridBagConstraints.RELATIVE
to specify
that the component's display area will be from gridx
to the next to the last cell in its row (for gridwidth
or from gridy
to the next to the last cell in its
column (for gridheight
).
GridBagConstraints.fill
- Used when the component's display area
is larger than the component's requested size
to determine whether (and how) to resize the component.
Possible values are
GridBagConstraints.NONE
(the default),
GridBagConstraints.HORIZONTAL
(make the component wide enough to fill its display area
horizontally, but don't change its height),
GridBagConstraints.VERTICAL
(make the component tall enough to fill its display area
vertically, but don't change its width), and
GridBagConstraints.BOTH
(make the component fill its display area entirely).
GridBagConstraints.ipadx
,
GridBagConstraints.ipady
- Specifies the component's internal padding within the layout,
how much to add to the minimum size of the component.
The width of the component will be at least its minimum width
plus
ipadx
pixels. Similarly, the height of
the component will be at least the minimum height plus
ipady
pixels.
GridBagConstraints.insets
- Specifies the component's external padding, the minimum
amount of space between the component and the edges of its display area.
GridBagConstraints.anchor
- Specifies where the component should be positioned in its display area.
There are three kinds of possible values: absolute, orientation-relative,
and baseline-relative
Orientation relative values are interpreted relative to the container's
ComponentOrientation
property while absolute values
are not. Baseline relative values are calculated relative to the
baseline. Valid values are:
Absolute Values |
Orientation Relative Values |
Baseline Relative Values |
GridBagConstraints.NORTH
GridBagConstraints.SOUTH
GridBagConstraints.WEST
GridBagConstraints.EAST
GridBagConstraints.NORTHWEST
GridBagConstraints.NORTHEAST
GridBagConstraints.SOUTHWEST
GridBagConstraints.SOUTHEAST
GridBagConstraints.CENTER (the default)
|
GridBagConstraints.PAGE_START
GridBagConstraints.PAGE_END
GridBagConstraints.LINE_START
GridBagConstraints.LINE_END
GridBagConstraints.FIRST_LINE_START
GridBagConstraints.FIRST_LINE_END
GridBagConstraints.LAST_LINE_START
GridBagConstraints.LAST_LINE_END
|
GridBagConstraints.BASELINE
GridBagConstraints.BASELINE_LEADING
GridBagConstraints.BASELINE_TRAILING
GridBagConstraints.ABOVE_BASELINE
GridBagConstraints.ABOVE_BASELINE_LEADING
GridBagConstraints.ABOVE_BASELINE_TRAILING
GridBagConstraints.BELOW_BASELINE
GridBagConstraints.BELOW_BASELINE_LEADING
GridBagConstraints.BELOW_BASELINE_TRAILING
|
GridBagConstraints.weightx
,
GridBagConstraints.weighty
- Used to determine how to distribute space, which is
important for specifying resizing behavior.
Unless you specify a weight for at least one component
in a row (
weightx
) and column (weighty
),
all the components clump together in the center of their container.
This is because when the weight is zero (the default),
the GridBagLayout
object puts any extra space
between its grid of cells and the edges of the container.
Each row may have a baseline; the baseline is determined by the
components in that row that have a valid baseline and are aligned
along the baseline (the component's anchor value is one of BASELINE
, BASELINE_LEADING
or BASELINE_TRAILING
).
If none of the components in the row has a valid baseline, the row
does not have a baseline.
If a component spans rows it is aligned either to the baseline of
the start row (if the baseline-resize behavior is CONSTANT_ASCENT
) or the end row (if the baseline-resize behavior
is CONSTANT_DESCENT
). The row that the component is
aligned to is called the prevailing row.
The following figure shows a baseline layout and includes a
component that spans rows:
This layout consists of three components:
- A panel that starts in row 0 and ends in row 1. The panel
has a baseline-resize behavior of
CONSTANT_DESCENT
and has
an anchor of BASELINE
. As the baseline-resize behavior
is CONSTANT_DESCENT
the prevailing row for the panel is
row 1.
- Two buttons, each with a baseline-resize behavior of
CENTER_OFFSET
and an anchor of BASELINE
.
Because the second button and the panel share the same prevailing row,
they are both aligned along their baseline.
Components positioned using one of the baseline-relative values resize
differently than when positioned using an absolute or orientation-relative
value. How components change is dictated by how the baseline of the
prevailing row changes. The baseline is anchored to the
bottom of the display area if any components with the same prevailing row
have a baseline-resize behavior of CONSTANT_DESCENT
,
otherwise the baseline is anchored to the top of the display area.
The following rules dictate the resize behavior:
- Resizable components positioned above the baseline can only
grow as tall as the baseline. For example, if the baseline is at 100
and anchored at the top, a resizable component positioned above the
baseline can never grow more than 100 units.
- Similarly, resizable components positioned below the baseline can
only grow as high as the difference between the display height and the
baseline.
- Resizable components positioned on the baseline with a
baseline-resize behavior of
OTHER
are only resized if
the baseline at the resized size fits within the display area. If
the baseline is such that it does not fit within the display area
the component is not resized.
- Components positioned on the baseline that do not have a
baseline-resize behavior of
OTHER
can only grow as tall as display height - baseline + baseline of component
.
If you position a component along the baseline, but the
component does not have a valid baseline, it will be vertically centered
in its space. Similarly if you have positioned a component relative
to the baseline and none of the components in the row have a valid
baseline the component is vertically centered.
The following figures show ten components (all buttons)
managed by a grid bag layout. Figure 2 shows the layout for a horizontal,
left-to-right container and Figure 3 shows the layout for a horizontal,
right-to-left container.
|
|
Figure 2: Horizontal, Left-to-Right |
Figure 3: Horizontal, Right-to-Left |
Each of the ten components has the fill
field
of its associated GridBagConstraints
object
set to GridBagConstraints.BOTH
.
In addition, the components have the following non-default constraints:
- Button1, Button2, Button3:
weightx = 1.0
- Button4:
weightx = 1.0
,
gridwidth = GridBagConstraints.REMAINDER
- Button5:
gridwidth = GridBagConstraints.REMAINDER
- Button6:
gridwidth = GridBagConstraints.RELATIVE
- Button7:
gridwidth = GridBagConstraints.REMAINDER
- Button8:
gridheight = 2
,
weighty = 1.0
- Button9, Button 10:
gridwidth = GridBagConstraints.REMAINDER
Here is the code that implements the example shown above:
import java.awt.*;
import java.util.*;
import java.applet.Applet;
public class GridBagEx1 extends Applet {
protected void makebutton(String name,
GridBagLayout gridbag,
GridBagConstraints c) {
Button button = new Button(name);
gridbag.setConstraints(button, c);
add(button);
}
public void init() {
GridBagLayout gridbag = new GridBagLayout();
GridBagConstraints c = new GridBagConstraints();
setFont(new Font("SansSerif", Font.PLAIN, 14));
setLayout(gridbag);
c.fill = GridBagConstraints.BOTH;
c.weightx = 1.0;
makebutton("Button1", gridbag, c);
makebutton("Button2", gridbag, c);
makebutton("Button3", gridbag, c);
c.gridwidth = GridBagConstraints.REMAINDER; //end row
makebutton("Button4", gridbag, c);
c.weightx = 0.0; //reset to the default
makebutton("Button5", gridbag, c); //another row
c.gridwidth = GridBagConstraints.RELATIVE; //next-to-last in row
makebutton("Button6", gridbag, c);
c.gridwidth = GridBagConstraints.REMAINDER; //end row
makebutton("Button7", gridbag, c);
c.gridwidth = 1; //reset to the default
c.gridheight = 2;
c.weighty = 1.0;
makebutton("Button8", gridbag, c);
c.weighty = 0.0; //reset to the default
c.gridwidth = GridBagConstraints.REMAINDER; //end row
c.gridheight = 1; //reset to the default
makebutton("Button9", gridbag, c);
makebutton("Button10", gridbag, c);
setSize(300, 100);
}
public static void main(String args[]) {
Frame f = new Frame("GridBag Layout Example");
GridBagEx1 ex1 = new GridBagEx1();
ex1.init();
f.add("Center", ex1);
f.pack();
f.setSize(f.getPreferredSize());
f.show();
}
}