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:

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 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();
     }
 }
 

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