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How to Use GridBagLayout: Specifying Constraints

Below is some of the code you'll typically see in a container that uses a GridBagLayout(in the API reference documentation). (You'll see an fleshed out example on the next page.)
GridBagLayout gridbag = new GridBagLayout();
GridBagConstraints c = new GridBagConstraints();

//For each component to be added to this container:
//...Create the component...
//...Set instance variables in the GridBagConstraints instance...
gridbag.setConstraints(theComponent, c);
As you might have guessed from the above example, you can reuse the same GridBagConstraints instance for multiple components, even if the components have different constraints. The GridBagLayout extracts the constraint values and doesn't use the GridBagConstraints again. You must be careful, however, to reset the GridBagConstraints instance variables to their default values when necessary.

You can set the following GridBagConstraints(in the API reference documentation) instance variables:

gridx, gridy
Specify the row and column at the upper left of the component. The leftmost column has address gridx=0, and the top row has address gridy=0. Use GridBagConstraints.RELATIVE (the default value) to specify that the component be placed just to the right of (for gridx) or just below (for gridy) the component that was added to the container just before this component was added.

gridwidth, gridheight
Specify the number of columns (for gridwidth) or rows (for gridheight) in the component's display area. These constraints specify the number of cells the component uses, not the number of pixels it uses. The default value is 1. Use GridBagConstraints.REMAINDER to specify that the component be the last one in its row (for gridwidth) or column (for gridheight). Use GridBagConstraints.RELATIVE to specify that the component be the next to last one in its row (for gridwidth) or column (for gridheight).

Note: Due to a bug in the the 1.0 release of Java, GridBagLayout doesn't allow components to span multiple rows unless the component is in the leftmost column.

Used when the component's display area is larger than the component's requested size to determine whether and how to resize the component. Valid 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).

ipadx, ipady
Specifies the internal padding: how much to add to the minimum size of the component. The default value is zero. The width of the component will be at least its minimum width plus ipadx*2 pixels (since the padding applies to both sides of the component). Similarly, the height of the component will be at least its minimum height plus ipady*2 pixels.

Specifies the external padding of the component -- the minimum amount of space between the component and the edges of its display area. The value is specified as an Insets(in the API reference documentation) object. By default, each component has no external padding.

Used when the component is smaller than its display area to determine where (within the area) to place the component. Valid values are GridBagConstraints.CENTER (the default), GridBagConstraints.NORTH, GridBagConstraints.NORTHEAST, GridBagConstraints.EAST, GridBagConstraints.SOUTHEAST, GridBagConstraints.SOUTH, GridBagConstraints.SOUTHWEST, GridBagConstraints.WEST, and GridBagConstraints.NORTHWEST.

weightx, weighty
Specifying weights is an art that can have a significant impact on the appearance of the components a GridBagLayout controls. Weights are used to determine how to distribute space among columns (weightx) and among rows (weighty); this is important for specifying resizing behavior.

Unless you specify at least one nonzero value for weightx or weighty, all the components clump together in the center of their container. This is because when the weight is 0.0 (the default), the GridBagLayout puts any extra space between its grid of cells and the edges of the container.

Generally weights are specified with 0.0 and 1.0 as the extremes, with numbers in between used as necessary. Larger numbers indicate that the component's row or column should get more space. For each column, the weight is related to the highest weightx specified for a component within that column (with each multi-column component's weight being split somehow between the columns the component is in). Similarly, each row's weight is related to the highest weighty specified for a component within that row. Extra space tends to go toward the rightmost column and bottom row.

The next page discusses constraints in depth, in the context of explaining how the example applet works.

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