java.awt
Class GridBagLayout

java.lang.Object
  |
  +--java.awt.GridBagLayout

All Implemented Interfaces:

LayoutManager, LayoutManager2, Serializable

public class GridBagLayout

extends Object

implements LayoutManager2, Serializable

The GridBagLayout class is a flexible layout manager that aligns components vertically and horizontally, 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 grid bag layout is associated with an instance of GridBagConstraints that specifies how the component is laid out within its display area.

How a GridBagLayout object places a set of components depends on the GridBagConstraints object associated with each component, and on the minimum size and the preferred size of the components' containers.

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 at the upper left of the component's display area, where the upper-left-most cell has address gridx = 0, gridy = 0. Use GridBagConstraints.RELATIVE (the default value) to specify that the component be just 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.

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 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).

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 * 2) pixels (since the padding applies to both sides of the component). Similarly, the height of the component will be at least the minimum height plus (ipady * 2) 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

Used when the component is smaller than its display area to determine where (within the display 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.

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.

The following figure shows ten components (all buttons) managed by a grid bag layout:

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("Helvetica", 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();
     }
 }
 

Since:

JDK1.0

See Also:

GridBagConstraints, Serialized Form

Field Summary

double[]	columnWeights This field holds the overrides to the column weights.
int[]	columnWidths This field holds the overrides to the column minimum width.
protected Hashtable	comptable This hashtable maintains the association between a component and its gridbag constraints.
protected GridBagConstraints	defaultConstraints This field holds a gridbag constraints instance containing the default values, so if a component does not have gridbag constraints associated with it, then the component will be assigned a copy of the defaultConstraints.
protected static int	MAXGRIDSIZE The maximum number of grid positions (both horizontally and vertically) that can be laid out by the grid bag layout.
protected static int	MINSIZE The smallest grid that can be laid out by the grid bag layout.
protected static int	PREFERREDSIZE
int[]	rowHeights This field holds the overrides to the row minimum heights.
double[]	rowWeights This field holds the overrides to the row weights.

Constructor Summary

GridBagLayout()
Creates a grid bag layout manager.

Method Summary

void	addLayoutComponent(Component comp, Object constraints) Adds the specified component to the layout, using the specified constraint object.
void	addLayoutComponent(String name, Component comp) Adds the specified component with the specified name to the layout.
protected void	AdjustForGravity(GridBagConstraints constraints, Rectangle r)
protected void	ArrangeGrid(Container parent)
GridBagConstraints	getConstraints(Component comp) Gets the constraints for the specified component.
float	getLayoutAlignmentX(Container parent) Returns the alignment along the x axis.
float	getLayoutAlignmentY(Container parent) Returns the alignment along the y axis.
int[][]	getLayoutDimensions() Determines column widths and row heights for the layout grid.
Point	getLayoutOrigin() Determines the origin of the layout grid.
double[][]	getLayoutWeights() Determines the weights of the layout grid's columns and rows.
void	invalidateLayout(Container target) Invalidates the layout, indicating that if the layout manager has cached information it should be discarded.
void	layoutContainer(Container parent) Lays out the specified container using this grid bag layout.
Point	location(int x, int y) Determines which cell in the layout grid contains the point specified by (x, y).
protected GridBagConstraints	lookupConstraints(Component comp) Retrieves the constraints for the specified component.
Dimension	maximumLayoutSize(Container target) Returns the maximum dimensions for this layout given the components in the specified target container.
Dimension	minimumLayoutSize(Container parent) Determines the minimum size of the target container using this grid bag layout.
Dimension	preferredLayoutSize(Container parent) Determines the preferred size of the target container using this grid bag layout.
void	removeLayoutComponent(Component comp) Removes the specified component from this layout.
void	setConstraints(Component comp, GridBagConstraints constraints) Sets the constraints for the specified component in this layout.
String	toString() Returns a string representation of this grid bag layout's values.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

MAXGRIDSIZE

protected static final int MAXGRIDSIZE

The maximum number of grid positions (both horizontally and vertically) that can be laid out by the grid bag layout.

See Also:

Constant Field Values

MINSIZE

protected static final int MINSIZE

The smallest grid that can be laid out by the grid bag layout.

See Also:

Constant Field Values

PREFERREDSIZE

protected static final int PREFERREDSIZE

See Also:

Constant Field Values

comptable

protected Hashtable comptable

This hashtable maintains the association between a component and its gridbag constraints. The Keys in comptable are the components and the values are the instances of GridBagConstraints.

See Also:

GridBagConstraints

defaultConstraints

protected GridBagConstraints defaultConstraints

This field holds a gridbag constraints instance containing the default values, so if a component does not have gridbag constraints associated with it, then the component will be assigned a copy of the defaultConstraints.

See Also:

getConstraints(Component), setConstraints(Component, GridBagConstraints), lookupConstraints(Component)

columnWidths

public int[] columnWidths

This field holds the overrides to the column minimum width. If this field is non-null the values are applied to the gridbag after all of the minimum columns widths have been calculated. If columnWidths has more elements than the number of columns, columns are added to the gridbag to match the number of elements in columnWidth.

See Also:

getLayoutDimensions()

rowHeights

public int[] rowHeights

This field holds the overrides to the row minimum heights. If this field is non-null the values are applied to the gridbag after all of the minimum row heights have been calculated. If rowHeights has more elements than the number of rows, rowa are added to the gridbag to match the number of elements in rowHeights.

See Also:

getLayoutDimensions()

columnWeights

public double[] columnWeights

This field holds the overrides to the column weights. If this field is non-null the values are applied to the gridbag after all of the columns weights have been calculated. If columnWeights[i] > weight for column i, then column i is assigned the weight in columnWeights[i]. If columnWeights has more elements than the number of columns, the excess elements are ignored - they do not cause more columns to be created.

rowWeights

public double[] rowWeights

This field holds the overrides to the row weights. If this field is non-null the values are applied to the gridbag after all of the rows weights have been calculated. If rowWeights[i] > weight for row i, then row i is assigned the weight in rowWeights[i]. If rowWeights has more elements than the number of rows, the excess elements are ignored - they do not cause more rows to be created.

Constructor Detail

GridBagLayout

public GridBagLayout()

Creates a grid bag layout manager.

Method Detail

setConstraints

public void setConstraints(Component comp,
                           GridBagConstraints constraints)

Sets the constraints for the specified component in this layout.

Parameters:

comp - the component to be modified.

constraints - the constraints to be applied.

getConstraints

public GridBagConstraints getConstraints(Component comp)

Gets the constraints for the specified component. A copy of the actual GridBagConstraints object is returned.

Parameters:

comp - the component to be queried.

Returns:

the constraint for the specified component in this grid bag layout; a copy of the actual constraint object is returned.

lookupConstraints

protected GridBagConstraints lookupConstraints(Component comp)

Retrieves the constraints for the specified component. The return value is not a copy, but is the actual GridBagConstraints object used by the layout mechanism.

Parameters:

comp - the component to be queried

Returns:

the contraints for the specified component.

getLayoutOrigin

public Point getLayoutOrigin()

Determines the origin of the layout grid. Most applications do not call this method directly.

Returns:

the origin of the cell in the top-left corner of the layout grid.

Since:

JDK1.1

getLayoutDimensions

public int[][] getLayoutDimensions()

Determines column widths and row heights for the layout grid.

Most applications do not call this method directly.

Returns:

an array of two arrays, containing the widths of the layout columns and the heights of the layout rows.

Since:

JDK1.1

getLayoutWeights

public double[][] getLayoutWeights()

Determines the weights of the layout grid's columns and rows. Weights are used to calculate how much a given column or row stretches beyond its preferred size, if the layout has extra room to fill.

Most applications do not call this method directly.

Returns:

an array of two arrays, representing the horizontal weights of the layout columns and the vertical weights of the layout rows.

Since:

JDK1.1

location

public Point location(int x,
                      int y)

Determines which cell in the layout grid contains the point specified by (x, y). Each cell is identified by its column index (ranging from 0 to the number of columns minus 1) and its row index (ranging from 0 to the number of rows minus 1).

If the (x, y) point lies outside the grid, the following rules are used. The column index is returned as zero if x lies to the left of the layout, and as the number of columns if x lies to the right of the layout. The row index is returned as zero if y lies above the layout, and as the number of rows if y lies below the layout.

Parameters:

x - the x coordinate of a point.

y - the y coordinate of a point.

Returns:

an ordered pair of indexes that indicate which cell in the layout grid contains the point (x, y).

Since:

JDK1.1

addLayoutComponent

public void addLayoutComponent(String name,
                               Component comp)

Adds the specified component with the specified name to the layout.

Specified by:

addLayoutComponent in interface LayoutManager

Parameters:

name - the name of the component.

comp - the component to be added.

addLayoutComponent

public void addLayoutComponent(Component comp,
                               Object constraints)

Adds the specified component to the layout, using the specified constraint object.

Specified by:

addLayoutComponent in interface LayoutManager2

Parameters:

comp - the component to be added.

constraints - an object that determines how the component is added to the layout.

removeLayoutComponent

public void removeLayoutComponent(Component comp)

Removes the specified component from this layout.

Most applications do not call this method directly.

Specified by:

removeLayoutComponent in interface LayoutManager

Parameters:

comp - the component to be removed.

See Also:

Container.remove(java.awt.Component), Container.removeAll()

preferredLayoutSize

public Dimension preferredLayoutSize(Container parent)

Determines the preferred size of the target container using this grid bag layout.

Most applications do not call this method directly.

Specified by:

preferredLayoutSize in interface LayoutManager

Parameters:

parent - the component to be laid out

See Also:

Container.getPreferredSize()

minimumLayoutSize

public Dimension minimumLayoutSize(Container parent)

Determines the minimum size of the target container using this grid bag layout.

Most applications do not call this method directly.

Specified by:

minimumLayoutSize in interface LayoutManager

Parameters:

parent - the component to be laid out

See Also:

Container.doLayout()

maximumLayoutSize

public Dimension maximumLayoutSize(Container target)

Returns the maximum dimensions for this layout given the components in the specified target container.

Specified by:

maximumLayoutSize in interface LayoutManager2

Parameters:

target - the component which needs to be laid out

See Also:

Container, minimumLayoutSize(Container), preferredLayoutSize(Container)

getLayoutAlignmentX

public float getLayoutAlignmentX(Container parent)

Returns the alignment along the x axis. This specifies how the component would like to be aligned relative to other components. The value should be a number between 0 and 1 where 0 represents alignment along the origin, 1 is aligned the furthest away from the origin, 0.5 is centered, etc.

Specified by:

getLayoutAlignmentX in interface LayoutManager2

getLayoutAlignmentY

public float getLayoutAlignmentY(Container parent)

Returns the alignment along the y axis. This specifies how the component would like to be aligned relative to other components. The value should be a number between 0 and 1 where 0 represents alignment along the origin, 1 is aligned the furthest away from the origin, 0.5 is centered, etc.

Specified by:

getLayoutAlignmentY in interface LayoutManager2

invalidateLayout

public void invalidateLayout(Container target)

Invalidates the layout, indicating that if the layout manager has cached information it should be discarded.

Specified by:

invalidateLayout in interface LayoutManager2

layoutContainer

public void layoutContainer(Container parent)

Lays out the specified container using this grid bag layout. This method reshapes components in the specified container in order to satisfy the contraints of this GridBagLayout object.

Most applications do not call this method directly.

Specified by:

layoutContainer in interface LayoutManager

Parameters:

parent - the container in which to do the layout.

See Also:

Container, Container.doLayout()

toString

public String toString()

Returns a string representation of this grid bag layout's values.

Overrides:

toString in class Object

Returns:

a string representation of this grid bag layout.

AdjustForGravity

protected void AdjustForGravity(GridBagConstraints constraints,
                                Rectangle r)

ArrangeGrid

protected void ArrangeGrid(Container parent)