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BoxLayout.javaAPI DocJava SE 5 API18566Fri Aug 26 14:57:52 BST 2005javax.swing

BoxLayout

public class BoxLayout extends Object implements Serializable, LayoutManager2
A layout manager that allows multiple components to be laid out either vertically or horizontally. The components will not wrap so, for example, a vertical arrangement of components will stay vertically arranged when the frame is resized.

The following text describes this graphic.

Nesting multiple panels with different combinations of horizontal and vertical gives an effect similar to GridBagLayout, without the complexity. The diagram shows two panels arranged horizontally, each of which contains 3 components arranged vertically.

The BoxLayout manager is constructed with an axis parameter that specifies the type of layout that will be done. There are four choices:

X_AXIS - Components are laid out horizontally from left to right.
Y_AXIS - Components are laid out vertically from top to bottom.
LINE_AXIS - Components are laid out the way words are laid out in a line, based on the container's ComponentOrientation property. If the container's ComponentOrientation is horizontal then components are laid out horizontally, otherwise they are laid out vertically. For horizontal orientations, if the container's ComponentOrientation is left to right then components are laid out left to right, otherwise they are laid out right to left. For vertical orientations components are always laid out from top to bottom.
PAGE_AXIS - Components are laid out the way text lines are laid out on a page, based on the container's ComponentOrientation property. If the container's ComponentOrientation is horizontal then components are laid out vertically, otherwise they are laid out horizontally. For horizontal orientations, if the container's ComponentOrientation is left to right then components are laid out left to right, otherwise they are laid out right to left.  For vertical orientations components are always laid out from top to bottom.

For all directions, components are arranged in the same order as they were added to the container.

BoxLayout attempts to arrange components at their preferred widths (for horizontal layout) or heights (for vertical layout). For a horizontal layout, if not all the components are the same height, BoxLayout attempts to make all the components as high as the highest component. If that's not possible for a particular component, then BoxLayout aligns that component vertically, according to the component's Y alignment. By default, a component has a Y alignment of 0.5, which means that the vertical center of the component should have the same Y coordinate as the vertical centers of other components with 0.5 Y alignment.

Similarly, for a vertical layout, BoxLayout attempts to make all components in the column as wide as the widest component. If that fails, it aligns them horizontally according to their X alignments. For PAGE_AXIS layout, horizontal alignment is done based on the leading edge of the component. In other words, an X alignment value of 0.0 means the left edge of a component if the container's ComponentOrientation is left to right and it means the right edge of the component otherwise.

Instead of using BoxLayout directly, many programs use the Box class. The Box class is a lightweight container that uses a BoxLayout. It also provides handy methods to help you use BoxLayout well. Adding components to multiple nested boxes is a powerful way to get the arrangement you want.

For further information and examples see How to Use BoxLayout, a section in The Java Tutorial.

Warning: Serialized objects of this class will not be compatible with future Swing releases. The current serialization support is appropriate for short term storage or RMI between applications running the same version of Swing. As of 1.4, support for long term storage of all JavaBeansTM has been added to the java.beans package. Please see {@link java.beans.XMLEncoder}.

see
Box
see
java.awt.ComponentOrientation
see
JComponent#getAlignmentX
see
JComponent#getAlignmentY
author
Timothy Prinzing
version
1.34 12/19/03

Fields Summary
public static final int
X_AXIS
Specifies that components should be laid out left to right.
public static final int
Y_AXIS
Specifies that components should be laid out top to bottom.
public static final int
LINE_AXIS
Specifies that components should be laid out in the direction of a line of text as determined by the target container's ComponentOrientation property.
public static final int
PAGE_AXIS
Specifies that components should be laid out in the direction that lines flow across a page as determined by the target container's ComponentOrientation property.
private int
axis
private Container
target
private transient SizeRequirements[]
xChildren
private transient SizeRequirements[]
yChildren
private transient SizeRequirements
xTotal
private transient SizeRequirements
yTotal
private transient PrintStream
dbg
Constructors Summary
public BoxLayout(Container target, int axis)
Creates a layout manager that will lay out components along the given axis.

param
target the container that needs to be laid out
param
axis the axis to lay out components along. Can be one of: BoxLayout.X_AXIS, BoxLayout.Y_AXIS, BoxLayout.LINE_AXIS or BoxLayout.PAGE_AXIS
exception
AWTError if the value of axis is invalid


                                                                                                                     
         
        if (axis != X_AXIS && axis != Y_AXIS && 
            axis != LINE_AXIS && axis != PAGE_AXIS) {
            throw new AWTError("Invalid axis");
        }
        this.axis = axis;
        this.target = target;
BoxLayout(Container target, int axis, PrintStream dbg)
Constructs a BoxLayout that produces debugging messages.

param
target the container that needs to be laid out
param
axis the axis to lay out components along. Can be one of: BoxLayout.X_AXIS, BoxLayout.Y_AXIS, BoxLayout.LINE_AXIS or BoxLayout.PAGE_AXIS
param
dbg the stream to which debugging messages should be sent, null if none

        this(target, axis);
        this.dbg = dbg;
Methods Summary
public voidaddLayoutComponent(java.lang.String name, java.awt.Component comp)
Not used by this class.

param
name the name of the component
param
comp the component
public voidaddLayoutComponent(java.awt.Component comp, java.lang.Object constraints)
Not used by this class.

param
comp the component
param
constraints constraints
voidcheckContainer(java.awt.Container target)

        if (this.target != target) {
            throw new AWTError("BoxLayout can't be shared");
        }
voidcheckRequests()

        if (xChildren == null || yChildren == null) {
            // The requests have been invalidated... recalculate
            // the request information.
            int n = target.getComponentCount();
            xChildren = new SizeRequirements[n];
            yChildren = new SizeRequirements[n];
            for (int i = 0; i < n; i++) {
                Component c = target.getComponent(i);
		if (!c.isVisible()) {
		    xChildren[i] = new SizeRequirements(0,0,0, c.getAlignmentX());
		    yChildren[i] = new SizeRequirements(0,0,0, c.getAlignmentY());
		    continue;
		}
                Dimension min = c.getMinimumSize();
                Dimension typ = c.getPreferredSize();
                Dimension max = c.getMaximumSize();
                xChildren[i] = new SizeRequirements(min.width, typ.width, 
                                                    max.width, 
                                                    c.getAlignmentX());
                yChildren[i] = new SizeRequirements(min.height, typ.height, 
                                                    max.height, 
                                                    c.getAlignmentY());
            }
            
            // Resolve axis to an absolute value (either X_AXIS or Y_AXIS)
            int absoluteAxis = resolveAxis(axis,target.getComponentOrientation());

            if (absoluteAxis == X_AXIS) {
                xTotal = SizeRequirements.getTiledSizeRequirements(xChildren);
                yTotal = SizeRequirements.getAlignedSizeRequirements(yChildren);
            } else {
                xTotal = SizeRequirements.getAlignedSizeRequirements(xChildren);
                yTotal = SizeRequirements.getTiledSizeRequirements(yChildren);
            }
        }
public synchronized floatgetLayoutAlignmentX(java.awt.Container target)
Returns the alignment along the X axis for the container. If the box is horizontal, the default alignment will be returned. Otherwise, the alignment needed to place the children along the X axis will be returned.

param
target the container
return
the alignment >= 0.0f && <= 1.0f
exception
AWTError if the target isn't the container specified to the BoxLayout constructor

        checkContainer(target);
        checkRequests();
        return xTotal.alignment;
public synchronized floatgetLayoutAlignmentY(java.awt.Container target)
Returns the alignment along the Y axis for the container. If the box is vertical, the default alignment will be returned. Otherwise, the alignment needed to place the children along the Y axis will be returned.

param
target the container
return
the alignment >= 0.0f && <= 1.0f
exception
AWTError if the target isn't the container specified to the BoxLayout constructor

        checkContainer(target);
        checkRequests();
        return yTotal.alignment;
public synchronized voidinvalidateLayout(java.awt.Container target)
Indicates that a child has changed its layout related information, and thus any cached calculations should be flushed.

This method is called by AWT when the invalidate method is called on the Container. Since the invalidate method may be called asynchronously to the event thread, this method may be called asynchronously.

param
target the affected container
exception
AWTError if the target isn't the container specified to the BoxLayout constructor

        checkContainer(target);
        xChildren = null;
        yChildren = null;
        xTotal = null;
        yTotal = null;
public voidlayoutContainer(java.awt.Container target)
Called by the AWT when the specified container needs to be laid out.

param
target the container to lay out
exception
AWTError if the target isn't the container specified to the BoxLayout constructor

	checkContainer(target);
	int nChildren = target.getComponentCount();
	int[] xOffsets = new int[nChildren];
	int[] xSpans = new int[nChildren];
	int[] yOffsets = new int[nChildren];
	int[] ySpans = new int[nChildren];
	    
	Dimension alloc = target.getSize();
	Insets in = target.getInsets();
	alloc.width -= in.left + in.right;
	alloc.height -= in.top + in.bottom;

        // Resolve axis to an absolute value (either X_AXIS or Y_AXIS)
        ComponentOrientation o = target.getComponentOrientation();
        int absoluteAxis = resolveAxis( axis, o );
        boolean ltr = (absoluteAxis != axis) ? o.isLeftToRight() : true;


	// determine the child placements
	synchronized(this) {
	    checkRequests();
        
	    if (absoluteAxis == X_AXIS) {
		SizeRequirements.calculateTiledPositions(alloc.width, xTotal,
							 xChildren, xOffsets,
							 xSpans, ltr);
		SizeRequirements.calculateAlignedPositions(alloc.height, yTotal,
							   yChildren, yOffsets,
							   ySpans);
	    } else {
		SizeRequirements.calculateAlignedPositions(alloc.width, xTotal,
							   xChildren, xOffsets,
							   xSpans, ltr);
		SizeRequirements.calculateTiledPositions(alloc.height, yTotal,
							 yChildren, yOffsets,
							 ySpans);
	    }
	}

        // flush changes to the container
        for (int i = 0; i < nChildren; i++) {
            Component c = target.getComponent(i);
            c.setBounds((int) Math.min((long) in.left + (long) xOffsets[i], Integer.MAX_VALUE),
                        (int) Math.min((long) in.top + (long) yOffsets[i], Integer.MAX_VALUE),
                        xSpans[i], ySpans[i]);

        }
        if (dbg != null) {
            for (int i = 0; i < nChildren; i++) {
                Component c = target.getComponent(i);
                dbg.println(c.toString());
                dbg.println("X: " + xChildren[i]);
                dbg.println("Y: " + yChildren[i]);
            }
        }
public java.awt.DimensionmaximumLayoutSize(java.awt.Container target)
Returns the maximum dimensions the target container can use to lay out the components it contains.

param
target the container that needs to be laid out
return
the dimenions >= 0 && <= Integer.MAX_VALUE
exception
AWTError if the target isn't the container specified to the BoxLayout constructor
see
#preferredLayoutSize
see
#minimumLayoutSize

	Dimension size;
	synchronized(this) {
	    checkContainer(target);
	    checkRequests();
	    size = new Dimension(xTotal.maximum, yTotal.maximum);
	}

        Insets insets = target.getInsets();
        size.width = (int) Math.min((long) size.width + (long) insets.left + (long) insets.right, Integer.MAX_VALUE);
        size.height = (int) Math.min((long) size.height + (long) insets.top + (long) insets.bottom, Integer.MAX_VALUE);
        return size;
public java.awt.DimensionminimumLayoutSize(java.awt.Container target)
Returns the minimum dimensions needed to lay out the components contained in the specified target container.

param
target the container that needs to be laid out
return
the dimensions >= 0 && <= Integer.MAX_VALUE
exception
AWTError if the target isn't the container specified to the BoxLayout constructor
see
#preferredLayoutSize
see
#maximumLayoutSize

	Dimension size;
	synchronized(this) {
	    checkContainer(target);
	    checkRequests();
	    size = new Dimension(xTotal.minimum, yTotal.minimum);
	}

        Insets insets = target.getInsets();
        size.width = (int) Math.min((long) size.width + (long) insets.left + (long) insets.right, Integer.MAX_VALUE);
        size.height = (int) Math.min((long) size.height + (long) insets.top + (long) insets.bottom, Integer.MAX_VALUE);
        return size;
public java.awt.DimensionpreferredLayoutSize(java.awt.Container target)
Returns the preferred dimensions for this layout, given the components in the specified target container.

param
target the container that needs to be laid out
return
the dimensions >= 0 && <= Integer.MAX_VALUE
exception
AWTError if the target isn't the container specified to the BoxLayout constructor
see
Container
see
#minimumLayoutSize
see
#maximumLayoutSize

	Dimension size;
	synchronized(this) {
	    checkContainer(target);
	    checkRequests();
	    size = new Dimension(xTotal.preferred, yTotal.preferred);
	}

        Insets insets = target.getInsets();
        size.width = (int) Math.min((long) size.width + (long) insets.left + (long) insets.right, Integer.MAX_VALUE);
        size.height = (int) Math.min((long) size.height + (long) insets.top + (long) insets.bottom, Integer.MAX_VALUE);
        return size;
public voidremoveLayoutComponent(java.awt.Component comp)
Not used by this class.

param
comp the component
private intresolveAxis(int axis, java.awt.ComponentOrientation o)
Given one of the 4 axis values, resolve it to an absolute axis. The relative axis values, PAGE_AXIS and LINE_AXIS are converted to their absolute couterpart given the target's ComponentOrientation value. The absolute axes, X_AXIS and Y_AXIS are returned unmodified.

param
axis the axis to resolve
param
o the ComponentOrientation to resolve against
return
the resolved axis

        int absoluteAxis;
        if( axis == LINE_AXIS ) {
            absoluteAxis = o.isHorizontal() ? X_AXIS : Y_AXIS;
        } else if( axis == PAGE_AXIS ) {
            absoluteAxis = o.isHorizontal() ? Y_AXIS : X_AXIS;
        } else {
            absoluteAxis = axis;
        } 
        return absoluteAxis;