FileDocCategorySizeDatePackage
BorderLayout.javaAPI DocJava SE 5 API28679Fri Aug 26 14:56:44 BST 2005java.awt

BorderLayout

public class BorderLayout extends Object implements LayoutManager2, Serializable
A border layout lays out a container, arranging and resizing its components to fit in five regions: north, south, east, west, and center. Each region may contain no more than one component, and is identified by a corresponding constant: NORTH, SOUTH, EAST, WEST, and CENTER. When adding a component to a container with a border layout, use one of these five constants, for example: 
Panel p = new Panel();
p.setLayout(new BorderLayout());
p.add(new Button("Okay"), BorderLayout.SOUTH);
As a convenience, BorderLayout interprets the absence of a string specification the same as the constant CENTER: 
Panel p2 = new Panel();
p2.setLayout(new BorderLayout());
p2.add(new TextArea()); // Same as p.add(new TextArea(), BorderLayout.CENTER);

In addition, BorderLayout supports the relative positioning constants, PAGE_START, PAGE_END, LINE_START, and LINE_END. In a container whose ComponentOrientation is set to ComponentOrientation.LEFT_TO_RIGHT, these constants map to NORTH, SOUTH, WEST, and EAST, respectively.

For compatibility with previous releases, BorderLayout also includes the relative positioning constants BEFORE_FIRST_LINE, AFTER_LAST_LINE, BEFORE_LINE_BEGINS and AFTER_LINE_ENDS. These are equivalent to PAGE_START, PAGE_END, LINE_START and LINE_END respectively. For consistency with the relative positioning constants used by other components, the latter constants are preferred.

Mixing both absolute and relative positioning constants can lead to unpredicable results. If you use both types, the relative constants will take precedence. For example, if you add components using both the NORTH and PAGE_START constants in a container whose orientation is LEFT_TO_RIGHT, only the PAGE_START will be layed out.

NOTE: Currently (in the Java 2 platform v1.2), BorderLayout does not support vertical orientations. The isVertical setting on the container's ComponentOrientation is not respected.

The components are laid out according to their preferred sizes and the constraints of the container's size. The NORTH and SOUTH components may be stretched horizontally; the EAST and WEST components may be stretched vertically; the CENTER component may stretch both horizontally and vertically to fill any space left over.

Here is an example of five buttons in an applet laid out using the BorderLayout layout manager:

Diagram of an applet demonstrating BorderLayout. Each section of the BorderLayout contains a Button corresponding to its position in the layout, one of: North, West, Center, East, or South.

The code for this applet is as follows: 

import java.awt.*;
import java.applet.Applet;

public class buttonDir extends Applet {
public void init() {
setLayout(new BorderLayout());
add(new Button("North"), BorderLayout.NORTH);
add(new Button("South"), BorderLayout.SOUTH);
add(new Button("East"), BorderLayout.EAST);
add(new Button("West"), BorderLayout.WEST);
add(new Button("Center"), BorderLayout.CENTER);
}
}

version
1.56, 05/18/04
author
Arthur van Hoff
see
java.awt.Container#add(String, Component)
see
java.awt.ComponentOrientation
since
JDK1.0

Fields Summary
int
hgap
Constructs a border layout with the horizontal gaps between components. The horizontal gap is specified by hgap.
int
vgap
Constructs a border layout with the vertical gaps between components. The vertical gap is specified by vgap.
Component
north
Constant to specify components location to be the north portion of the border layout.
Component
west
Constant to specify components location to be the west portion of the border layout.
Component
east
Constant to specify components location to be the east portion of the border layout.
Component
south
Constant to specify components location to be the south portion of the border layout.
Component
center
Constant to specify components location to be the center portion of the border layout.
Component
firstLine
A relative positioning constant, that can be used instead of north, south, east, west or center. mixing the two types of constants can lead to unpredicable results. If you use both types, the relative constants will take precedence. For example, if you add components using both the NORTH and BEFORE_FIRST_LINE constants in a container whose orientation is LEFT_TO_RIGHT, only the BEFORE_FIRST_LINE will be layed out. This will be the same for lastLine, firstItem, lastItem.
Component
lastLine
A relative positioning constant, that can be used instead of north, south, east, west or center. Please read Description for firstLine.
Component
firstItem
A relative positioning constant, that can be used instead of north, south, east, west or center. Please read Description for firstLine.
Component
lastItem
A relative positioning constant, that can be used instead of north, south, east, west or center. Please read Description for firstLine.
public static final String
NORTH
The north layout constraint (top of container).
public static final String
SOUTH
The south layout constraint (bottom of container).
public static final String
EAST
The east layout constraint (right side of container).
public static final String
WEST
The west layout constraint (left side of container).
public static final String
CENTER
The center layout constraint (middle of container).
public static final String
BEFORE_FIRST_LINE
Synonym for PAGE_START. Exists for compatibility with previous versions. PAGE_START is preferred.
public static final String
AFTER_LAST_LINE
Synonym for PAGE_END. Exists for compatibility with previous versions. PAGE_END is preferred.
public static final String
BEFORE_LINE_BEGINS
Synonym for LINE_START. Exists for compatibility with previous versions. LINE_START is preferred.
public static final String
AFTER_LINE_ENDS
Synonym for LINE_END. Exists for compatibility with previous versions. LINE_END is preferred.
public static final String
PAGE_START
The component comes before the first line of the layout's content. For Western, left-to-right and top-to-bottom orientations, this is equivalent to NORTH.
public static final String
PAGE_END
The component comes after the last line of the layout's content. For Western, left-to-right and top-to-bottom orientations, this is equivalent to SOUTH.
public static final String
LINE_START
The component goes at the beginning of the line direction for the layout. For Western, left-to-right and top-to-bottom orientations, this is equivalent to WEST.
public static final String
LINE_END
The component goes at the end of the line direction for the layout. For Western, left-to-right and top-to-bottom orientations, this is equivalent to EAST.
private static final long
serialVersionUID
Constructors Summary
public BorderLayout()
Constructs a new border layout with no gaps between components.


                   
      
	this(0, 0);
public BorderLayout(int hgap, int vgap)
Constructs a border layout with the specified gaps between components. The horizontal gap is specified by hgap and the vertical gap is specified by vgap.

param
hgap the horizontal gap.
param
vgap the vertical gap.

	this.hgap = hgap;
	this.vgap = vgap;
Methods Summary
public voidaddLayoutComponent(java.awt.Component comp, java.lang.Object constraints)
Adds the specified component to the layout, using the specified constraint object. For border layouts, the constraint must be one of the following constants: NORTH, SOUTH, EAST, WEST, or CENTER.

Most applications do not call this method directly. This method is called when a component is added to a container using the Container.add method with the same argument types.

param
comp the component to be added.
param
constraints an object that specifies how and where the component is added to the layout.
see
java.awt.Container#add(java.awt.Component, java.lang.Object)
exception
IllegalArgumentException if the constraint object is not a string, or if it not one of the five specified constants.
since
JDK1.1

      synchronized (comp.getTreeLock()) {
	if ((constraints == null) || (constraints instanceof String)) {
	    addLayoutComponent((String)constraints, comp);
	} else {
	    throw new IllegalArgumentException("cannot add to layout: constraint must be a string (or null)");
	}
      }
public voidaddLayoutComponent(java.lang.String name, java.awt.Component comp)

deprecated
replaced by addLayoutComponent(Component, Object).

      synchronized (comp.getTreeLock()) {
	/* Special case:  treat null the same as "Center". */
	if (name == null) {
	    name = "Center";
	}

	/* Assign the component to one of the known regions of the layout.
	 */
	if ("Center".equals(name)) {
	    center = comp;
	} else if ("North".equals(name)) {
	    north = comp;
	} else if ("South".equals(name)) {
	    south = comp;
	} else if ("East".equals(name)) {
	    east = comp;
	} else if ("West".equals(name)) {
	    west = comp;
	} else if (BEFORE_FIRST_LINE.equals(name)) {
	    firstLine = comp;
	} else if (AFTER_LAST_LINE.equals(name)) {
	    lastLine = comp;
	} else if (BEFORE_LINE_BEGINS.equals(name)) {
	    firstItem = comp;
	} else if (AFTER_LINE_ENDS.equals(name)) {
	    lastItem = comp;
	} else {
	    throw new IllegalArgumentException("cannot add to layout: unknown constraint: " + name);
	}
      }
private java.awt.ComponentgetChild(java.lang.String key, boolean ltr)
Get the component that corresponds to the given constraint location

param
key The desired absolute position, either NORTH, SOUTH, EAST, or WEST.
param
ltr Is the component line direction left-to-right?

        Component result = null;

        if (key == NORTH) {
            result = (firstLine != null) ? firstLine : north;
        }
        else if (key == SOUTH) {
            result = (lastLine != null) ? lastLine : south;
        }
        else if (key == WEST) {
            result = ltr ? firstItem : lastItem;
            if (result == null) {
                result = west;
            }
        }
        else if (key == EAST) {
            result = ltr ? lastItem : firstItem;
            if (result == null) {
                result = east;
            }
        }
        else if (key == CENTER) {
            result = center;
        }
        if (result != null && !result.visible) {
            result = null;
        }
        return result;
public java.lang.ObjectgetConstraints(java.awt.Component comp)
Gets the constraints for the specified component

param
comp the component to be queried
return
the constraint for the specified component, or null if component is null or is not present in this layout
see
#addLayoutComponent(java.awt.Component, java.lang.Object)
since
1.5

	if (comp == center) {
	    return CENTER;
	} else if (comp == north) {
	    return NORTH;
	} else if (comp == south) {
	    return SOUTH;
	} else if (comp == west) {
	    return WEST;
	} else if (comp == east) {
	    return EAST;
	} else if (comp == firstLine) {
	    return PAGE_START;
	} else if (comp == lastLine) {
	    return PAGE_END;
	} else if (comp == firstItem) {
	    return LINE_START;
	} else if (comp == lastItem) {
	    return LINE_END;
	}
	return null;
public intgetHgap()
Returns the horizontal gap between components.

since
JDK1.1

	return hgap;
public floatgetLayoutAlignmentX(java.awt.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.

	return 0.5f;
public floatgetLayoutAlignmentY(java.awt.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.

	return 0.5f;
public java.awt.ComponentgetLayoutComponent(java.lang.Object constraints)
Gets the component that was added using the given constraint

param
constraints the desired constraint, one of CENTER, NORTH, SOUTH, WEST, EAST, PAGE_START, PAGE_END, LINE_START, LINE_END
return
the component at the given location, or null if the location is empty
exception
IllegalArgumentException if the constraint object is not one of the nine specified constants
see
#addLayoutComponent(java.awt.Component, java.lang.Object)
since
1.5

	if (CENTER.equals(constraints)) {
	    return center;
	} else if (NORTH.equals(constraints)) {
	    return north;
	} else if (SOUTH.equals(constraints)) {
	    return south;
	} else if (WEST.equals(constraints)) {
	    return west;
	} else if (EAST.equals(constraints)) {
	    return east;
	} else if (PAGE_START.equals(constraints)) {
	    return firstLine;
	} else if (PAGE_END.equals(constraints)) {
	    return lastLine;
	} else if (LINE_START.equals(constraints)) {
	    return firstItem;
	} else if (LINE_END.equals(constraints)) {
	    return lastItem;
	} else {
	    throw new IllegalArgumentException("cannot get component: unknown constraint: " + constraints);
	}
public java.awt.ComponentgetLayoutComponent(java.awt.Container target, java.lang.Object constraints)
Gets the component that corresponds to the given constraint location based on the target Container's component orientation

param
constraints the desired absolute position, one of CENTER, one of NORTH, SOUTH, EAST, WEST
param
target the Container using this BorderLayout
return
the component at the given location, or null if the location is empty
exception
IllegalArgumentException if the constraint object is not one of the five specified constants
exception
NullPointerException if the target parameter is null
see
#addLayoutComponent(java.awt.Component, java.lang.Object)
since
1.5

	boolean ltr = target.getComponentOrientation().isLeftToRight();
        Component result = null;

        if (NORTH.equals(constraints)) {
            result = (firstLine != null) ? firstLine : north;
        } else if (SOUTH.equals(constraints)) {
            result = (lastLine != null) ? lastLine : south;
        } else if (WEST.equals(constraints)) {
            result = ltr ? firstItem : lastItem;
            if (result == null) {
		result = west;
            }
        } else if (EAST.equals(constraints)) {
            result = ltr ? lastItem : firstItem;
            if (result == null) {
		result = east;
            }
        } else if (CENTER.equals(constraints)) {
            result = center;
	} else {
	    throw new IllegalArgumentException("cannot get component: invalid constraint: " + constraints);
        }

        return result;
public intgetVgap()
Returns the vertical gap between components.

since
JDK1.1

	return vgap;
public voidinvalidateLayout(java.awt.Container target)
Invalidates the layout, indicating that if the layout manager has cached information it should be discarded.
public voidlayoutContainer(java.awt.Container target)
Lays out the container argument using this border layout.

This method actually reshapes the components in the specified container in order to satisfy the constraints of this BorderLayout object. The NORTH and SOUTH components, if any, are placed at the top and bottom of the container, respectively. The WEST and EAST components are then placed on the left and right, respectively. Finally, the CENTER object is placed in any remaining space in the middle.

Most applications do not call this method directly. This method is called when a container calls its doLayout method.

param
target the container in which to do the layout.
see
java.awt.Container
see
java.awt.Container#doLayout()

      synchronized (target.getTreeLock()) {
	Insets insets = target.getInsets();
	int top = insets.top;
	int bottom = target.height - insets.bottom;
	int left = insets.left;
	int right = target.width - insets.right;

        boolean ltr = target.getComponentOrientation().isLeftToRight();
        Component c = null;

	if ((c=getChild(NORTH,ltr)) != null) {
	    c.setSize(right - left, c.height);
	    Dimension d = c.getPreferredSize();
	    c.setBounds(left, top, right - left, d.height);
	    top += d.height + vgap;
	}
	if ((c=getChild(SOUTH,ltr)) != null) {
	    c.setSize(right - left, c.height);
	    Dimension d = c.getPreferredSize();
	    c.setBounds(left, bottom - d.height, right - left, d.height);
	    bottom -= d.height + vgap;
	}
	if ((c=getChild(EAST,ltr)) != null) {
	    c.setSize(c.width, bottom - top);
	    Dimension d = c.getPreferredSize();
	    c.setBounds(right - d.width, top, d.width, bottom - top);
	    right -= d.width + hgap;
	}
	if ((c=getChild(WEST,ltr)) != null) {
	    c.setSize(c.width, bottom - top);
	    Dimension d = c.getPreferredSize();
	    c.setBounds(left, top, d.width, bottom - top);
	    left += d.width + hgap;
	}
	if ((c=getChild(CENTER,ltr)) != null) {
	    c.setBounds(left, top, right - left, bottom - top);
	}
      }
public java.awt.DimensionmaximumLayoutSize(java.awt.Container target)
Returns the maximum dimensions for this layout given the components in the specified target container.

param
target the component which needs to be laid out
see
Container
see
#minimumLayoutSize
see
#preferredLayoutSize

	return new Dimension(Integer.MAX_VALUE, Integer.MAX_VALUE);
public java.awt.DimensionminimumLayoutSize(java.awt.Container target)
Determines the minimum size of the target container using this layout manager.

This method is called when a container calls its getMinimumSize method. Most applications do not call this method directly.

param
target the container in which to do the layout.
return
the minimum dimensions needed to lay out the subcomponents of the specified container.
see
java.awt.Container
see
java.awt.BorderLayout#preferredLayoutSize
see
java.awt.Container#getMinimumSize()

      synchronized (target.getTreeLock()) {
	Dimension dim = new Dimension(0, 0);

        boolean ltr = target.getComponentOrientation().isLeftToRight();
        Component c = null;

	if ((c=getChild(EAST,ltr)) != null) {
	    Dimension d = c.getMinimumSize();
	    dim.width += d.width + hgap;
	    dim.height = Math.max(d.height, dim.height);
	}
	if ((c=getChild(WEST,ltr)) != null) {
	    Dimension d = c.getMinimumSize();
	    dim.width += d.width + hgap;
	    dim.height = Math.max(d.height, dim.height);
	}
	if ((c=getChild(CENTER,ltr)) != null) {
	    Dimension d = c.getMinimumSize();
	    dim.width += d.width;
	    dim.height = Math.max(d.height, dim.height);
	}
	if ((c=getChild(NORTH,ltr)) != null) {
	    Dimension d = c.getMinimumSize();
	    dim.width = Math.max(d.width, dim.width);
	    dim.height += d.height + vgap;
	}
	if ((c=getChild(SOUTH,ltr)) != null) {
	    Dimension d = c.getMinimumSize();
	    dim.width = Math.max(d.width, dim.width);
	    dim.height += d.height + vgap;
	}

	Insets insets = target.getInsets();
	dim.width += insets.left + insets.right;
	dim.height += insets.top + insets.bottom;

	return dim;
      }
public java.awt.DimensionpreferredLayoutSize(java.awt.Container target)
Determines the preferred size of the target container using this layout manager, based on the components in the container.

Most applications do not call this method directly. This method is called when a container calls its getPreferredSize method.

param
target the container in which to do the layout.
return
the preferred dimensions to lay out the subcomponents of the specified container.
see
java.awt.Container
see
java.awt.BorderLayout#minimumLayoutSize
see
java.awt.Container#getPreferredSize()

      synchronized (target.getTreeLock()) {
	Dimension dim = new Dimension(0, 0);

        boolean ltr = target.getComponentOrientation().isLeftToRight();
        Component c = null;

	if ((c=getChild(EAST,ltr)) != null) {
	    Dimension d = c.getPreferredSize();
	    dim.width += d.width + hgap;
	    dim.height = Math.max(d.height, dim.height);
	}
	if ((c=getChild(WEST,ltr)) != null) {
	    Dimension d = c.getPreferredSize();
	    dim.width += d.width + hgap;
	    dim.height = Math.max(d.height, dim.height);
	}
	if ((c=getChild(CENTER,ltr)) != null) {
	    Dimension d = c.getPreferredSize();
	    dim.width += d.width;
	    dim.height = Math.max(d.height, dim.height);
	}
	if ((c=getChild(NORTH,ltr)) != null) {
	    Dimension d = c.getPreferredSize();
	    dim.width = Math.max(d.width, dim.width);
	    dim.height += d.height + vgap;
	}
	if ((c=getChild(SOUTH,ltr)) != null) {
	    Dimension d = c.getPreferredSize();
	    dim.width = Math.max(d.width, dim.width);
	    dim.height += d.height + vgap;
	}

	Insets insets = target.getInsets();
	dim.width += insets.left + insets.right;
	dim.height += insets.top + insets.bottom;

	return dim;
      }
public voidremoveLayoutComponent(java.awt.Component comp)
Removes the specified component from this border layout. This method is called when a container calls its remove or removeAll methods. Most applications do not call this method directly.

param
comp the component to be removed.
see
java.awt.Container#remove(java.awt.Component)
see
java.awt.Container#removeAll()

      synchronized (comp.getTreeLock()) {
	if (comp == center) {
	    center = null;
	} else if (comp == north) {
	    north = null;
	} else if (comp == south) {
	    south = null;
	} else if (comp == east) {
	    east = null;
	} else if (comp == west) {
	    west = null;
	}
	if (comp == firstLine) {
	    firstLine = null;
	} else if (comp == lastLine) {
	    lastLine = null;
	} else if (comp == firstItem) {
	    firstItem = null;
	} else if (comp == lastItem) {
	    lastItem = null;
	}
      }
public voidsetHgap(int hgap)
Sets the horizontal gap between components.

param
hgap the horizontal gap between components
since
JDK1.1

	this.hgap = hgap;
public voidsetVgap(int vgap)
Sets the vertical gap between components.

param
vgap the vertical gap between components
since
JDK1.1

	this.vgap = vgap;
public java.lang.StringtoString()
Returns a string representation of the state of this border layout.

return
a string representation of this border layout.

	return getClass().getName() + "[hgap=" + hgap + ",vgap=" + vgap + "]";