src/java.desktop/share/classes/java/awt/BorderLayout.java - master - githubmirror/panama-vector

src/java.desktop/share/classes/java/awt/BorderLayout.java
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Phil Race
8357176: java.awt javadoc code examples still use Applet API
22 май 2025, 22:19
22 май 2025, 22:19139a05d
/*
 * Copyright (c) 1995, 2025, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.awt;

import java.io.Serial;

/**
 * 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:
 * {@code NORTH}, {@code SOUTH}, {@code EAST},
 * {@code WEST}, and {@code CENTER}.  When adding a
 * component to a container with a border layout, use one of these
 * five constants, for example:
 * <pre>
 *    Panel p = new Panel();
 *    p.setLayout(new BorderLayout());
 *    p.add(new Button("Okay"), BorderLayout.SOUTH);
 * </pre>
 * As a convenience, {@code BorderLayout} interprets the
 * absence of a string specification the same as the constant
 * {@code CENTER}:
 * <pre>
 *    Panel p2 = new Panel();
 *    p2.setLayout(new BorderLayout());
 *    p2.add(new TextArea());  // Same as p.add(new TextArea(), BorderLayout.CENTER);
 * </pre>
 * <p>
 * In addition, {@code BorderLayout} supports the relative
 * positioning constants, {@code PAGE_START}, {@code PAGE_END},
 * {@code LINE_START}, and {@code LINE_END}.
 * In a container whose {@code ComponentOrientation} is set to
 * {@code ComponentOrientation.LEFT_TO_RIGHT}, these constants map to
 * {@code NORTH}, {@code SOUTH}, {@code WEST}, and
 * {@code EAST}, respectively.
 * <p>
 * For compatibility with previous releases, {@code BorderLayout}
 * also includes the relative positioning constants {@code BEFORE_FIRST_LINE},
 * {@code AFTER_LAST_LINE}, {@code BEFORE_LINE_BEGINS} and
 * {@code AFTER_LINE_ENDS}.  These are equivalent to
 * {@code PAGE_START}, {@code PAGE_END}, {@code LINE_START}
 * and {@code LINE_END} respectively.  For
 * consistency with the relative positioning constants used by other
 * components, the latter constants are preferred.
 * <p>
 * Mixing both absolute and relative positioning constants can lead to
 * unpredictable results.  If
 * you use both types, the relative constants will take precedence.
 * For example, if you add components using both the {@code NORTH}
 * and {@code PAGE_START} constants in a container whose
 * orientation is {@code LEFT_TO_RIGHT}, only the
 * {@code PAGE_START} will be laid out.
 * <p>
 * NOTE: Currently,
 * {@code BorderLayout} does not support vertical
 * orientations.  The {@code isVertical} setting on the container's
 * {@code ComponentOrientation} is not respected.
 * <p>
 * The components are laid out according to their
 * preferred sizes and the constraints of the container's size.
 * The {@code NORTH} and {@code SOUTH} components may
 * be stretched horizontally; the {@code EAST} and
 * {@code WEST} components may be stretched vertically;
 * the {@code CENTER} component may stretch both horizontally
 * and vertically to fill any space left over.
 * <p>
 * Here is an example of five buttons in a window laid out using
 * the {@code BorderLayout} layout manager:
 * <p>
 * <img src="doc-files/BorderLayout-1.png" alt="Diagram of a window
 * 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.">
 * <p>
 * The code for this program is as follows:
 *
 * {@snippet lang='java':
 * import java.awt.BorderLayout;
 * import java.awt.Button;
 * import java.awt.EventQueue;
 * import java.awt.Frame;
 *
 * public class BorderLayoutExample {
 *
 *     public static void main(String[] args) throws Exception {
 *         EventQueue.invokeAndWait(() -> {
 *             Frame frame = new Frame("BorderLayout");
 *             frame.setLayout(new BorderLayout());
 *             frame.add(new Button("North"), BorderLayout.NORTH);
 *             frame.add(new Button("South"), BorderLayout.SOUTH);
 *             frame.add(new Button("East"), BorderLayout.EAST);
 *             frame.add(new Button("West"), BorderLayout.WEST);
 *             frame.add(new Button("Center"), BorderLayout.CENTER);
 *             frame.setSize(300, 300);
 *             frame.setVisible(true);
 *         });
 *     }
 * }
 * }
 *
 * @author      Arthur van Hoff
 * @see         java.awt.Container#add(String, Component)
 * @see         java.awt.ComponentOrientation
 * @since       1.0
 */
public class BorderLayout implements LayoutManager2,
                                     java.io.Serializable {
    /**
     * Constructs a border layout with the horizontal gaps
     * between components.
     * The horizontal gap is specified by {@code hgap}.
     *
     * @see #getHgap()
     * @see #setHgap(int)
     *
     * @serial
     */
        int hgap;

    /**
     * Constructs a border layout with the vertical gaps
     * between components.
     * The vertical gap is specified by {@code vgap}.
     *
     * @see #getVgap()
     * @see #setVgap(int)
     * @serial
     */
        int vgap;

    /**
     * Constant to specify components location to be the
     *      north portion of the border layout.
     * @serial
     * @see #getChild(String, boolean)
     * @see #addLayoutComponent
     * @see #getLayoutAlignmentX
     * @see #getLayoutAlignmentY
     * @see #removeLayoutComponent
     */
        Component north;
     /**
     * Constant to specify components location to be the
     *      west portion of the border layout.
     * @serial
     * @see #getChild(String, boolean)
     * @see #addLayoutComponent
     * @see #getLayoutAlignmentX
     * @see #getLayoutAlignmentY
     * @see #removeLayoutComponent
     */
        Component west;
    /**
     * Constant to specify components location to be the
     *      east portion of the border layout.
     * @serial
     * @see #getChild(String, boolean)
     * @see #addLayoutComponent
     * @see #getLayoutAlignmentX
     * @see #getLayoutAlignmentY
     * @see #removeLayoutComponent
     */
        Component east;
    /**
     * Constant to specify components location to be the
     *      south portion of the border layout.
     * @serial
     * @see #getChild(String, boolean)
     * @see #addLayoutComponent
     * @see #getLayoutAlignmentX
     * @see #getLayoutAlignmentY
     * @see #removeLayoutComponent
     */
    Component south;
    /**
     * Constant to specify components location to be the
     *      center portion of the border layout.
     * @serial
     * @see #getChild(String, boolean)
     * @see #addLayoutComponent
     * @see #getLayoutAlignmentX
     * @see #getLayoutAlignmentY
     * @see #removeLayoutComponent
     */
        Component center;

    /**
     *
     * 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 unpredictable results.  If
     * you use both types, the relative constants will take precedence.
     * For example, if you add components using both the {@code NORTH}
     * and {@code BEFORE_FIRST_LINE} constants in a container whose
     * orientation is {@code LEFT_TO_RIGHT}, only the
     * {@code BEFORE_FIRST_LINE} will be laid out.
     * This will be the same for lastLine, firstItem, lastItem.
     * @serial
     */
    Component firstLine;
     /**
     * A relative positioning constant, that can be used instead of
     * north, south, east, west or center.
     * Please read Description for firstLine.
     * @serial
     */
        Component lastLine;
     /**
     * A relative positioning constant, that can be used instead of
     * north, south, east, west or center.
     * Please read Description for firstLine.
     * @serial
     */
        Component firstItem;
    /**
     * A relative positioning constant, that can be used instead of
     * north, south, east, west or center.
     * Please read Description for firstLine.
     * @serial
     */
        Component lastItem;

    /**
     * The north layout constraint (top of container).
     */
    public static final String NORTH  = "North";

    /**
     * The south layout constraint (bottom of container).
     */
    public static final String SOUTH  = "South";

    /**
     * The east layout constraint (right side of container).
     */
    public static final String EAST   = "East";

    /**
     * The west layout constraint (left side of container).
     */
    public static final String WEST   = "West";

    /**
     * The center layout constraint (middle of container).
     */
    public static final String CENTER = "Center";

    /**
     * Synonym for PAGE_START.  Exists for compatibility with previous
     * versions.  PAGE_START is preferred.
     *
     * @see #PAGE_START
     * @since 1.2
     */
    public static final String BEFORE_FIRST_LINE = "First";

    /**
     * Synonym for PAGE_END.  Exists for compatibility with previous
     * versions.  PAGE_END is preferred.
     *
     * @see #PAGE_END
     * @since 1.2
     */
    public static final String AFTER_LAST_LINE = "Last";

    /**
     * Synonym for LINE_START.  Exists for compatibility with previous
     * versions.  LINE_START is preferred.
     *
     * @see #LINE_START
     * @since 1.2
     */
    public static final String BEFORE_LINE_BEGINS = "Before";

    /**
     * Synonym for LINE_END.  Exists for compatibility with previous
     * versions.  LINE_END is preferred.
     *
     * @see #LINE_END
     * @since 1.2
     */
    public static final String AFTER_LINE_ENDS = "After";

    /**
     * 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.
     *
     * @see java.awt.Component#getComponentOrientation
     * @since 1.4
     */
    public static final String PAGE_START = BEFORE_FIRST_LINE;

    /**
     * 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.
     *
     * @see java.awt.Component#getComponentOrientation
     * @since 1.4
     */
    public static final String PAGE_END = AFTER_LAST_LINE;

    /**
     * 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.
     *
     * @see java.awt.Component#getComponentOrientation
     * @since 1.4
     */
    public static final String LINE_START = BEFORE_LINE_BEGINS;

    /**
     * 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.
     *
     * @see java.awt.Component#getComponentOrientation
     * @since 1.4
     */
    public static final String LINE_END = AFTER_LINE_ENDS;

    /**
     * Use serialVersionUID from JDK 1.1 for interoperability.
     */
     @Serial
     private static final long serialVersionUID = -8658291919501921765L;

    /**
     * Constructs a new border layout with
     * no gaps between components.
     */
    public BorderLayout() {
        this(0, 0);
    }

    /**
     * Constructs a border layout with the specified gaps
     * between components.
     * The horizontal gap is specified by {@code hgap}
     * and the vertical gap is specified by {@code vgap}.
     * @param   hgap   the horizontal gap.
     * @param   vgap   the vertical gap.
     */
    public BorderLayout(int hgap, int vgap) {
        this.hgap = hgap;
        this.vgap = vgap;
    }

    /**
     * Returns the horizontal gap between components.
     *
     * @return the horizontal gap between components
     * @since   1.1
     */
    public int getHgap() {
        return hgap;
    }

    /**
     * Sets the horizontal gap between components.
     *
     * @param hgap the horizontal gap between components
     * @since   1.1
     */
    public void setHgap(int hgap) {
        this.hgap = hgap;
    }

    /**
     * Returns the vertical gap between components.
     *
     * @return the vertical gap between components
     * @since   1.1
     */
    public int getVgap() {
        return vgap;
    }

    /**
     * Sets the vertical gap between components.
     *
     * @param vgap the vertical gap between components
     * @since   1.1
     */
    public void setVgap(int vgap) {
        this.vgap = vgap;
    }

    /**
     * Adds the specified component to the layout, using the specified
     * constraint object.  For border layouts, the constraint must be
     * one of the following constants:  {@code NORTH},
     * {@code SOUTH}, {@code EAST},
     * {@code WEST}, or {@code CENTER}.
     * <p>
     * Most applications do not call this method directly. This method
     * is called when a component is added to a container using the
     * {@code 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)
     * @throws   IllegalArgumentException  if the constraint object is not
     *              a string, or if it not one of the five specified constants.
     * @since   1.1
     */
    public void addLayoutComponent(Component comp, Object constraints) {
      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)");
        }
      }
    }

    /**
     * @deprecated  replaced by {@code addLayoutComponent(Component, Object)}.
     */
    @Deprecated
    public void addLayoutComponent(String name, Component comp) {
      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);
        }
      }
    }

    /**
     * Removes the specified component from this border layout. This
     * method is called when a container calls its {@code remove} or
     * {@code 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()
     */
    public void removeLayoutComponent(Component comp) {
      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;
        }
      }
    }

    /**
     * Gets the component that was added using the given constraint
     *
     * @param   constraints  the desired constraint, one of {@code CENTER},
     *                       {@code NORTH}, {@code SOUTH},
     *                       {@code WEST}, {@code EAST},
     *                       {@code PAGE_START}, {@code PAGE_END},
     *                       {@code LINE_START}, {@code LINE_END}
     * @return  the component at the given location, or {@code null} if
     *          the location is empty
     * @throws   IllegalArgumentException  if the constraint object is
     *              not one of the nine specified constants
     * @see     #addLayoutComponent(java.awt.Component, java.lang.Object)
     * @since 1.5
     */
    public Component getLayoutComponent(Object constraints) {
        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);
        }
    }


    /**
     * Returns the component that corresponds to the given constraint location
     * based on the target {@code Container}'s component orientation.
     * Components added with the relative constraints {@code PAGE_START},
     * {@code PAGE_END}, {@code LINE_START}, and {@code LINE_END}
     * take precedence over components added with the explicit constraints
     * {@code NORTH}, {@code SOUTH}, {@code WEST}, and {@code EAST}.
     * The {@code Container}'s component orientation is used to determine the location of components
     * added with {@code LINE_START} and {@code LINE_END}.
     *
     * @param   target     the {@code Container} used to obtain
     *                     the constraint location based on the target
     *                     {@code Container}'s component orientation.
     * @param   constraints     the desired absolute position, one of {@code CENTER},
     *                          {@code NORTH}, {@code SOUTH},
     *                          {@code EAST}, {@code WEST}
     * @return  the component at the given location, or {@code null} if
     *          the location is empty
     * @throws   IllegalArgumentException  if the constraint object is
     *              not one of the five specified constants
     * @throws   NullPointerException  if the target parameter is null
     * @see     #addLayoutComponent(java.awt.Component, java.lang.Object)
     * @since 1.5
     */
    public Component getLayoutComponent(Container target, Object constraints) {
        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;
    }


    /**
     * 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
     */
    public Object getConstraints(Component comp) {
        //fix for 6242148 : API method java.awt.BorderLayout.getConstraints(null) should return null
        if (comp == null){
            return null;
        }
        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;
    }

    /**
     * Determines the minimum size of the {@code target} container
     * using this layout manager.
     * <p>
     * This method is called when a container calls its
     * {@code 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()
     */
    public Dimension minimumLayoutSize(Container target) {
      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;
      }
    }

    /**
     * Determines the preferred size of the {@code target}
     * container using this layout manager, based on the components
     * in the container.
     * <p>
     * Most applications do not call this method directly. This method
     * is called when a container calls its {@code 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()
     */
    public Dimension preferredLayoutSize(Container target) {
      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;
      }
    }

    /**
     * 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
     */
    public Dimension maximumLayoutSize(Container target) {
        return new Dimension(Integer.MAX_VALUE, Integer.MAX_VALUE);
    }

    /**
     * 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.
     */
    public float getLayoutAlignmentX(Container parent) {
        return 0.5f;
    }

    /**
     * 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.
     */
    public float getLayoutAlignmentY(Container parent) {
        return 0.5f;
    }

    /**
     * Invalidates the layout, indicating that if the layout manager
     * has cached information it should be discarded.
     */
    public void invalidateLayout(Container target) {
    }

    /**
     * Lays out the container argument using this border layout.
     * <p>
     * This method actually reshapes the components in the specified
     * container in order to satisfy the constraints of this
     * {@code BorderLayout} object. The {@code NORTH}
     * and {@code SOUTH} components, if any, are placed at
     * the top and bottom of the container, respectively. The
     * {@code WEST} and {@code EAST} components are
     * then placed on the left and right, respectively. Finally,
     * the {@code CENTER} object is placed in any remaining
     * space in the middle.
     * <p>
     * Most applications do not call this method directly. This method
     * is called when a container calls its {@code doLayout} method.
     * @param   target   the container in which to do the layout.
     * @see     java.awt.Container
     * @see     java.awt.Container#doLayout()
     */
    public void layoutContainer(Container target) {
      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);
        }
      }
    }

    /**
     * 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?
     */
    private Component getChild(String key, boolean ltr) {
        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;
    }

    /**
     * Returns a string representation of the state of this border layout.
     * @return    a string representation of this border layout.
     */
    public String toString() {
        return getClass().getName() + "[hgap=" + hgap + ",vgap=" + vgap + "]";
    }
}