Is it possible to align components horizontally when using WrapLayout/FlowLayout?

Question

I use WrapLayout which extends FlowLayout

Now, I have this GUI:

What I want is this:

I tried some things like: label.setVerticalAlignment(JLabel.TOP); but the layout does not seem to respect it. I guess this behavior is inherited from FlowLayout?

Full code:

public class WrapLayoutExample {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(WrapLayoutExample::runGui);
    }

    private static void runGui() {
        JFrame frame = new JFrame("A");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel panel = new JPanel(new WrapLayout());

        JLabel l = new JLabel("CCC");
        l.setBorder(BorderFactory.createLineBorder(Color.red, 10));
        l.setVerticalAlignment(JLabel.TOP);
        panel.add(l);

        l = new JLabel("BBBB");
        l.setBorder(BorderFactory.createLineBorder(Color.red, 20));
        panel.add(l);

        frame.setLayout(new BorderLayout());
        frame.add(new JScrollPane(panel));

        frame.pack();
        frame.setVisible(true);
    }
}

Answer 1

From @camickr's comment in my question:

You should be able to write your own layout manager. Just copy the FlowLayout and replace the logic that centers the component within the row, to position the component at the top.

In FlowLayout, in moveComponents method, there is this line:

cy = y + (height - m.height) / 2;

Changing the line to:

cy = y;

achieves what I want.

It probably breaks the flowLayout.setAlignOnBaseline functionality. But I don't use it anyway.

Layout code (WrapLayout and custom FlowLayout combined):

public class WrapAndAlignHorizontallyTopLayout implements LayoutManager, java.io.Serializable {

    /**
     * This value indicates that each row of components should be left-justified.
     */
    public static final int LEFT = 0;

    /**
     * This value indicates that each row of components should be centered.
     */
    public static final int CENTER = 1;

    /**
     * This value indicates that each row of components should be right-justified.
     */
    public static final int RIGHT = 2;

    /**
     * This value indicates that each row of components should be justified to the
     * leading edge of the container's orientation, for example, to the left in
     * left-to-right orientations.
     *
     * @see java.awt.Component#getComponentOrientation
     * @see java.awt.ComponentOrientation
     * @since 1.2
     */
    public static final int LEADING = 3;

    /**
     * This value indicates that each row of components should be justified to the
     * trailing edge of the container's orientation, for example, to the right in
     * left-to-right orientations.
     *
     * @see java.awt.Component#getComponentOrientation
     * @see java.awt.ComponentOrientation
     * @since 1.2
     */
    public static final int TRAILING = 4;

    /**
     * <code>align</code> is the property that determines how each row distributes
     * empty space. It can be one of the following values:
     * <ul>
     * <li><code>LEFT</code>
     * <li><code>RIGHT</code>
     * <li><code>CENTER</code>
     * </ul>
     *
     * @serial
     * @see #getAlignment
     * @see #setAlignment
     */
    int align; // This is for 1.1 serialization compatibility

    /**
     * <code>newAlign</code> is the property that determines how each row
     * distributes empty space for the Java 2 platform, v1.2 and greater. It can be
     * one of the following three values:
     * <ul>
     * <li><code>LEFT</code>
     * <li><code>RIGHT</code>
     * <li><code>CENTER</code>
     * <li><code>LEADING</code>
     * <li><code>TRAILING</code>
     * </ul>
     *
     * @serial
     * @since 1.2
     * @see #getAlignment
     * @see #setAlignment
     */
    int newAlign; // This is the one we actually use

    /**
     * The flow layout manager allows a seperation of components with gaps. The
     * horizontal gap will specify the space between components and between the
     * components and the borders of the <code>Container</code>.
     *
     * @serial
     * @see #getHgap()
     * @see #setHgap(int)
     */
    int hgap;

    /**
     * The flow layout manager allows a seperation of components with gaps. The
     * vertical gap will specify the space between rows and between the the rows and
     * the borders of the <code>Container</code>.
     *
     * @serial
     * @see #getHgap()
     * @see #setHgap(int)
     */
    int vgap;

    /**
     * If true, components will be aligned on their baseline.
     */
    private boolean alignOnBaseline;

    /*
     * JDK 1.1 serialVersionUID
     */
    private static final long serialVersionUID = -7262534875583282631L;

    /**
     * Constructs a new <code>FlowLayout</code> with a centered alignment and a
     * default 5-unit horizontal and vertical gap.
     */
    public WrapAndAlignHorizontallyTopLayout() {
        this(CENTER, 5, 5);
    }

    /**
     * Constructs a new <code>FlowLayout</code> with the specified alignment and a
     * default 5-unit horizontal and vertical gap. The value of the alignment
     * argument must be one of <code>FlowLayout.LEFT</code>,
     * <code>FlowLayout.RIGHT</code>, <code>FlowLayout.CENTER</code>,
     * <code>FlowLayout.LEADING</code>, or <code>FlowLayout.TRAILING</code>.
     * 
     * @param align the alignment value
     */
    public WrapAndAlignHorizontallyTopLayout(int align) {
        this(align, 5, 5);
    }

    /**
     * Creates a new flow layout manager with the indicated alignment and the
     * indicated horizontal and vertical gaps.
     * <p>
     * The value of the alignment argument must be one of
     * <code>FlowLayout.LEFT</code>, <code>FlowLayout.RIGHT</code>,
     * <code>FlowLayout.CENTER</code>, <code>FlowLayout.LEADING</code>, or
     * <code>FlowLayout.TRAILING</code>.
     * 
     * @param align the alignment value
     * @param hgap  the horizontal gap between components and between the components
     *              and the borders of the <code>Container</code>
     * @param vgap  the vertical gap between components and between the components
     *              and the borders of the <code>Container</code>
     */
    public WrapAndAlignHorizontallyTopLayout(int align, int hgap, int vgap) {
        this.hgap = hgap;
        this.vgap = vgap;
        setAlignment(align);
    }

    /**
     * Gets the alignment for this layout. Possible values are
     * <code>FlowLayout.LEFT</code>, <code>FlowLayout.RIGHT</code>,
     * <code>FlowLayout.CENTER</code>, <code>FlowLayout.LEADING</code>, or
     * <code>FlowLayout.TRAILING</code>.
     * 
     * @return the alignment value for this layout
     * @see java.awt.FlowLayout#setAlignment
     * @since JDK1.1
     */
    public int getAlignment() {
        return newAlign;
    }

    /**
     * Sets the alignment for this layout. Possible values are
     * <ul>
     * <li><code>FlowLayout.LEFT</code>
     * <li><code>FlowLayout.RIGHT</code>
     * <li><code>FlowLayout.CENTER</code>
     * <li><code>FlowLayout.LEADING</code>
     * <li><code>FlowLayout.TRAILING</code>
     * </ul>
     * 
     * @param align one of the alignment values shown above
     * @see #getAlignment()
     * @since JDK1.1
     */
    public void setAlignment(int align) {
        this.newAlign = align;

        // this.align is used only for serialization compatibility,
        // so set it to a value compatible with the 1.1 version
        // of the class

        switch (align) {
        case LEADING:
            this.align = LEFT;
            break;
        case TRAILING:
            this.align = RIGHT;
            break;
        default:
            this.align = align;
            break;
        }
    }

    /**
     * Gets the horizontal gap between components and between the components and the
     * borders of the <code>Container</code>
     *
     * @return the horizontal gap between components and between the components and
     *         the borders of the <code>Container</code>
     * @see java.awt.FlowLayout#setHgap
     * @since JDK1.1
     */
    public int getHgap() {
        return hgap;
    }

    /**
     * Sets the horizontal gap between components and between the components and the
     * borders of the <code>Container</code>.
     *
     * @param hgap the horizontal gap between components and between the components
     *             and the borders of the <code>Container</code>
     * @see java.awt.FlowLayout#getHgap
     * @since JDK1.1
     */
    public void setHgap(int hgap) {
        this.hgap = hgap;
    }

    /**
     * Gets the vertical gap between components and between the components and the
     * borders of the <code>Container</code>.
     *
     * @return the vertical gap between components and between the components and
     *         the borders of the <code>Container</code>
     * @see java.awt.FlowLayout#setVgap
     * @since JDK1.1
     */
    public int getVgap() {
        return vgap;
    }

    /**
     * Sets the vertical gap between components and between the components and the
     * borders of the <code>Container</code>.
     *
     * @param vgap the vertical gap between components and between the components
     *             and the borders of the <code>Container</code>
     * @see java.awt.FlowLayout#getVgap
     * @since JDK1.1
     */
    public void setVgap(int vgap) {
        this.vgap = vgap;
    }

    /**
     * Sets whether or not components should be vertically aligned along their
     * baseline. Components that do not have a baseline will be centered. The
     * default is false.
     *
     * @param alignOnBaseline whether or not components should be vertically aligned
     *                        on their baseline
     * @since 1.6
     */
    public void setAlignOnBaseline(boolean alignOnBaseline) {
        this.alignOnBaseline = alignOnBaseline;
    }

    /**
     * Returns true if components are to be vertically aligned along their baseline.
     * The default is false.
     *
     * @return true if components are to be vertically aligned along their baseline
     * @since 1.6
     */
    public boolean getAlignOnBaseline() {
        return alignOnBaseline;
    }

    /**
     * Adds the specified component to the layout. Not used by this class.
     * 
     * @param name the name of the component
     * @param comp the component to be added
     */
    @Override
    public void addLayoutComponent(String name, Component comp) {
    }

    /**
     * Removes the specified component from the layout. Not used by this class.
     * 
     * @param comp the component to remove
     * @see java.awt.Container#removeAll
     */
    @Override
    public void removeLayoutComponent(Component comp) {
    }

    /**
     * Returns the preferred dimensions for this layout given the <i>visible</i>
     * components in the specified target container.
     * 
     * @param target the component which needs to be laid out
     * @return the preferred dimensions to lay out the subcomponents of the
     *         specified container
     */
    @Override
    public Dimension preferredLayoutSize(Container target) {
        return layoutSize(target, true);
    }

    /**
     * Returns the minimum dimensions needed to layout the <i>visible</i> components
     * contained in the specified target container.
     * 
     * @param target the component which needs to be laid out
     * @return the minimum dimensions to lay out the subcomponents of the specified
     *         container
     */
    @Override
    public Dimension minimumLayoutSize(Container target) {
        Dimension minimum = layoutSize(target, false);
        minimum.width -= (getHgap() + 1);
        return minimum;
    }

    /**
     * Returns the minimum or preferred dimension needed to layout the target
     * container.
     *
     * @param target    target to get layout size for
     * @param preferred should preferred size be calculated
     * @return the dimension to layout the target container
     */
    private Dimension layoutSize(Container target, boolean preferred) {
        synchronized (target.getTreeLock()) {
            // Each row must fit with the width allocated to the containter.
            // When the container width = 0, the preferred width of the container
            // has not yet been calculated so lets ask for the maximum.

            int targetWidth = target.getSize().width;
            Container container = target;

            while (container.getSize().width == 0 && container.getParent() != null) {
                container = container.getParent();
            }

            targetWidth = container.getSize().width;

            if (targetWidth == 0)
                targetWidth = Integer.MAX_VALUE;

            int hgap = getHgap();
            int vgap = getVgap();
            Insets insets = target.getInsets();
            int horizontalInsetsAndGap = insets.left + insets.right + (hgap * 2);
            int maxWidth = targetWidth - horizontalInsetsAndGap;

            // Fit components into the allowed width

            Dimension dim = new Dimension(0, 0);
            int rowWidth = 0;
            int rowHeight = 0;

            int nmembers = target.getComponentCount();

            for (int i = 0; i < nmembers; i++) {
                Component m = target.getComponent(i);

                if (m.isVisible()) {
                    Dimension d = preferred ? m.getPreferredSize() : m.getMinimumSize();

                    // Can't add the component to current row. Start a new row.

                    if (rowWidth + d.width > maxWidth) {
                        addRow(dim, rowWidth, rowHeight);
                        rowWidth = 0;
                        rowHeight = 0;
                    }

                    // Add a horizontal gap for all components after the first

                    if (rowWidth != 0) {
                        rowWidth += hgap;
                    }

                    rowWidth += d.width;
                    rowHeight = Math.max(rowHeight, d.height);
                }
            }

            addRow(dim, rowWidth, rowHeight);

            dim.width += horizontalInsetsAndGap;
            dim.height += insets.top + insets.bottom + vgap * 2;

            // When using a scroll pane or the DecoratedLookAndFeel we need to
            // make sure the preferred size is less than the size of the
            // target containter so shrinking the container size works
            // correctly. Removing the horizontal gap is an easy way to do this.

            Container scrollPane = SwingUtilities.getAncestorOfClass(JScrollPane.class, target);

            if (scrollPane != null && target.isValid()) {
                dim.width -= (hgap + 1);
            }

            return dim;
        }
    }

    /*
     * A new row has been completed. Use the dimensions of this row to update the
     * preferred size for the container.
     *
     * @param dim update the width and height when appropriate
     * 
     * @param rowWidth the width of the row to add
     * 
     * @param rowHeight the height of the row to add
     */
    private void addRow(Dimension dim, int rowWidth, int rowHeight) {
        dim.width = Math.max(dim.width, rowWidth);

        if (dim.height > 0) {
            dim.height += getVgap();
        }

        dim.height += rowHeight;
    }

    /**
     * Centers the elements in the specified row, if there is any slack.
     * 
     * @param target      the component which needs to be moved
     * @param x           the x coordinate
     * @param y           the y coordinate
     * @param width       the width dimensions
     * @param height      the height dimensions
     * @param rowStart    the beginning of the row
     * @param rowEnd      the the ending of the row
     * @param useBaseline Whether or not to align on baseline.
     * @param ascent      Ascent for the components. This is only valid if
     *                    useBaseline is true.
     * @param descent     Ascent for the components. This is only valid if
     *                    useBaseline is true.
     * @return actual row height
     */
    private int moveComponents(Container target, int x, int y, int width, int height, int rowStart, int rowEnd,
            boolean ltr, boolean useBaseline, int[] ascent, int[] descent) {
        switch (newAlign) {
        case LEFT:
            x += ltr ? 0 : width;
            break;
        case CENTER:
            x += width / 2;
            break;
        case RIGHT:
            x += ltr ? width : 0;
            break;
        case LEADING:
            break;
        case TRAILING:
            x += width;
            break;
        }
        int maxAscent = 0;
        int nonbaselineHeight = 0;
        int baselineOffset = 0;
        if (useBaseline) {
            int maxDescent = 0;
            for (int i = rowStart; i < rowEnd; i++) {
                Component m = target.getComponent(i);
                if (m.isVisible()) {
                    if (ascent[i] >= 0) {
                        maxAscent = Math.max(maxAscent, ascent[i]);
                        maxDescent = Math.max(maxDescent, descent[i]);
                    } else {
                        nonbaselineHeight = Math.max(m.getHeight(), nonbaselineHeight);
                    }
                }
            }
            height = Math.max(maxAscent + maxDescent, nonbaselineHeight);
            baselineOffset = (height - maxAscent - maxDescent) / 2;
        }
        for (int i = rowStart; i < rowEnd; i++) {
            Component m = target.getComponent(i);
            if (m.isVisible()) {
                int cy;
                if (useBaseline && ascent[i] >= 0) {
                    cy = y + baselineOffset + maxAscent - ascent[i];
                } else {
                    cy = y;
                }
                if (ltr) {
                    m.setLocation(x, cy);
                } else {
                    m.setLocation(target.getWidth() - x - m.getWidth(), cy);
                }
                x += m.getWidth() + hgap;
            }
        }
        return height;
    }

    /**
     * Lays out the container. This method lets each <i>visible</i> component take
     * its preferred size by reshaping the components in the target container in
     * order to satisfy the alignment of this <code>FlowLayout</code> object.
     *
     * @param target the specified component being laid out
     * @see Container
     * @see java.awt.Container#doLayout
     */
    @Override
    public void layoutContainer(Container target) {
        synchronized (target.getTreeLock()) {
            Insets insets = target.getInsets();
            int maxwidth = target.getWidth() - (insets.left + insets.right + hgap * 2);
            int nmembers = target.getComponentCount();
            int x = 0, y = insets.top + vgap;
            int rowh = 0, start = 0;

            boolean ltr = target.getComponentOrientation().isLeftToRight();

            boolean useBaseline = getAlignOnBaseline();
            int[] ascent = null;
            int[] descent = null;

            if (useBaseline) {
                ascent = new int[nmembers];
                descent = new int[nmembers];
            }

            for (int i = 0; i < nmembers; i++) {
                Component m = target.getComponent(i);
                if (m.isVisible()) {
                    Dimension d = m.getPreferredSize();
                    m.setSize(d.width, d.height);

                    if (useBaseline) {
                        int baseline = m.getBaseline(d.width, d.height);
                        if (baseline >= 0) {
                            ascent[i] = baseline;
                            descent[i] = d.height - baseline;
                        } else {
                            ascent[i] = -1;
                        }
                    }
                    if ((x == 0) || ((x + d.width) <= maxwidth)) {
                        if (x > 0) {
                            x += hgap;
                        }
                        x += d.width;
                        rowh = Math.max(rowh, d.height);
                    } else {
                        rowh = moveComponents(target, insets.left + hgap, y, maxwidth - x, rowh, start, i, ltr,
                                useBaseline, ascent, descent);
                        x = d.width;
                        y += vgap + rowh;
                        rowh = d.height;
                        start = i;
                    }
                }
            }
            moveComponents(target, insets.left + hgap, y, maxwidth - x, rowh, start, nmembers, ltr, useBaseline, ascent,
                    descent);
        }
    }

    //
    // the internal serial version which says which version was written
    // - 0 (default) for versions before the Java 2 platform, v1.2
    // - 1 for version >= Java 2 platform v1.2, which includes "newAlign" field
    //
    private static final int currentSerialVersion = 1;
    /**
     * This represent the <code>currentSerialVersion</code> which is bein used. It
     * will be one of two values : <code>0</code> versions before Java 2 platform
     * v1.2.. <code>1</code> versions after Java 2 platform v1.2..
     *
     * @serial
     * @since 1.2
     */
    private int serialVersionOnStream = currentSerialVersion;

    /**
     * Reads this object out of a serialization stream, handling objects written by
     * older versions of the class that didn't contain all of the fields we use
     * now..
     */
    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
        stream.defaultReadObject();

        if (serialVersionOnStream < 1) {
            // "newAlign" field wasn't present, so use the old "align" field.
            setAlignment(this.align);
        }
        serialVersionOnStream = currentSerialVersion;
    }

    /**
     * Returns a string representation of this <code>FlowLayout</code> object and
     * its values.
     * 
     * @return a string representation of this layout
     */
    @Override
    public String toString() {
        String str = "";
        switch (align) {
        case LEFT:
            str = ",align=left";
            break;
        case CENTER:
            str = ",align=center";
            break;
        case RIGHT:
            str = ",align=right";
            break;
        case LEADING:
            str = ",align=leading";
            break;
        case TRAILING:
            str = ",align=trailing";
            break;
        }
        return getClass().getName() + "[hgap=" + hgap + ",vgap=" + vgap + str + "]";
    }

}

Example code:

public class WrapLayoutExample {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(WrapLayoutExample::runGui);
    }

    private static void runGui() {
        JFrame frame = new JFrame("A");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel panel = new JPanel(new WrapAndAlignHorizontallyTopLayout());

        JLabel l = new JLabel("CCC");
        l.setBorder(BorderFactory.createLineBorder(Color.red, 10));
        l.setVerticalAlignment(JLabel.TOP);

        panel.add(l);

        for (int i = 0; i < 44; i++) {
            l = new JLabel("BBBB");
            l.setBorder(BorderFactory.createLineBorder(Color.red, (int) (Math.random() * 50)));
            panel.add(l);
        }

        frame.setSize(300, 300);
        frame.setLayout(new BorderLayout());
        frame.add(new JScrollPane(panel));
        frame.setVisible(true);
    }
}

Result: