Forcing a viewport to only scroll vertically

Question

I've created a custom LayoutManager that extends FlowLayout. Unlike FlowLayout, this layout manager creates as many columns as it can in the bounding area and justifies the component in the columns. Unlike any of the other layout managers, the number of columns is dynamic, changing as the user stretches the component.
The problem I have occurs when the developer wraps the panel in a JScrollPane. At this point, the manel inside the scroll pane has no bounded fixed size. The scrollpane basically allows the panel to be any size and it will scroll the contents appropriately. I would like the scroll pane to use the panel's preferred width and scroll the contents only in the vertical direction.
Can I accomplish this using only my layout manager, and not having to create a special JPanel class that implements the Scrollable interface or such? The attached code is my layout manager class along with a main method for testing it. Uncomment the commented line to see the problem I am having with the JScrollPane.
import javax.swing.*;
import java.awt.*;
* ColumnarFlowLayout
* @author <a href="mailto:[email protected]">James Cook</a>
public class ColumnarFlowLayout extends FlowLayout {
   // instance ----------------------------------------------------------------
   int _maxColumns = Integer.MAX_VALUE - 1;
   // constructor -------------------------------------------------------------
   public ColumnarFlowLayout() {
   public ColumnarFlowLayout(int maxColumns) {
      _maxColumns = maxColumns;
   public ColumnarFlowLayout(int hgap, int vgap) {
      super(LEFT, hgap, vgap);
   public ColumnarFlowLayout(int hgap, int vgap, int maxColumns) {
      super(LEFT, hgap, vgap);
      _maxColumns = maxColumns;
    // FlowLayout override -----------------------------------------------------
    * Lays out the container. This method lets each component take
    * its preferred size by reshaping the components in the
    * target container.
    * @param target the specified component being laid out
    * @see Container
    * @see       java.awt.Container#doLayout
   public void layoutContainer(Container target) {
      synchronized (target.getTreeLock()) {
         Insets insets = target.getInsets();
         int maxwidth = target.getWidth() - (insets.left + insets.right);
         // Stuff all of the visible components into an array
         int index = -1;
         int nmembers = target.getComponentCount();
         Component[] tempComps = new Component[nmembers];
         for (int i = 0; i < nmembers; i++) {
            Component m = target.getComponent(i);
            if (m.isVisible()) {
               index++;
               Dimension d = m.getPreferredSize();
               m.setSize(d.width, d.height);
               tempComps[index] = m;
         // Get rid of any unused component cells
         Component[] comps = new Component[index+1];
         System.arraycopy(tempComps, 0, comps, 0, comps.length);
         tempComps = null;
         // Determine the appropriate number of columns (and their widths)
         // for the given maxwidth
         int[] columnWidths = getColumnWidths(comps, maxwidth, getHgap(), _maxColumns);
         int x = 0, y = 0, maxHeight = 0;
         for (int i = 0; i < comps.length; i++) {
            Component m = comps;
int columnIndex = i % columnWidths.length;
x += (columnIndex == 0) ? 0 : columnWidths[columnIndex - 1] + getHgap();
m.setLocation(insets.left + x, insets.top + y);
maxHeight = Math.max(maxHeight, m.getHeight());
if (columnIndex == columnWidths.length - 1) {
y += maxHeight + getVgap();
x = 0;
// private -----------------------------------------------------------------
* Determine the maximum widths per column of the greatest number of
* components that will fit into a maxWidth space.
* @param comps The components to layout.
* @param maxWidth The maximum width available to layout components.
* @param hgap The horizontal space (in pixels) between components.
* @param maxColumns The maximum number of columns to use.
* @return An array that contains the widths of the resulting columns.
private int[] getColumnWidths(Component[] comps, int maxWidth, int hgap, int maxColumns) {
int[] results;
int columnCount = Math.min(comps.length + 1, maxColumns + 1);
int width = 0;
// Keep reducing the number of columns until all of the components fit
// into the allowable space. Note: The minimum number of columns will
// be one, regardless of whether it fits or not.
do {
columnCount--;
results = new int[columnCount];
for (int i = 0; i < comps.length; i++) {
int columnIndex = i % columnCount;
int componentWidth = comps[i].getSize().width;
results[columnIndex] = Math.max(results[columnIndex], componentWidth);
width = hgap * (columnCount - 1);
for (int i = 0; i < results.length; i++)
width += results[i];
} while (width > maxWidth && columnCount > 1);
return results;
// static ------------------------------------------------------------------
public static void main(String[] args) {
JFrame frame = new JFrame("ColumnarFlowLayout Example");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JComponent panel = new JPanel(new ColumnarFlowLayout(4));
String s = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
StringBuffer sb = new StringBuffer();
for (int i = 0; i < 50; i++) {
sb.setLength(0);
for (int j = 0, c = 1 + (int)(Math.random() * 10); j < c; j++)
sb.append(s.charAt((int)(Math.random() * 26)));
panel.add(new JCheckBox(sb.toString()));
// Uncomment the following line to see the problem. A scrollpane provides the
// underlying layout manager with no limits on space. Thus, the layout manager
// isn't able to respect any boundaries.
// panel = new JScrollPane(panel);
frame.getContentPane().add(panel);
frame.pack();
frame.setSize(600,600);
frame.setVisible(true);

Answer 1

This may, or may not, help you. In my [url http://www.discoverteenergy.com/files/WrapLayout.java]WrapLayout class I had some success doing something similiar to what you need. It may give you some ideas.