Implementing a Scrollable JPanel

Question

Does anyone have any suggestions for making a JPanel scrollable? Apparently, simply placing the JPanel inside a JScrollPane doesn't work, since JPanel does not implement Scrollable. I guess the solution would be to implement a custom JScrollablePanel. Has anyone else come across this problem?
Thanks.

Answer 1

Ok - I figured it out - It's the FlowLayout that I'm
using for the JPanel. It doesn't support wrapping.hmm, no, that's not it. FlowLayout is actually a simple/limited layout manager - it's preferred size is always calculated as though it should be on one line, but it will wrap if the panel's actual width is too small and it's height can accomodate more than 1 row. When you put a JPanel in a JScrollPane, the panel's preferred-size is used to size the panel, hence, a single line that goes on forever. The best solution is to subclass your JPanel and implement Scrollable - that is the whole reason that Scrollable exists, so that you can tell the JScrollPane how to scroll. Here's a sample that obviously only works with FlowLayout: import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class ScrollablePanel extends JPanel implements Scrollable {
     public ScrollablePanel() {
          this(FlowLayout.LEFT);
     public ScrollablePanel(int flowLayoutType) {
          super(new FlowLayout(flowLayoutType));
     public ScrollablePanel(int flowLayoutType, int hgap, int vgap) {
          super(new FlowLayout(flowLayoutType, hgap, vgap));
     public Dimension getPreferredSize() {
          if (getParent() == null)
               return getPreferredSize();
                // calculate the preferred size based on the flow of components
          FlowLayout flow = (FlowLayout)getLayout();
          int w = getParent().getWidth();
          int h = flow.getVgap();
          int x = flow.getHgap();
          int rowH = 0;
          Dimension d;
          Component[] comps = getComponents();
          for (int i = 0; i < comps.length; i++) {
               if (comps.isVisible()) {
                    d = comps[i].getPreferredSize();
                    if (x + d.width > w && x > flow.getHgap()) {
                         x = flow.getHgap();
                         h += rowH;
                         rowH = 0;
                         h += flow.getVgap();
                    rowH = Math.max(d.height, rowH);
                    x += d.width + flow.getHgap();
          h += rowH;
          return new Dimension(w, h);
     public Dimension getPreferredScrollableViewportSize() {
          return getPreferredSize();
     public int getScrollableBlockIncrement(Rectangle visibleRect, int orientation, int direction) {
          // shift the view so that you see the next logical page
          if (orientation == SwingConstants.HORIZONTAL) {
               // this won't happen in this case since we never scroll horizontally
               return visibleRect.width;
          return visibleRect.height;
     public boolean getScrollableTracksViewportHeight() {
          // only force the height if the preferred size is too short to fill the viewport
          return getPreferredSize().height < getParent().getHeight();
     public boolean getScrollableTracksViewportWidth() {
          // always force this panel's width to the viewport's width
// this effectively disables horizontal scrolling
          return true;
     public int getScrollableUnitIncrement(Rectangle visibleRect, int orientation, int direction) {
          return 10;
     private static JScrollPane scroller;
     private static ScrollablePanel sp;
     public static void main(String[] args) {
          try {
               SwingUtilities.invokeAndWait(new Runnable() {
                    public void run() {
                         sp = new ScrollablePanel(FlowLayout.LEFT, 20, 20);
                         scroller = new JScrollPane(sp);
                         final JButton button = new JButton("Add Label");
                         button.addActionListener(new ActionListener() {
                              public void actionPerformed(ActionEvent e) {
                                   sp.add(new JLabel("HELLO"));
                                   scroller.validate();
                         JPanel content = new JPanel(new BorderLayout());
                         content.add(button, BorderLayout.NORTH);
                         content.add(scroller);
                         JFrame f = new JFrame("Test");
                         f.setContentPane(content);
                         f.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
                         f.setSize(600, 400);
                         f.setLocationRelativeTo(null);
                         f.setVisible(true);
          catch (Exception e) { e.printStackTrace(); }