How to restrict the maximum size of a java.awt.ScrollPane

Question

Dear all,
I would like to implement a scroll pane which is resizable, but not to a size exceeding the maximum size of the java.awt.Canvas that it contains.
I've sort of managed to do this by writing a subclass of java.awt.ScrollPane which implements java.awt.event.ComponentListener and has a componentResized method that checks whether the ScrollPane's viewport width (height) exceeds the content's preferred size, and if so, resizes the pane appropriately (see code below).
It seems to me, however, that there ought to be a simpler way to achieve this.
One slightly weird thing is that when the downsizing of the pane happens, the content can once be moved to the left by sliding the horizontal scrollbar, but not by clicking on the arrows. This causes one column of gray pixels to disappear and the rightmost column of the content to appear; subsequent actions on the scrollbar does not have any further effect. Likewise, the vertical scrollbar can also be moved up once.
Also, I would like a java.awt.Frame containing such a restrictedly resizable scrollpane, such that the Frame cannot be resized by the user such that its inside is larger than the maximum size of the scrollpane. The difficulty I encountered with that is that setSize on a Frame appears to set the size of the window including the decorations provided by the window manager (fvwm2, if that matters), and I haven't been able to find anything similar to getViewportSize, which would let me find out the size of the area inside the Frame which is available for the scrollpane which the frame contains.
Thanks in advance for hints and advice.
Here's the code of the componentResized method:
  public void componentResized(java.awt.event.ComponentEvent e)
    java.awt.Dimension contentSize = this.content.getPreferredSize();
    this.content.setSize(contentSize);
    java.awt.Dimension viewportSize = getViewportSize();
    System.err.println("MaxSizeScrollPane: contentSize = " + contentSize);
    System.err.println("MaxSizeScrollPane: viewportSize = " + viewportSize);
    int dx = Math.max(0, (int) (viewportSize.getWidth() - contentSize.getWidth()));
    int dy = Math.max(0, (int) (viewportSize.getHeight() - contentSize.getHeight()));
    System.err.println("MaxSizeScrollPane: dx = " + dx + ", dy = " + dy);
    if ((dx > 0) || (dy > 0))
      java.awt.Dimension currentSize = getSize();
      System.err.println("MaxSizeScrollPane: currentSize = " + currentSize);
      setSize(new java.awt.Dimension(((int) currentSize.getWidth()) - dx, ((int) currentSize.getHeight()) - dy));
    System.err.println();
  }Best regards, Jan

Answer 1

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
public class ScrollPaneTest
    GraphicCanvas canvas;
    CustomScrollPane scrollPane;
    private Panel getScrollPanel()
        canvas = new GraphicCanvas();
        scrollPane = new CustomScrollPane();
        scrollPane.add(canvas);
        // GridBagLayout allows scrollPane to remain at
        // its preferred size during resizing activity
        Panel panel = new Panel(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        panel.add(scrollPane, gbc);
        return panel;
    private WindowListener closer = new WindowAdapter()
        public void windowClosing(WindowEvent e)
            System.exit(0);
    private Panel getUIPanel()
        int w = canvas.width;
        int h = canvas.height;
        int visible = 100;
        int minimum = 200;
        int maximum = 500;
        final Scrollbar
            width  = new Scrollbar(Scrollbar.HORIZONTAL, w,
                                   visible, minimum, maximum),
            height = new Scrollbar(Scrollbar.HORIZONTAL, h,
                                   visible, minimum, maximum);
        AdjustmentListener l = new AdjustmentListener()
            public void adjustmentValueChanged(AdjustmentEvent e)
                Scrollbar scrollbar = (Scrollbar)e.getSource();
                int value = scrollbar.getValue();
                if(scrollbar == width)
                    canvas.setWidth(value);
                if(scrollbar == height)
                    canvas.setHeight(value);
                canvas.invalidate();
                scrollPane.validate();
        width.addAdjustmentListener(l);
        height.addAdjustmentListener(l);
        Panel panel = new Panel(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.insets = new Insets(2,2,2,2);
        gbc.weightx = 1.0;
        addComponents(new Label("width"),  width,  panel, gbc);
        addComponents(new Label("height"), height, panel, gbc);
        gbc.anchor = GridBagConstraints.CENTER;
        return panel;
    private void addComponents(Component c1, Component c2, Container c,
                               GridBagConstraints gbc)
        gbc.anchor = GridBagConstraints.EAST;
        c.add(c1, gbc);
        gbc.anchor = GridBagConstraints.WEST;
        c.add(c2, gbc);
    public static void main(String[] args)
        ScrollPaneTest test = new ScrollPaneTest();
        Frame f = new Frame();
        f.addWindowListener(test.closer);
        f.add(test.getScrollPanel());
        f.add(test.getUIPanel(), "South");
        f.pack();
        f.setLocation(200,200);
        f.setVisible(true);
        f.addComponentListener(new FrameSizer(f));
class GraphicCanvas extends Canvas
    int width, height;
    public GraphicCanvas()
        width = 300;
        height = 300;
    public void paint(Graphics g)
        super.paint(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        int dia = Math.min(width, height)*7/8;
        g2.setPaint(Color.blue);
        g2.draw(new Rectangle2D.Double(width/16, height/16, width*7/8, height*7/8));
        g2.setPaint(Color.green.darker());
        g2.draw(new Ellipse2D.Double(width/2 - dia/2, height/2 - dia/2, dia-1, dia-1));
        g2.setPaint(Color.red);
        g2.draw(new Line2D.Double(width/16, height*15/16-1, width*15/16-1, height/16));
    public Dimension getPreferredSize()
        return new Dimension(width, height);
    public Dimension getMaximumSize()
        return getPreferredSize();
    public void setWidth(int w)
        width = w;
        repaint();
    public void setHeight(int h)
        height = h;
        repaint();
class CustomScrollPane extends ScrollPane
    Dimension minimumSize;
    public Dimension getPreferredSize()
        Component child = getComponent(0);
        if(child != null)
            Dimension d = child.getPreferredSize();
            if(minimumSize == null)
                minimumSize = (Dimension)d.clone();
            Insets insets = getInsets();
            d.width  += insets.left + insets.right;
            d.height += insets.top + insets.bottom;
            return d;
        return null;
    public Dimension getMinimumSize()
        return minimumSize;
    public Dimension getMaximumSize()
        Component child = getComponent(0);
        if(child != null)
            return child.getMaximumSize();
        return null;
class FrameSizer extends ComponentAdapter
    Frame f;
    public FrameSizer(Frame f)
        this.f = f;
    public void componentResized(ComponentEvent e)
        Dimension needed = getSizeForViewport();
        Dimension size = f.getSize();
        if(size.width > needed.width || size.height > needed.height)
            f.setSize(needed);
            f.pack();
     * returns the minimum required frame size that will allow
     * the scrollPane to be displayed at its preferred size
    private Dimension getSizeForViewport()
        ScrollPane scrollPane = getScrollPane(f);
        Insets insets = f.getInsets();
        int w = scrollPane.getWidth() + insets.left + insets.right;
        int h = getHeightOfChildren() + insets.top + insets.bottom;
        return new Dimension(w, h);
    private ScrollPane getScrollPane(Container cont)
        Component[] c = cont.getComponents();
        for(int j = 0; j < c.length; j++)
            if(c[j] instanceof ScrollPane)
                return (ScrollPane)c[j];
            if(((Container)c[j]).getComponentCount() > 0)
                return getScrollPane((Container)c[j]);
        return null;
    private int getHeightOfChildren()
        Component[] c = f.getComponents();
        int extraHeight = 0;
        for(int j = 0; j < c.length; j++)
            int height;
            if(((Container)c[j]).getComponent(0) instanceof ScrollPane)
                height = ((Container)c[j]).getComponent(0).getHeight();
            else
                height = c[j].getHeight();
            extraHeight += height;
        return extraHeight;
}