Question

I have a JPanel with a custom LayoutManager and trying to get it inside a JScrollPane, however, the scroll pane doesn’t seem to know it’s supposed to enable scrolling. No matter how I set preferred size of the components (that is what the javadoc for JScrollPane says it needs), it makes absolutely no difference. Here is the code:

    canvas = new JPanel();
    JScrollPane scroll = new JScrollPane();
    scroll.setViewportView(canvas);
    scroll.setPreferredSize(new Dimension(924,700));
    canvas.setPreferredSize(new Dimension(2000,2000));

The scrollbars don’t appear. The container of the JScrollPane is set to respect its preferred size. I have checked through the debugger that the preferred size of the canvas is indeed 2000 x 2000 at all times (the LayoutManager ensures this), so what might be going on here?

Answer 1

This is not really an answer, but is a demo sscce that shows that again, your code works fine for me:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Paint;
import java.awt.RenderingHints;

import javax.swing.*;

public class ScrollBarEg {
   protected static final Paint GRADIENT_PAINT = new GradientPaint(0, 0,
         Color.blue, 50, 50, Color.red, true);

   private static void createAndShowGui() {
      JPanel mainPanel = new JPanel();

      JPanel canvas = new JPanel() {
         @Override
         protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2 = (Graphics2D) g.create();
            g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                  RenderingHints.VALUE_ANTIALIAS_ON);
            g2.setPaint(GRADIENT_PAINT);
            g2.fillOval(0, 0, getWidth(), getHeight());
            g2.dispose();
         }
      };
      JScrollPane scroll = new JScrollPane();
      scroll.setViewportView(canvas);
      scroll.setPreferredSize(new Dimension(924, 700));
      canvas.setPreferredSize(new Dimension(2000, 2000));

      mainPanel.add(scroll);

      JFrame frame = new JFrame("ScrollBar Eg");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

This suggests to me that your error lies elsewhere in code that you’ve not posted.