JPanel size setting

Question

I have been programming in java for a while, but haven't done any gui. Lately, I had to develop a gui for my program so i went through the Sun tutorial and have been practicing gui programming for a while. However, I couldn't figure out how to solve my following problem. I did some googling, but haven't been able to find a solution. My problem description is as follows:
I have a JPanel which is added to the JScrollPane and which is added to the JFrame.
I have set a preferred size for the JPanel. I want to draw on this JPanel. Suppose say I am drawing lines.
If the coordinate for the two points of the line exceed the preferred size of the JPanel or the screen size I cannot see the drawings and no JScrollPane. Better explained by the following SSCCE code.
import java.awt.Dimension;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
public class MainFrame extends JFrame{
     private DrawPanel drawPanel;
     private JScrollPane drawScrollPane;
     public MainFrame(){
          createGui();
     public static void main(String[] args){
          javax.swing.SwingUtilities.invokeLater(new Runnable(){
               public void run(){
                    new MainFrame().setVisible(true);
     private void createGui(){
          setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          setSize(400,400);
          drawPanel = new DrawPanel();
          drawPanel.setPreferredSize(new Dimension(500,500));
          drawScrollPane = new JScrollPane(drawPanel);
          drawScrollPane.setPreferredSize(new Dimension(400,400));
          getContentPane().add(drawScrollPane);
          pack();
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JPanel;
public class DrawPanel extends JPanel{
     public void paintComponent(Graphics g){
          super.paintComponent(g);
          Graphics2D g2d = (Graphics2D)g;
          g2d.drawLine(10, 10, 400, 400); //visible
          g2d.drawLine(550,550,650,650); //not visible in default size, but visible in full screen
          g2d.drawLine(2000,2000,2100,2100); // not visible at all
}In my real program, I want to draw few lines, but the maximum coordinates for the lines are not known and it
depends on case by case. In such situation, how could I display drawings? What am i doing wrong?
Any help on what I should be searching for, or any links I should refer to be will be very helpful.
Thank you in advance for any help. If I need to provide more, then please let me know.

Answer 1

My question when I read your post is "why do you wanna see something that is still off screen?"
If you want to see your drawing then you need to scale the drawing to fit the panel. To do this effectively, without much flicker, then using a BufferedImage in addition to the custom painting might help with double buffering (I think Swing primarily takes care of that. I added it just in case, didn't have too much time to test that)
import java.awt.*;
import java.awt.image.*;
import java.awt.geom.*;
import javax.swing.*;
public class MainFrame extends JFrame{
    private DrawPanel drawPanel;
    private JScrollPane drawScrollPane;
    public MainFrame(){
        createGui();
    public static void main(String[] args){
        javax.swing.SwingUtilities.invokeLater(new Runnable(){
            public void run(){
                new MainFrame().setVisible(true);
    private void createGui(){
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(400,400);
        drawPanel = new DrawPanel();
        drawPanel.setPreferredSize(new Dimension(500,500));
        drawScrollPane = new JScrollPane(drawPanel);
        drawScrollPane.setPreferredSize(new Dimension(400,400));
        getContentPane().add(drawScrollPane);
        pack();
class DrawPanel extends JPanel{
    BufferedImage bgImage = null;
    public void paintComponent(Graphics g){
        super.paintComponent(g);
        if( bgImage == null || (getWidth() != bgImage.getWidth()) ) {
            bgImage = (BufferedImage)createImage( getWidth(), getHeight() );
        Graphics2D g2d = bgImage.createGraphics();
        // perform image scaling on the buffered to fit panel size
        double maxCoord = 2100;
        double xscale = bgImage.getWidth() / maxCoord;
        double yscale = bgImage.getHeight() / maxCoord;
        g2d.scale( xscale, yscale );
        g2d.drawLine(10, 10, 400, 400); //visible
        g2d.drawLine(550,550,650,650); //not visible in default size, but visible in full screen
        g2d.drawLine(2000,2000,2100,2100); // not visible at all
        g.drawImage( bgImage, 0, 0, this);
}ICE