JScrollPane (added to a JPanel, which paints) is visible but does nothing

Question

Hi. I am writing a program that reads data from files and does paiting using the info. The top level window is a JFrame, which is divided into two parts - A Box which contains the buttons through which the user interacts and a JPanel on which the painting is done. I have added the JPanel to a JScrollPane, but have run into problems there. The JScrollPane is visible (both policies always visible) but it does nothing. I understand that when the drawing is fitting, the ScrollPane is not used. But even if I make the window small or draw something that is not visible in the normal are, the JScrollPane doesn't work. In fact, no knob is visible on either of the two scrollbars. I am pasting the relevant code below:
//import
public class MainWindow extends JFrame {
    public static void Main(String[] args) {
        new MainWindow();
    //Declare all the variables here.
    private Box buttionBox;
    private JScrollPane scroller;
    private HelloPanel drawingPanel;   
    //other variables
    //The constructor for the class MainWindow.
    public MainWindow() {
       initComponents();
        this.setLayout(new BorderLayout());
        this.setPreferredSize(new Dimension(900,670));
        //buttonBox containts the buttons - not very relevant to this problem.
        this.getContentPane().add(buttonBox, BorderLayout.WEST);
        //scroller is the JScrollPane
        this.getContentPane().add(scroller, BorderLayout.CENTER);       
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setTitle("My Title");       
        this.setVisible(true);
        this.pack();       
    public void initComponents() {
        buttonBox = Box.createVerticalBox();
        //instantiate the buttons here and add them to the ButtonBox.
        //The event listeners instantiated. Not relevant.       
        //The various components are assigned their appropriate event listeners here.
         //Not relevant.
//Now adding all the buttons to the box with proper spacing.
        buttonBox.add(Box.createVerticalStrut(20));
        //This is the drawing panel on which the drawing will be done.
        drawingPanel = new HelloPanel();
        scroller = new JScrollPane(drawingPanel);
                scroller.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        scroller.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
        scroller.getHorizontalScrollBar().setUnitIncrement(10);
        scroller.getVerticalScrollBar().setUnitIncrement(10);
    //This inner class is used to define and implement the event listener for handling the checkboxes.
    private class checkBoxListener implements ItemListener{
        public void itemStateChanged(ItemEvent e){
             drawingPanel.repaint();
                //Implement actions. Irrelevant
    //This private class is used to define and implement the event listener for the buttons.
    private class buttonListener implements ActionListener {
        //Do this when the button which has an instance of this class as its listener is clicked.       
        public void actionPerformed(ActionEvent e) {
            if (e.getSource() == saveStructureButton){
              //Implement action. Irrelvant.
    //The panel on which the drawings are done.
    private class HelloPanel extends JPanel {
        //This JPanel is used for drawing the image (for the purpose of saving to disk).
        JComponent c;    //For image. Irrelevant.
        public HelloPanel(){           
            c = this;   //This is necessary for drawing the image to be saved to disk.
            this.setBackground(Color.WHITE);
        //This is the method that actually paints all the drawings whenever a.
        //The shapes theselves can be defined somewhere else, but that paint method must be invoked from here.
        public void paintComponent(Graphics g){
            super.paintComponent(g);    //First of all, clear the panel.
            Graphics2D g2 = (Graphics2D) g;
            g2.fill(new Rectangle2D.Double(40,40,100,100));    //Just for this post.
            g2.drawString("Text", 750, 750);    //To test the scrollpane.
} I hope this helps you get an idea of what I am trying to do and what the problem might be. Any help would be really appreciated. I have spent hours on this and I have no idea why it doesn't work.
The actual code is much bigger, so if you need any extra information, please tell me.

Answer 1

HelloPanel should provide a "public Dimension getPreferredSize()" method.
With your code you create a simple JPanel and want to draw outside of it, but you never tell the JScrollPane that your HelloPanel is actually bigger than it seems, that's why it does not feel the need to add scroll bars.
Here's the working code:
//import
import javax.swing.*;
import java.awt.*;
import java.awt.geom.*;
import java.awt.event.*;
public class MainWindow extends JFrame {
    public static void main(String[] args) {
        new MainWindow();
    //Declare all the variables here.
    private Box buttionBox;
    private JScrollPane scroller;
    private HelloPanel drawingPanel; 
    //other variables
    //The constructor for the class MainWindow.
    public MainWindow() {
       initComponents();
        this.setLayout(new BorderLayout());
        this.setPreferredSize(new Dimension(900,670));
        //buttonBox containts the buttons - not very relevant to this problem.
        //this.getContentPane().add(buttonBox, BorderLayout.WEST);
        //scroller is the JScrollPane
        this.getContentPane().add(scroller, BorderLayout.CENTER);       
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setTitle("My Title");       
        this.setVisible(true);
        this.pack();       
    public void initComponents() {
        //buttonBox = Box.createVerticalBox();
        //instantiate the buttons here and add them to the ButtonBox.
        //The event listeners instantiated. Not relevant.       
        //The various components are assigned their appropriate event listeners here.
         //Not relevant.
//Now adding all the buttons to the box with proper spacing.
        //buttonBox.add(Box.createVerticalStrut(20));
        //This is the drawing panel on which the drawing will be done.
        drawingPanel = new HelloPanel();
        scroller = new JScrollPane(drawingPanel);
                scroller.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        scroller.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
        scroller.getHorizontalScrollBar().setUnitIncrement(10);
        scroller.getVerticalScrollBar().setUnitIncrement(10);
    //This inner class is used to define and implement the event listener for handling the checkboxes.
    private class checkBoxListener implements ItemListener{
        public void itemStateChanged(ItemEvent e){
             drawingPanel.repaint();
                //Implement actions. Irrelevant
    //This private class is used to define and implement the event listener for the buttons.
    private class buttonListener implements ActionListener {
        //Do this when the button which has an instance of this class as its listener is clicked.       
        public void actionPerformed(ActionEvent e) {
            //if (e.getSource() == saveStructureButton){
              //Implement action. Irrelvant.
    //The panel on which the drawings are done.
    private class HelloPanel extends JPanel {
        //This JPanel is used for drawing the image (for the purpose of saving to disk).
        JComponent c;    //For image. Irrelevant.
        public HelloPanel(){           
            c = this;   //This is necessary for drawing the image to be saved to disk.
            this.setBackground(Color.WHITE);
        //This is the method that actually paints all the drawings whenever a.
        //The shapes theselves can be defined somewhere else, but that paint method must be invoked from here.
        public void paintComponent(Graphics g){
            super.paintComponent(g);    //First of all, clear the panel.
            Graphics2D g2 = (Graphics2D) g;
            g2.fill(new Rectangle2D.Double(40,40,100,100));    //Just for this post.
            g2.drawString("Text", 750, 750);    //To test the scrollpane.
     public Dimension getPreferredSize()
     return new Dimension(750,750);
}