Scrollbars with painting - why this doesn't work properly ?

Question

Hello I written such a small program:
/* Test.java */
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Test extends JFrame {
     private JPanel mainPanel;
     private InnerPanel innerPanel; // Panel w/ JScrollPane
     private JScrollPane scroll;
     private JButton addItem;
     public static final int WINDOW_WIDTH = 200;
     public static final int WINDOW_HEIGHT = 200;
     public Test () {
          buildPanel();
          add(mainPanel);
          setDefaultCloseOperation(EXIT_ON_CLOSE);
          setSize(WINDOW_WIDTH, WINDOW_HEIGHT);
          setVisible(true);
     public void buildPanel() {
          mainPanel = new JPanel(new BorderLayout());
          innerPanel = new InnerPanel();
          scroll = new JScrollPane(innerPanel);
          scroll.setPreferredSize(new Dimension(100, 100));
          addItem = new JButton("Add an item to JPanel");
          addItem.addActionListener(new ButtonListener());
          mainPanel.add(scroll, BorderLayout.CENTER);
          mainPanel.add(addItem, BorderLayout.PAGE_START);
     private class ButtonListener implements ActionListener {
          public void actionPerformed(ActionEvent e) {
               innerPanel.increase();
               innerPanel.revalidate();
               scroll.revalidate();
               scroll.getVerticalScrollBar().setValue(
                    (scroll.getVerticalScrollBar()).getMaximum());
     public static void main(String[] args) {
          Test test = new Test();
class InnerPanel extends JPanel {
     int x0 = 50, y0 = 50, x1 = 100, y1 = 50;
     public void paintComponent(Graphics g) {
          g.drawLine(x0,y0,x1,y1);
     public void increase() {
          x1 += 50;
          setPreferredSize(new Dimension(x1 + 100, y0));
          repaint();
}I click for example 5 times on the button and then move the scroll max to the right.
The end of the line should be already visible, but it's not. However it will be,
when you resize the frame just a bit.
Now when I try to move scroll max to the left but the beginning of the line doesn't show up. However it will , when you resize the frame just a bit.
Can anybody point where I made a mistake ?

Answer 1

Not sure I understand the description of the problem. I didn't notice any change in painting when I resized the frame, but anyway I made the following changes to help with the painting:
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Test6 extends JFrame {
     private JPanel mainPanel;
     private InnerPanel innerPanel; // Panel w/ JScrollPane
     private JScrollPane scroll;
     private JButton addItem;
     public static final int WINDOW_WIDTH = 200;
     public static final int WINDOW_HEIGHT = 200;
     public Test6 () {
          buildPanel();
          getContentPane().add(mainPanel);
          setDefaultCloseOperation(EXIT_ON_CLOSE);
          setSize(WINDOW_WIDTH, WINDOW_HEIGHT);
          setVisible(true);
     public void buildPanel() {
          mainPanel = new JPanel(new BorderLayout());
          innerPanel = new InnerPanel();
          scroll = new JScrollPane(innerPanel);
          scroll.setPreferredSize(new Dimension(100, 100));
          addItem = new JButton("Add an item to JPanel");
          addItem.addActionListener(new ButtonListener());
          mainPanel.add(scroll, BorderLayout.CENTER);
          mainPanel.add(addItem, BorderLayout.PAGE_START);
     private class ButtonListener implements ActionListener {
          public void actionPerformed(ActionEvent e) {
               innerPanel.increase();
//               innerPanel.revalidate();
//               scroll.revalidate();
               scroll.getVerticalScrollBar().setValue(
                    (scroll.getVerticalScrollBar()).getMaximum());
     public static void main(String[] args) {
          Test6 test = new Test6();
class InnerPanel extends JPanel {
     int x0 = 50, y0 = 50, x1 = 100, y1 = 50;
     public void paintComponent(Graphics g) {
          super.paintComponent(g); // added
          g.drawLine(x0,y0,x1,y1);
     public void increase() {
          x1 += 50;
          setPreferredSize(new Dimension(x1 + 100, y0));
          revalidate();  // added
//          repaint();
}