Painting works fine under OS X, but vanishes under Linux (Fedora 6)

Question

I've written some code to draw arrays and objects, which I'm using in the CS2 class I teach (where the students are learning about references). It works fine under OS X, but (to my horror) when I tried using it in front of the students in our Linux labs, the diagrams would disappear (i.e., the window would become blank) after a few milliseconds. Sometimes we could get them to stick around by resizing the window, but this was not reliable.
I should note that my OS X testing is under Java 1.5 (because Macs don't have 1.6 yet) and the Linux (Fedora 6, I believe) is using Java 1.6.
The code is given below. There are two classes, ViewPanel and ObjectView. Run the main() method in ViewPanel. Both of these are in the cs2tools package.
Any help would be greatly appreciated.
package cs2tools;
import java.awt.*;
import java.awt.font.*;
import javax.swing.*;
import java.util.*;
import java.util.List; // For disambiguation
/** Provides methods to graphically display objects and arrays. */
public class ViewPanel extends JPanel {
     /** Singleton instance to allow static method calls, hiding OOP details from students. */
     private static ViewPanel instance = null;
     private static final long serialVersionUID = 1L;
     protected static void createInstance() {
          if (instance == null) {
               JFrame frame = new JFrame();
               frame.setSize(640, 480);
               frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
               instance = new ViewPanel();
               frame.add(instance);
               frame.setVisible(true);
     /** Displays object at position x, y. */
     public static void display(Object object, int x, int y) {
          display(object, x, y, "");
     /** Displays object at position x, y, using name in the title. */
     public static void display(Object object, int x, int y, String name) {
          createInstance();
          instance.addObject(x, y, object, name);
          instance.repaint();
     /** Refreshes the graphics to show any changes to displayed objects. */
     public static void refresh() {
          if (instance != null) {
               instance.repaint();
     private FontRenderContext fontRenderContext;
     private Graphics2D graphics;
     private List<Object> objects;
     private List<ObjectView> views;
     protected ViewPanel() {
          objects = new ArrayList<Object>();
          views = new ArrayList<ObjectView>();
          setBackground(new Color(0x8fbfff)); // A tasteful light blue
     protected void addObject(int x, int y, Object object, String instanceName) {
          if (objects.contains(object)) {
               throw new Error("The object " + object + " has already been displayed");
          objects.add(object);
          views.add(new ObjectView(this, x, y, object, instanceName));
     protected FontRenderContext getFontRenderContext() {
          return fontRenderContext;
     protected void paintComponent(Graphics g) {
          super.paintComponent(g);
          graphics = (Graphics2D)g;
          fontRenderContext = graphics.getFontRenderContext();
          for (ObjectView view : views) {
               view.draw();
          for (ObjectView view : views) {
               view.drawPointers(objects, views);
     public static void main(String[] args) {
          int[][] arr = new int[2][3];
          arr[0] = arr[1];
          display(arr, 100, 100);
          display(arr[0], 300, 100);          
package cs2tools;
import java.awt.*;
import java.awt.geom.*;
import java.lang.reflect.*;
import static java.awt.Color.*;
import static java.lang.Math.*;
import java.util.List; // For disambiguation
import static java.lang.reflect.Array.*;
/** View of a single object, used by ViewPanel. */
public class ObjectView extends Rectangle2D.Double {
     /** Height of each field box. */
     public static final int BOX_HEIGHT = 20;
     /** Width of each field box. */
     public static final int BOX_WIDTH = BOX_HEIGHT * 3;
     private static final long serialVersionUID = 1L;
     private Field[] fields;
     private Rectangle2D[] nameRectangles;
     private Object object;
     private ViewPanel panel;
     private String title;
     private Rectangle2D titleBar;
     private Rectangle2D[] valueRectangles;
     public ObjectView(ViewPanel panel, int x, int y, Object object, String name) {
          super(x, y, 0, 0);
          this.object = object;
          this.panel = panel;
          fields = object.getClass().getDeclaredFields();
          // Create title bar
          titleBar = new Rectangle2D.Double(x, y, BOX_WIDTH * 2 + BOX_HEIGHT, BOX_HEIGHT);
          title = name + ":" + object.getClass().getSimpleName();
          if (object.getClass().isArray()) {
               // Adjust size of the entire cs2tools to accomodate fields
               setRect(getX(), getY(), BOX_WIDTH * 2 + BOX_HEIGHT, BOX_HEIGHT * (3 + getLength(object)));
               // Create field name and value rectangles
               nameRectangles = new Rectangle2D[getLength(object)];
               valueRectangles = new Rectangle2D[getLength(object)];
               y += BOX_HEIGHT;
               for (int i = 0; i < getLength(object); i++) {
                    y += BOX_HEIGHT;
                    nameRectangles[i] = new Rectangle2D.Double(x, y, BOX_WIDTH, BOX_HEIGHT);
                    valueRectangles[i] = new Rectangle2D.Double(x + BOX_WIDTH, y, BOX_WIDTH, BOX_HEIGHT);
          } else { // Not an array
               // Adjust size of the entire cs2tools to accomodate fields
               setRect(getX(), getY(), BOX_WIDTH * 2 + BOX_HEIGHT, BOX_HEIGHT * (3 + fields.length));
               // Create field name and value rectangles
               nameRectangles = new Rectangle2D[fields.length];
               valueRectangles = new Rectangle2D[fields.length];
               y += BOX_HEIGHT;
               for (int i = 0; i < fields.length; i++) {
                    fields.setAccessible(true);
                    y += BOX_HEIGHT;
                    nameRectangles[i] = new Rectangle2D.Double(x, y, BOX_WIDTH, BOX_HEIGHT);
                    valueRectangles[i] = new Rectangle2D.Double(x + BOX_WIDTH, y, BOX_WIDTH, BOX_HEIGHT);
     protected void centerDot(Rectangle2D rect, Graphics2D graphics) {
          double radius = min(rect.getWidth() / 4, rect.getHeight() / 4);
          double x = rect.getCenterX();
          double y = rect.getCenterY();
          graphics.fillOval((int)(x - radius), (int)(y - radius), (int)(radius * 2), (int)(radius * 2));
     protected void centerText(String text, Rectangle2D rect, Graphics2D graphics) {
          Rectangle2D bounds = panel.getFont().getStringBounds(text, panel.getFontRenderContext());
          double x = (rect.getWidth() - bounds.getWidth()) / 2;
          double y = ((rect.getHeight() - bounds.getHeight()) / 2) - bounds.getY();
          graphics.drawString(text, (int)(rect.getX() + x), (int)(rect.getY() + y));
     /** Draw this ObjectView, but not its pointers. */
     public void draw() {
          try {
               Graphics2D graphics = (Graphics2D) (panel.getGraphics());
               // Draw the outer box
               graphics.setColor(LIGHT_GRAY);
               graphics.fill(this);
               graphics.setColor(BLACK);
               graphics.draw(this);
               // Draw the title bar
               graphics.setColor(WHITE);
               graphics.fill(titleBar);
               graphics.setColor(BLACK);
               graphics.draw(titleBar);
               graphics.setColor(BLACK);
               centerText(title, titleBar, graphics);
               // Draw the fields
               for (int i = 0; i < nameRectangles.length; i++) {
                    String name;
                    String value;
                    boolean isPrimitive;
                    if (object.getClass().isArray()) {
                         name = i + "";
                         value = get(object, i) + "";
                         isPrimitive = object.getClass().getComponentType().isPrimitive();
                    } else {                         
                         Field field = fields[i];
                         name = field.getName();
                         value = field.get(object) + "";
                         isPrimitive = field.getType().isPrimitive();
                    graphics.setColor(WHITE);
                    graphics.fill(valueRectangles[i]);
                    graphics.setColor(BLACK);
                    graphics.draw(valueRectangles[i]);
                    centerText(name, nameRectangles[i], graphics);
                    if (isPrimitive) {
                         centerText(value, valueRectangles[i], graphics);
                    } else if (value.equals("null")) {
                         graphics.setColor(BLUE);
                         centerText("null", valueRectangles[i], graphics);
                    } else {
                         graphics.setColor(BLUE);
                         centerDot(valueRectangles[i], graphics);
          } catch (IllegalAccessException e) {
               // We checked for this, so it should never happen
               e.printStackTrace();
               System.exit(1);
     /** Draw lines for any pointers from this ObjectView's fields to other ObjectViews in views. */
     public void drawPointers(List<Object> objects, List<ObjectView> views) {
          try {
               Graphics2D graphics = (Graphics2D) (panel.getGraphics());
               graphics.setColor(BLUE);
               for (int i = 0; i < nameRectangles.length; i++) {
                    boolean isPrimitive;
                    Object target;
                    if (object.getClass().isArray()) {
                         isPrimitive = object.getClass().getComponentType().isPrimitive();
                         target = get(object, i);
                    } else {                         
                         Field field = fields[i];
                         isPrimitive = field.getType().isPrimitive();
                         target = field.get(object);
                    if (!isPrimitive && (target != null)) {
                         for (int j = 0; j < objects.size(); j++) {
                              if (target == objects.get(j)) {
                                   ObjectView targetView = views.get(j);
                                   int x = (int)(valueRectangles[i].getCenterX());
                                   int y = (int)(valueRectangles[i].getCenterY());
                                   int tx = (int)(targetView.getCenterX());
                                   int ty = (int)(targetView.getCenterY());
                                   if (abs(x - tx) > abs(y - ty)) { // X difference larger -- horizontal line
                                        if (x < tx) { // Draw line to left side of target
                                             graphics.drawLine(x, y, (int)(targetView.getX()), ty);
                                        } else { // Draw line to right side of target
                                             graphics.drawLine(x, y, (int)(targetView.getX() + targetView.getWidth()), ty);
                                   } else { // Y difference larger -- vertical line
                                        if (y < ty) { // Draw line to top of target
                                             graphics.drawLine(x, y, tx, (int)(targetView.getY()));
                                        } else { // Draw line to bottom of target
                                             graphics.drawLine(x, y, tx, (int)(targetView.getY() + targetView.getHeight()));
          } catch (IllegalAccessException e) {
               // We checked for this, so it should never happen
               e.printStackTrace();
               System.exit(1);

Answer 1

Put your posted code inside code tags, or things like array element access expressions using common indices such as 'i' will be interpreted as italic by the forum software. Code tags are the word 'code' between square braces at the start and '/code' between square braces at the end.
The fact that your code worked on a Mac is dumb luck. The code has a fatal flaw that causes the behavior you see on the Linux machines. Incidentally, Windows shows the same 'disappearing' problem.
The error is in your draw() and drawPointers() methods. Both methods call
panel.getGraphics()There is no guarantee that the Graphics object returned by this call is the same Graphics object being used to paint to the screen at any given moment. On a Mac, it appears to be. Elsewhere it clearly is not. You should be passing along the Graphics object handed to you in the paintComponent() method. This is the same Graphics object associated with the screen.
protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        graphics = (Graphics2D) g;
        fontRenderContext = graphics.getFontRenderContext();
        for (ObjectView view : views) {
            view.draw(g);
        for (ObjectView view : views) {
            view.drawPointers(g, objects, views);
    } public void draw(Graphics g) {
        try {
            Graphics2D graphics = (Graphics2D) g;
public void drawPointers(Graphics g, List<Object> objects, List<ObjectView> views) {
        try {
            Graphics2D graphics = (Graphics2D) g;