Add the same JComponent in more than one JPanel

Question

Hello!
I'm making a simple drawing application where the user can draw common shapes, move and delete them. However, I'm trying to make two windows view/observe the same "picture". So all shapes added, moved or removed in the picture will happen in both places. My problem seems to be a restriction in Swing; when I add the same shape (same reference) to two different JPanels, only one of the JPanels draw the shape. However if I copy the shape (different references) and add the copy to the other JPanel it gets drawn. This code/application will demonstrate the problem:
import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JComponent;
public class Square extends JComponent
     protected void paintComponent(Graphics g)
          super.paintComponent(g);
          int x = 0;
          int y = 0;
          int width = this.getWidth();
          int height = this.getHeight();
          g.setColor(Color.RED);          
          g.fillRect(x, y, width, height);
import javax.swing.*;
public class SwingHelp
     public static void main(String[] args)
          JFrame frame1 = new JFrame("1");
          JFrame frame2 = new JFrame("2");
          JPanel panel1 = new JPanel();
          JPanel panel2 = new JPanel();
          Square mySquare = new Square();
          mySquare.setBounds(50, 50, 75, 75);
          panel1.setLayout(null);
          panel2.setLayout(null);
          panel1.add(mySquare);          
          panel2.add(mySquare);
          frame1.add(panel1);
          frame2.add(panel2);
          frame1.setSize(500, 500);
          frame2.setSize(500, 500);
          frame1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          frame2.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          frame1.setVisible(true);
          frame2.setVisible(true);
}Is there any setting or workaround to get the desired effect I want? Any help is appreciated :-)

Answer 1

Here's a simple example. I have a program called RectangleDrawMaster that draws and displays rectangles on mouse drag and a program called RectangleDrawSlave that uses the master's "model" an ArrayList of ColorRect (objects that have rectangle coordinates and colors). So while both classes use different drawing JPanels, these JPanels share the same model and the images look the same.
ColorRect.java: holds color and rectangle data
import java.awt.Color;
import java.awt.geom.Rectangle2D;
class ColorRect
    private Color c;
    private Rectangle2D rect;
    ColorRect(Color c, Rectangle2D rect)
        this.c = c;
        this.rect = rect;
    public Color getColor()
        return c;
    public Rectangle2D getRect()
        return rect;
import java.awt.*;
import java.awt.event.*;
import java.awt.geom.Rectangle2D;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import javax.swing.*;
import javax.swing.event.*;
public class RectangleDrawMaster
  private static final int COLOR_MIN = 0;
  private static final int COLOR_MAX = 255;
  private static final Dimension PANEL_SIZE = new Dimension(500, 500);
  //private BufferedImage bImage;
  private Point firstPoint;
  private Point lastPoint;
  private List<ColorRect> rectList = new ArrayList<ColorRect>();
  private Random random = new Random();
  private JPanel mainPanel = new JPanel();
  private List<ChangeListener> changeList = new ArrayList<ChangeListener>();
  private JPanel drawingPanel = new JPanel()
    @Override
    protected void paintComponent(Graphics g)
      super.paintComponent(g);
      panelDraw(g);
  public RectangleDrawMaster()
    drawingPanel.addMouseListener(new MyMouseListener());
    JButton clearBtn = new JButton("Clear");
    clearBtn.addActionListener(new ActionListener()
      @Override
      public void actionPerformed(ActionEvent e)
        clearAction();
    mainPanel.setPreferredSize(PANEL_SIZE);
    mainPanel.setLayout(new BorderLayout());
    mainPanel.add(drawingPanel, BorderLayout.CENTER);
    mainPanel.add(clearBtn, BorderLayout.SOUTH);
  public void addChangeListener(ChangeListener cl)
    changeList.add(cl);
  public List<ColorRect> getRectList()
    return rectList;
  private void clearAction()
    rectList.clear();
    drawingPanel.repaint();
    invokeChangeListeners();
  private void invokeChangeListeners()
    ChangeEvent e = new ChangeEvent(this);
    for (ChangeListener cl : changeList)
      cl.stateChanged(e);
  private void panelDraw(Graphics g)
    Graphics2D g2 = (Graphics2D) g;
    for (ColorRect rect : rectList)
      g2.setColor(rect.getColor());
      g2.fill(rect.getRect());
  private Color randomColor()
    Color c = new Color(
        random.nextInt(COLOR_MAX - COLOR_MIN) + COLOR_MIN,
        random.nextInt(COLOR_MAX - COLOR_MIN) + COLOR_MIN,
        random.nextInt(COLOR_MAX - COLOR_MIN) + COLOR_MIN);
    return c;
  public JPanel getMainPanel()
    return mainPanel;
  private void drawRectangle()
    int width = Math.abs(firstPoint.x - lastPoint.x);
    int height = Math.abs(firstPoint.y - lastPoint.y);
    int x = Math.min(firstPoint.x, lastPoint.x);
    int y = Math.min(firstPoint.y, lastPoint.y);
    Rectangle2D rect = new Rectangle2D.Double(x, y, width, height);
    Color c = randomColor();
    rectList.add(new ColorRect(c, rect));
    drawingPanel.repaint();
    invokeChangeListeners();
  private class MyMouseListener extends MouseAdapter
    public void mousePressed(MouseEvent e)
      firstPoint = e.getPoint();
    @Override
    public void mouseReleased(MouseEvent e)
      lastPoint = e.getPoint();
      drawRectangle();
  private static void createAndShowUI()
    RectangleDrawMaster rectdraw = new RectangleDrawMaster();
    JFrame frame = new JFrame("Rect Draw Frame");
    frame.getContentPane().add(rectdraw.getMainPanel());
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
    final RectangleDrawSlave rectdraw2 = new RectangleDrawSlave(rectdraw.getRectList());
    rectdraw.addChangeListener(new ChangeListener()
      public void stateChanged(ChangeEvent e)
        rectdraw2.repaint();
    JDialog dialog = new JDialog(frame, "Rect Draw Dialog", false);
    dialog.getContentPane().add(rectdraw2.getMainPanel());
    dialog.pack();
    dialog.setVisible(true);
  public static void main(String[] args)
    java.awt.EventQueue.invokeLater(new Runnable()
      public void run()
        createAndShowUI();
import java.awt.*;
import java.util.List;
import javax.swing.JPanel;
public class RectangleDrawSlave
    private static final Dimension PANEL_SIZE = new Dimension(500, 500);
    //private BufferedImage bImage;
    private List<ColorRect> rectList;
    private JPanel mainPanel = new JPanel();
    private JPanel drawingPanel = new JPanel()
        @Override
        protected void paintComponent(Graphics g)
            super.paintComponent(g);
            panelDraw(g);
    public RectangleDrawSlave(List<ColorRect> rectList)
        this.rectList = rectList;
        mainPanel.setPreferredSize(PANEL_SIZE);
        mainPanel.setLayout(new BorderLayout());
        mainPanel.add(drawingPanel, BorderLayout.CENTER);
    public void repaint()
      drawingPanel.repaint();
    private void panelDraw(Graphics g)
        Graphics2D g2 = (Graphics2D)g;
        for (ColorRect rect : rectList)
            g2.setColor(rect.getColor());
            g2.fill(rect.getRect());
    public JPanel getMainPanel()
        return mainPanel;
}