Layering two active JPanels on top of each other

I'm trying to create a simple animation whereby a swimmer follows a path underwater. I have two JPanels - one for the swimmer and one for the background. Both are active, the swimmer is moving its limbs and the background has fish/obstacles that need to be updated.

I am trying to put the swimmer on top of the background. I'm trying to use a JLayeredPane, so I create the two layers and add them:

JLayeredPane lp = frame.getLayeredPane();
lp.setPreferredSize(new Dimension(650, 550));

underwaterScene.setSize(lp.getPreferredSize());
underwaterScene.setLocation(0,0);

swimmer.setSize(lp.getPreferredSize());
swimmer.setLocation(0,0);
swimmer.setOpaque(false);

lp.add(underwaterScene, Integer.valueOf(1));
lp.add(swimmer, Integer.valueOf(2));

The swimmer's paintComponent method simply draws the swimmer in the correct place:

g.drawImage(swimmer.sprite, swimmer.x, swimmer.y, 150, 100, null);

Currently, I only see the swimmer layer. If I remove the lp.add(swimmer ... call then I can see the background layer, so I know that it's being added.

My question is: why isn't the swimmer appearing on top of the background without completely blocking it?

Thanks in advance for any help!

Solution

Here is a working example.

Also, you can find out its context here: https://github.com/fzoli/RemoteControlCar/blob/master/desktop/BrowserTest/src/ConstrainVisibility.java#L58

public class ConstrainVisibility extends JPanel {

  protected static final String LS = System.getProperty("line.separator");
  protected static final int OFFSET_X = 150;
  protected static final int OFFSET_Y = 120;
  protected static final int WIDTH = 200;
  protected static final int HEIGHT = 200;

  public ConstrainVisibility() {
    super(new BorderLayout());
    JLayeredPane layeredPane = new JLayeredPane();
    layeredPane.setOpaque(true);
    layeredPane.setBackground(new Color(200, 200, 255));
    int layerIndex = 0;
    // A flash player

    // A swing panel
    JPanel swingPanel = new JPanel();
    swingPanel.setBorder(BorderFactory.createTitledBorder("Swing JPanel"));
    swingPanel.setBackground(Color.GREEN);
    swingPanel.setBounds(OFFSET_X * layerIndex, OFFSET_Y * layerIndex, WIDTH, HEIGHT);
    layeredPane.setLayer(swingPanel, layerIndex++);
    layeredPane.add(swingPanel);
    // A web browser
    JWebBrowser webBrowser = new JWebBrowser(JWebBrowser.constrainVisibility());
    webBrowser.setBarsVisible(false);
    webBrowser.setStatusBarVisible(true);
    webBrowser.setHTMLContent(
        "<html>" + LS +
        "  <body>" + LS +
        "    <h1>A web page</h1>" + LS +
        "    <p>A paragraph with a <a href=\"http://www.google.com\">link</a>.</p>" + LS +
        "  </body>" + LS +
        "</html>");
    webBrowser.setBounds(OFFSET_X * layerIndex, OFFSET_Y * layerIndex, WIDTH, HEIGHT);
    layeredPane.setLayer(webBrowser, layerIndex++);
    // A swing button
    JButton swingButton = new JButton("Swing JButton");
    swingButton.setBounds(OFFSET_X * layerIndex, OFFSET_Y * layerIndex, WIDTH, HEIGHT);
    layeredPane.setLayer(swingButton, layerIndex++);
    layeredPane.add(swingButton);
    layeredPane.add(webBrowser);
    layeredPane.setPreferredSize(new Dimension(WIDTH + OFFSET_X * (layerIndex - 1), HEIGHT + OFFSET_Y * (layerIndex - 1)));
    add(new JScrollPane(layeredPane), BorderLayout.CENTER);
  }

  /* Standard main method to try that test as a standalone application. */
  public static void main(String[] args) {
    UIUtils.setPreferredLookAndFeel();
    NativeInterface.open();
    SwingUtilities.invokeLater(new Runnable() {
      public void run() {
        JFrame frame = new JFrame("DJ Native Swing Test");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(new ConstrainVisibility(), BorderLayout.CENTER);
        frame.setSize(800, 600);
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
      }
    });
    NativeInterface.runEventPump();
  }

}