A little more help with image

Question

I have refered to this thread to help me create my background image.
http://forum.java.sun.com/thread.jspa?threadID=599393&messageID=3196643
However I am stuck mostly because of not really fully grasping all the objects and how they work yet.
Basically right now I have a JFrame and then I have a container which I put all my objects in.
First question:
What exactly does the following line of code do?
Container contentPane = getContentPane();
Does the "getContentPane()" method get the JFrame, since this is the class this line of code is embedded in?
It sets contentPane which is a container to the JFrame?
Second question...
So using the above link I created a class called BackgroundImage and it works ...sort of. I have two windows - one is my JFrame game and the other is this background frame.
Now I knew this would happen and I realize what I am supposed to do but I have been unsuccessful. Basically I need to make this JPanel my contentPane right?
But when I say something like
JPanel contentPane = panel my JFrame has nothing in it...
How do I take this seperate window and incorporate it into what I already have.
I want to use the contentPane variable since this is the variable where everything gets added throughout the code? I can't seem to get the JPanel to work.
Take a look at some code snippets:
public class DungMast extends JFrame
    /** Creates a new instance of EventPress */
    public DungMast() {
        Animation_Complete=false;
        Current_Level = 1;
        Level_Complete = false;
        CreateUserInterface();
        JPanel panel = new JPanel()
        protected void paintComponent(Graphics g)
       //  Dispaly image at at full size
     g.drawImage(img_BG.getImage(), 0, 0, null);
     //  Scale image to size of component
//                    Dimension d = getSize();
//                    g.drawImage icon.getImage(), 0, 0, d.width, d.height, null);
                    //  Fix the image position in the scroll pane
//                    Point p = scrollPane.getViewport().getViewPosition();
//                    g.drawImage(icon.getImage(), p.x, p.y, null);
                    super.paintComponent(g);
         panel.setOpaque( false );
         panel.setPreferredSize( new Dimension(400, 400) );
         scrollPane = new JScrollPane( panel );
         getContentPane().add( scrollPane );
         JButton button = new JButton( "Hello" );
         panel.add( button );
    }  //end constructor for main JFrame
//Then I have something like
JPanel contentPane = panel;
}

Answer 1

What exactly does the following line of code do?
Container contentPane = getContentPane();
It gets the content pane for the JFrame and saves a reference to it for later use, viz,
the reference "contentPane" can be used to call methods in the Container class.
Lightweight (Swing) top–level containers have a content pane that holds the child
components for the parent container. See the comments section of the JRootPane class api
for a birds–eye view.
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.*;
import javax.imageio.ImageIO;
import javax.swing.*;
public class BackgroundImage {
    private JPanel getContent(BufferedImage image) {
        // Make up our own content pane with an image
        // in the background.
        ContentPanel cp = new ContentPanel(image);
        // Add some components.
        cp.setLayout(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.weightx = 1.0;
        gbc.weighty = 1.0;
        for(int j = 0; j < 10; j++) {
            gbc.gridwidth = (j+1)%3 == 0 ? gbc.REMAINDER : 1;
            cp.add(new JButton(String.valueOf(j+1)), gbc);
        return cp;
    public static void main(String[] args) throws IOException {
        BufferedImage image = ImageIO.read(new File("images/cougar.jpg"));
        BackgroundImage test = new BackgroundImage();
        JFrame f = new JFrame();
        System.out.println("Default contentPane = " +
                            f.getContentPane().getClass().getName());
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setContentPane(test.getContent(image));
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
class ContentPanel extends JPanel {
    BufferedImage image;
    public ContentPanel(BufferedImage image) {
        this.image = image;
        // This is required for JComponents that are used
        // as content panes since the Ocean LookAndFeel
        // (which can have non-opaque panels) was introduced.
        // Content panes must be opaque so they can draw
        // themseves and their child components.
        setOpaque(true);
        System.out.println("Default layout manager = " +
                            getLayout().getClass().getName());
    protected void paintComponent(Graphics g) {
        // Fill background with opaque color.
        super.paintComponent(g);
        int w = getWidth();
        int h = getHeight();
        int x = (w - image.getWidth())/2;
        int y = (h - image.getHeight())/2;
        g.drawImage(image, x, y, this);
}