How can I print a single JPanel's contents?

Here is an example to print any Swing component.

public void printComponenet(Component component){
  PrinterJob pj = PrinterJob.getPrinterJob();
  pj.setJobName(" Print Component ");

  pj.setPrintable (new Printable() {    
    public int print(Graphics pg, PageFormat pf, int pageNum){
      if (pageNum > 0){
      return Printable.NO_SUCH_PAGE;
      }

      Graphics2D g2 = (Graphics2D) pg;
      g2.translate(pf.getImageableX(), pf.getImageableY());
      component.paint(g2);
      return Printable.PAGE_EXISTS;
    }
  });
  if (pj.printDialog() == false)
  return;

  try {
        pj.print();
  } catch (PrinterException ex) {
        // handle exception
  }
}

A simple way to do it would be implementing the Printable interface (in java.awt.print) and adding the specified print method (it works similar to paint—in here, you could specify what components you would want to draw onto the printed page). And when you want to actually print the contents of the panel, obtain a PrinterJob instance and call its setPrintable method, passing the object that implemented Printable.

That's just a quick overview, though. I'd recommend taking a look at Sun's tutorial on printing for further information.