Hello, I had the problem that I did not want multiple instances of my
application to start. I would like instead to have the command line
parameters (file paths in my case) of a second starting instance to be
forwarded to the first already running app (eg. if someone double-clicks
on an associated file type).
Here is the solution I came up with (posting it for future reference).
Any comments/corrections are welcome (this is my first try at RMI).

Phil

-- RemoteContactInterface.java --
import java.io.File;
import java.rmi.Remote;
import java.rmi.RemoteException;
public interface RemoteContactInterface extends Remote {
 void loadFile(File f) throws RemoteException;
}

-- MyMainClass.java --
import java.io.File;
import java.rmi.RemoteException;
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.rmi.server.UnicastRemoteObject;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.SwingUtilities;

public class MyMainClass extends JFrame implements RemoteContactInterface {

 MyMainClass(){
//  start RMI server
  try {
   int PORT = 1099;
   LocateRegistry.createRegistry(PORT);
   RemoteContactInterface stub =
(RemoteContactInterface)UnicastRemoteObject.exportObject(this, PORT);
   Registry registry = LocateRegistry.getRegistry();
   registry.bind("myClassID", stub);
   System.out.println("Server ready");
  } catch (Exception e) {
   System.err.println("RMI Server exception: " + e.toString());
   e.printStackTrace();
  }

  setDefaultCloseOperation(EXIT_ON_CLOSE);
  this.getContentPane().add(new JLabel("Hello World"));
  pack();
  setVisible(true);

 }

 public static void main(String args[]) {
  try {
   Registry registry = LocateRegistry.getRegistry(null);
   RemoteContactInterface stub = (RemoteContactInterface)
registry.lookup("myClassID");
   for(String s: args){
    File f = new File(s);
    stub.loadFile(f);
   }
   System.out.println("App already running");
   System.exit(0);
  } catch (Exception e) {
   // if exception is thrown, no other process already exists
   // go on with normal loading
  }

  SwingUtilities.invokeLater(new Runnable() {
   public void run() {
    MyMainClass myClass = new MyMainClass();
   }
  });

 }

 public void loadFile(File f) throws RemoteException {
  // load the file here;
 }
}