Java RMI Proxy issue

Question

I am getting this error:

 java.lang.ClassCastException: $Proxy0 cannot be cast to rmi.engine.Call
        at Main.main(Main.java:39)

My Abstract and Call class both extend Remote.

Call:

public class Call extends UnicastRemoteObject implements rmi.engine.Abstract {

    public Call() throws Exception {
        super(Store.PORT, new RClient(), new RServer());
    }

    public String getHello() {
        System.out.println("CONN");
        return "HEY";
    }
} 

Abstract:

public interface Abstract extends Remote {

    String getHello() throws RemoteException;

}

This is my main:

public static void main(String[] args) {
    if (args.length == 0) {
        try {
            System.out.println("We are slave ");
            InetAddress ip = InetAddress.getLocalHost();
            Registry rr = LocateRegistry.
                getRegistry(ip.getHostAddress(), Store.PORT, new RClient());

            Object ss = rr.lookup("FILLER");

            System.out.println(ss.getClass().getCanonicalName());
            System.out.println(((Call)ss).getHello());

        } catch (Exception e) {
            e.printStackTrace();
        }
    } else {
        if (args[0].equals("master")) {
            // Start Master
            try {
                RMIServer.start();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }

Netbeans says the problem is on line 39 which is

    System.out.println(((Call)ss).getHello());

The output looks like this:

Run:

We are slave 
Connecting 10.0.0.212:5225
$Proxy0
java.lang.ClassCastException: $Proxy0 cannot be cast to rmi.engine.Call
        at Main.main(Main.java:39)

BUILD SUCCESSFUL (total time: 1 second)

I am running a master in cmd listening on port 5225.

Answer 1

The class $Proxy0 is a dynamic proxy, that is, a class is generate by java at run-time.

This class should implement the interface Abstract, but does not extend Call. So you can't downcast it to Call (this gives a class cast), but downcasting it to Abstract should be ok.

If you use RMI, you should only communicate with remote object through interfaces.

PS: Abstract is kind of a confusing name. I really thought that you were speaking of abstract class first. I would rename that if possible. Same for rmi.engine, prefer something like org.myproject, to avoid confusion with internal class of the JDK.