Java RMI - Passing Objects

Question

I'm running a Java program on a Raspberry Pi and an Android-App on a smartphone. The app should be able to invoke methods running on the Raspi. Since Android does not include the standard Java RMI package, I used LipeRMI: http://lipermi.sourceforge.net/

So far so good, LipeRMI does work well for passing primitive data types such as boolean or int.

What I want to do now is passing an ArrayList<String> from the Server to the app but every time I run the method, I receive following error:

LipeRMIException:Class java.util.ArrayList is not assignable from control.ServerInt
at lipermi.handler.CallHandler.exportObject(CallHandler.java:54)
at lipermi.handler.CallHandler.exportObject(CallHandler.java:48)
at control.RMIServer.createServer(RMIServer.java:26)
at control.Main.main(Main.java:17)

I do not understand what I'm doing wrong here.

RMIServer.java

public class RMIServer implements ServerInt{
    private static final long serialVersionUID = 1L;
    private ArrayList<String> list;

    public RMIServer()  {
    list = new ArrayList<String>();
        list.add("50:25:5:-1");
        list.add("99:42:6:4");

        createServer();
    }

    public void createServer() {
        try {
            CallHandler callHandler = new CallHandler();
            callHandler.registerGlobal(ServerInt.class, this);
            callHandler.exportObject(ServerInt.class, list);
            Server server = new Server();
            server.bind(7777, callHandler);

            server.addServerListener(new IServerListener() {

                @Override
                public void clientDisconnected(Socket socket) {}

                @Override
                public void clientConnected(Socket socket) {}
            });

        } catch (LipeRMIException | IOException e) {
            e.printStackTrace();
        }    
    }


    @Override
    public ArrayList<String> getPWMLines() {       
        return list;
    }   
}

ServerInt.java

import java.util.ArrayList;

public interface ServerInt{          
    public ArrayList<String> getPWMLines();   
}

Answer 1

You obviously misuse the method CallHandler#exportObject(Class, Object) the first parameter must be a class representing an interface and the second parameter must be an instance of a class that implements this interface. Here you provide an ArrayList as second parameter which is clearly not an instance of ServerInt that is why you get this exception.

Here is a good example of how to use this method:

MyRemoteListener listenerImplementation = new MyRemoteListenerImpl();
callHandler.exportObject(MyRemoteListener.class, listenerImplementation);

In this case the first parameter is the interface MyRemoteListener and the second is an instance of MyRemoteListenerImpl which is an implementation of the interface MyRemoteListener.

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To expose your List you can try callHandler.registerGlobal(List.class, list)