send command to UI thread using a message containing the command and a place to put the response. The sender waits for the UI using a CountDownLatch

solution.java

// In background thread:
Handler uiHandler;

public void test() {
    Command command = new Command();
    // fill command
    Response response = send(command);
}

public Response send(Command command) {
    CommandAndResponse car = new CommandAndResponse(command);
    sendMessage(uiHandler, MESSAGE_COMMAND_AND_RESPONSE, car);
    return car.getResponse();
}

public void sendMessage(Handler handler, int what, Object object) {
    if (object == null)
        return;

    handler.obtainMessage(what, object)
           .sendToTarget();
}

// in UI thread
public void handleMessage(Message msg) {
    if (msg.what == MESSAGE_COMMAND_AND_RESPONSE) {
        CommandAndResponse car = (CommandAndResponse) msg.obj;
        Response response = handleCommand(car.getCommand());
        car.setResponse(response);
    }
}

// helper class
public class CommandAndResponse {
    private final Command mCommand;
    private Response mResponse;
    private final CountDownLatch mCountDownLatch;

    public CommandAndResponse(Command command) {
        mCommand = new Command(command);
        mCountDownLatch = new CountDownLatch(1);
    }

    public void setResponse(Response response) {
        synchronized (this) {
            mResponse = new Response(response);
        }
        mCountDownLatch.countDown();
    }

    public Response getResponse() throws InterruptedException {
        mCountDownLatch.await();
        synchronized (this) {
            return new Response(mResponse);
        }
    }

    public Command getCommand() {
        return new Command(mCommand);
    }
}