Polymorphism and Collections

[diofanto]

Hi everybody, I'm quite sure this is a common question about OO design, but I'm really, really in hurry.. I hope you'll forgive me.

Let's say that I have an abstract class Message, with title and description:

public abstract class Message{

	private String title;
	private String body;
	
	public Messaggio(String title, String body) {
		super();
		this.title = title;
		this.body = body;
	}

	public String getTitle() {
		return title;
	}

	public String getBody() {
		return body;
	}
	
	public abstract String getPreview() ;

}

Let's say that I have also different kinds of Message with more attributes, for example:

public abstract class SignedMessage extends Message{

	private Person author;  
	
	public Messaggio(String title, String body, Person author) {
		super(title, body);
		this.author = author
	}

	public String getAuthor() {
		return author;
	}

	
	public String getPreview() {
		return "A preview";
	}

}

I'm writing an application which is meant to dispatch these Messages.
For each user to which i'm sending the messages, I have to provide a buffer wich *must* be unique (for reasons of timestamping and concurrent dispatching), and I mean I cannot have different kinds of Buffer for each user:

Buffer<SignedMessage>, Buffer<MessageWithDraw>....

but I *have* to have only one Buffer<Message> per user.

Now, my problem is: every message has to be treated differently by the receiver (it's a client-server system using RMI), but how can the sender or the receiver recognize the specific kind of message when it sends or receive it?

If I initialize Buffer<Message> aBuffer = new Buffer<Message>() and add a SignedMessage, then upcasting occurs: what if I want to call remote method onReceiveSignedMessage(SignedMessage) from the dispatcher, OR call onReceiveMessage(Message) and recognized SignedMessage inside the receiver?
Do I need to downcast? How?
Is there a better OO workaround?

Thank you sooo much!