import java.util.*;

public class MessageBoard extends Observable {
	private String message;

	public String getMessage() {
		return message;
	}
	public void changeMessage( String message ) {
		this.message = message;
		setChanged();
		notifyObservers( message );
	}
	public static void main( String [] args ) {
		MessageBoard board = new MessageBoard();
		Student bob = new Student();
		Student joe = new Student();
		board.addObserver( bob );
		board.addObserver( joe );
		board.changeMessage("More Homework!");
	}
}

class Student implements Observer {
	public void update(Observable o, Object arg) {
		System.out.println( "Message board changed: " + arg );
	}
}