Define an interface with getMark as the only method:

interface Marker{
  int getMark(int x, int y);
}

Your class would have a Marker reference, and instead of calling
getMark directly would call e.g. marker.getMark(x,y).

If there is a default Marker initialize accordingly:

Marker marker = new Marker(){
  public int getMark(int x, int y){
    return x + y;
  }
};

You can use a Marker as a constructor parameter, and/or provide a
setMarker method to change it for an existing object.

Patricia

If you not concern with performance, you can use a java scripting
language such as Beanshell (http://www.beanshell.org/), and do sth like:

int getMark(int x, int y)
{
    Context context= new Context();
    context.put("x", new Integer(x));
    context.put("y", new Integer(y));
    return (int) Beanshell.interpret(context, "some formula involving x and
y");
}

Your best bet is the Strategy Design Pattern (http://en.wikipedia.org/
wiki/Strategy_pattern)

T