Stopper.java

public class Stopper implements ControlObject
{
    public Stopper()
    {
    }

    public void doFunction(Object o)
    {
        System.out.println(this + ".doFunction(" + o + ")");
        if (o instanceof Stoppable)
            ((Stoppable)o).stop();
    }

    public String toString()
    {
        return "Stopper[]";
    }
}