1. Polymorphism and Observers
Chapter 4.9
Polymorphism and Observers
Karel J. Robot: A Gentle Introduction to the Art of Object-Oriented Programming in Java by Joseph Bergin et al.
By E.F. Teevan

2. Observers - 4.9
An Observer is an object which is controlled by an observable object
An interface is needed for the two to communicate: public interface RobotListeners { public void action(); }
An observer example: public class WalkListener extends ur_Robot implements RobotListener { public WalkListener(int street, int avenue, Directions.Direction dir, int beepers) { super(street, avenue, dir, beepers); } public void action() { move(); } }
Uses the RobotListener interface
WalkListeners move one block when
action method is called.
WalkListeners are Observers

3. Observers II - 4.9
The observable object class: public class ObservablePicker extends ur_Robot { private RobotListener myListener; public ObservablePicker(int street, int avenue, Directions.Direction direction, int beepers) { super(street, avenue, direction, beepers); } public void register(RobotListener listener) { myListener = listener; } public void pickBeeper() { super.pickBeeper(); myListener.action(); } }
instance variable
constructor
New method for ObservablePickers
Instantiates the instance variable myListener
New method for ObservablePicker
Calls action method of RobotListener class for myListener

4. Observers III - 4.9
The main method: public class ObserverTest implements Directions { public static void main(String args[]) { WalkListener2 tom = new WalkListener2(1, 1, North, 0); ObservablePicker2 jerry = new ObservablePicker2(2, 3, East, 0); jerry.register(tom); jerry.pickBeeper(); } }
CONTROLLED
tom is an Observer
jerry is observable
CONTROLLER
See fourNine in the Bowling workspace