Idea:

• permit an object to "observed" by any number of other objects.

Contract (between observed object and observer):

1. Each observer registers with the object to be observed

2. The observed object notifies all observers whenever it makes a significant change in state

Implementation:

1. The object to be observed extends the class java.util.Observable. This class provides the public methods
   • addObserver( Observer o ) // Used by observer

   • deleteObserver( Observer o ) // Used by observer

   • notifyObservers() // Used by observed object

   • protected setChanged() // Used by observed object

2. Each observer implements the interface java.util.Observer that requires
   • public void update( Observable o, Object x ) // Used by observer
   that is called by notifyObservers().

Next, we demo the use of this paradigm in the multiple display of an animated thermometer.