The "Status" and "Load" classes all extend the java.util.Observable class.
This means, that whenever they are updated with new values, an Observer
class is notified. This is done with the following two statements;
![\begin{lstlisting}[frame=trbl,caption=Extract from Load.java]{}
setChanged();
notifyObservers();
\end{lstlisting}](img40.png)
The Spy class implements the java.util.Observer interface. This class
observes the instances of "Load" and "Status" that are passed to it. To
implement an Observer interface, you must inform the Observable class
that it has an Observer. This can be done like this;
![\begin{lstlisting}[frame=trbl,caption=Extract from Spy.java]{}
status.addObserver(this);
\end{lstlisting}](img41.png)
The public void update(Observable o, Object args) method must also be implemented. As the "Spy" class observes two different classes, polymorphism is used to distinguish between them, e.g.:
![\begin{lstlisting}[frame=trbl,caption=Extract from Spy.java]{}
if(observable instanceof Names)
\end{lstlisting}](img42.png)
The observer pattern was implemented here because the "Spy" class must be kept updated at all times, as it is broadcast using RMI. The "Load" and "Status" classes should not have the responsibility of directly updating the object, as this is bad design. Using Observer here simplifies the design greatly, as updating is done automatically, and there is no need to pass the "Spy" class to the observable classes.