CSC241 Assignment 2

• Subjects maintain representations of things they model.
• Observers know how to graphically display things that subjects model.
• Each subject can have many observers, that display or otherwise use the modeled information in different ways. (But normally, each Observer observes only one subject.)
• When a subject changes its representation, it merely alerts all of its observers that it has changed. They are responsible for figuring out whether and how to redisplay those changes.

public interface Subject {
  /** Add x to the list of observers **/
  public void attach(Observer x);

  /** Remove x from the list of observers **/  
  public void detach(Observer x);

  /** Report the current represented value **/
  public float currentValue();
}

public interface Observer {
  /** Deal with the fact that Subject s has changed state **/
  public void changeNotification(Subject s);
}

public interface ObserverList {
  /** Standard list insert **/
  public void add(Observer x);

  /** Standard list remove **/
  public void remove(Observer x);

  /** For each element e, of the list, send e.changeNotification(s) **/
  public void reportChange(Subject s);
}

1. Write a class that implements ObserverList using a linked list.

2. Write a Temperature class that implements Subject, using the float to represent the current temperature, an instance of your ObserverList implementation to maintain the list, and warmBy and/or similar methods that use the list to do change notifications upon each update.

3. Implement a WeatherSimulator class based on threads, as discussed in class. Depending on the exact details of your Temperature class, the WeatherSimulator will look something like:

         public class WeatherSimulator implements Runnable {
           static final long period = 500;   // update period in millisecs
           static final float offset = 0.5f; // bias for random numbers
           static final float scale = 1.0f;   // scale factor for random numbers
           protected Temperature temp_;      // the temperature object to change
         
           public void run() {
             for (;;) {
               float delta = (float)((Math.random() - offset) * scale);
               temp_.warmBy(delta);
               try { Thread.sleep(period); }
               catch (InterruptedException ex) {}
             }
           }
         
           public WeatherSimulator(Temperature t) {
             temp_ = t;
             new Thread(this).start();
           }
         
         }
         

4. Write Three different kinds of classes that implement Observer. Here are the suggested classes, but you can substitute others:
   1. A Gauge class that looks at least vaguely like a thermometer.
   2. A DigitalDisplay class that displays the value as a number (for example using a textfield).
   3. A HeaterLight, that displays itself as on if the value is less than a desired temperature (as indicated in a constructor).

5. Write an Applet that includes (in any reasonable way):
   1. At least one instance each of the three Observer implementations.
   2. A Temperature object to which the Observers can attach and detach.
   3. Simple buttons or whatever to attach and detach observers.
   4. A start method that creates a WeatherSimulator.