Wiki**3

Observadores e Observados

<text>

 public interface Subject {
   public void registerObserver(Observer o);
   public void removeObserver(Observer o);
   public void notifyObservers();
 }

</text>

<text>

 public interface Observer {
   public void update(float temp, float humidity, float pressure);
 }

</text>

Apresentação

Esta interface define o método básico para apresentação de dados.

<java5>

 public interface DisplayElement {
   public void display();
 }

</java5>

As implementações da interface de apresentação, que implementam também a de observação, definem várias formas de exibição de dados.

<text>

 public class CurrentConditionsDisplay implements Observer, DisplayElement {
   private float _temperature;
   private float _humidity;
   private Subject _weatherData;
       
   public CurrentConditionsDisplay(Subject weatherData) {
     _weatherData = weatherData;
     _weatherData.registerObserver(this);
   }

   public void update(float temperature, float humidity, float pressure) {
     _temperature = temperature;
     _humidity    = humidity;
     display();
   }

   public void display() {
     System.out.println("Current conditions: " + _temperature + "F degrees and " + _humidity + "% humidity");
   }
 }

</text>

<text>

 public class ForecastDisplay implements Observer, DisplayElement {

   private float _currentPressure = 29.92f;  
   private float _lastPressure;
   private WeatherData _weatherData;

   public ForecastDisplay(WeatherData weatherData) {
     _weatherData = weatherData;
     _weatherData.registerObserver(this);
   }

   public void update(float temp, float humidity, float pressure) {
     _lastPressure    = currentPressure;
     _currentPressure = pressure;
     display();
   }

   public void display() {
     System.out.print("Forecast: ");
     if (_currentPressure > _lastPressure) {
       System.out.println("Improving weather on the way!");
     } else if  (_currentPressure == _lastPressure) {
       System.out.println("More of the same");
     } else if (_currentPressure < _lastPressure) {
       System.out.println("Watch out for cooler, rainy weather");
     }
   }
 }

</text>

Representação de dados

Os dados de meteorologia correspondem ao objecto observado.

<java5>

 public class WeatherData implements Subject {
   private ArrayList<Observer> _observers = new ArrayList<Observer>();
   private float _temperature;
   private float _humidity;
   private float _pressure;
        
   public WeatherData() { }
        
   public void registerObserver(Observer o) { _observers.add(o); }
        
   public void removeObserver(Observer o) {
     int i = _observers.indexOf(o);
     if (i >= 0) { _observers.remove(i); }
   }
         
   public void notifyObservers() {
     for (int i = 0; i < _observers.size(); i++) {
       Observer observer = _observers.get(i);
       observer.update(temperature, humidity, pressure);
     }
   }
         
   public void measurementsChanged() { notifyObservers(); }
         
   public void setMeasurements(float temperature, float humidity, float pressure) {
     _temperature = temperature;
     _humidity        = humidity;
     _pressure       = pressure;
     measurementsChanged();
   }

 }

</java5>

Dois contextos

Os dois contextos de utilização correspondem a duas estações meteorológicas: em cada uma são utilizados observadores diferentes sobre os mesmos dados.

 public class WeatherStation {
   public static void main(String[] args) {  
     WeatherData weatherData = new WeatherData();  
         
     CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData);  
     StatisticsDisplay     statisticsDisplay = new StatisticsDisplay(weatherData);  
     ForecastDisplay         forecastDisplay = new ForecastDisplay(weatherData);  

     weatherData.setMeasurements(80, 65, 30.4f);  
     weatherData.setMeasurements(82, 70, 29.2f);  
     weatherData.setMeasurements(78, 90, 29.2f);  
   }  
 }  

 public class WeatherStationHeatIndex {
   public static void main(String[] args) {
     WeatherData weatherData = new WeatherData();

     CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData);
     StatisticsDisplay     statisticsDisplay = new StatisticsDisplay(weatherData);
     ForecastDisplay         forecastDisplay = new ForecastDisplay(weatherData);
     HeatIndexDisplay       heatIndexDisplay = new HeatIndexDisplay(weatherData);

     weatherData.setMeasurements(80, 65, 30.4f);
     weatherData.setMeasurements(82, 70, 29.2f);
     weatherData.setMeasurements(78, 90, 29.2f);
   }
 }