Wiki**3

Observadores e Observados

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

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

Apresentação

Esta interface define o método básico para apresentação de dados.

  public interface DisplayElement {
    public void display();
  }

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

  public class CurrentConditionsDisplay implements Observer, DisplayElement {
    private float _temperature;
    private float _humidity;
    private WeatherData _weatherData;
        
    public CurrentConditionsDisplay(WeatherData 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");
    }
  }

  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");
      }
    }
  }

Representação de dados

Os dados de meteorologia correspondem ao objecto observado.

  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 (Observer observer: _observers) {
        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();
    }

  }

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);
   }
 }