(→Exemplo) |
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Line 127: | Line 127: | ||
} | } | ||
} | } | ||
+ | |||
+ | ===Dois contextos=== | ||
+ | |||
+ | Os dois contextos de utilização correspondem a duas estações meteorológicas: em cada uma são utilizados observadores diferentes sobre os mesmos dados. | ||
+ | |||
+ | <B>public</B> <B>class</B> WeatherStation { | ||
+ | <B>public</B> <FONT COLOR="#800000">static</FONT> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">main</FONT>(<B><FONT COLOR="#0095ff">String</FONT></B>[] args) { | ||
+ | WeatherData weatherData = <B>new</B> <FONT COLOR="#000080">WeatherData</FONT>(); | ||
+ | |||
+ | CurrentConditionsDisplay currentDisplay = <B>new</B> <FONT COLOR="#000080">CurrentConditionsDisplay</FONT>(weatherData); | ||
+ | StatisticsDisplay statisticsDisplay = <B>new</B> <FONT COLOR="#000080">StatisticsDisplay</FONT>(weatherData); | ||
+ | ForecastDisplay forecastDisplay = <B>new</B> <FONT COLOR="#000080">ForecastDisplay</FONT>(weatherData); | ||
+ | |||
+ | weatherData.<FONT COLOR="#000080">setMeasurements</FONT>(<FONT COLOR="#0000ff">80</FONT>, <FONT COLOR="#0000ff">65</FONT>, <FONT COLOR="#800080">30.4f</FONT>); | ||
+ | weatherData.<FONT COLOR="#000080">setMeasurements</FONT>(<FONT COLOR="#0000ff">82</FONT>, <FONT COLOR="#0000ff">70</FONT>, <FONT COLOR="#800080">29.2f</FONT>); | ||
+ | weatherData.<FONT COLOR="#000080">setMeasurements</FONT>(<FONT COLOR="#0000ff">78</FONT>, <FONT COLOR="#0000ff">90</FONT>, <FONT COLOR="#800080">29.2f</FONT>); | ||
+ | } | ||
+ | } | ||
+ | |||
+ | <B>public</B> <B>class</B> WeatherStationHeatIndex { | ||
+ | <B>public</B> <FONT COLOR="#800000">static</FONT> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">main</FONT>(<B><FONT COLOR="#0095ff">String</FONT></B>[] args) { | ||
+ | WeatherData weatherData = <B>new</B> <FONT COLOR="#000080">WeatherData</FONT>(); | ||
+ | |||
+ | CurrentConditionsDisplay currentDisplay = <B>new</B> <FONT COLOR="#000080">CurrentConditionsDisplay</FONT>(weatherData); | ||
+ | StatisticsDisplay statisticsDisplay = <B>new</B> <FONT COLOR="#000080">StatisticsDisplay</FONT>(weatherData); | ||
+ | ForecastDisplay forecastDisplay = <B>new</B> <FONT COLOR="#000080">ForecastDisplay</FONT>(weatherData); | ||
+ | HeatIndexDisplay heatIndexDisplay = <B>new</B> <FONT COLOR="#000080">HeatIndexDisplay</FONT>(weatherData); | ||
+ | |||
+ | weatherData.<FONT COLOR="#000080">setMeasurements</FONT>(<FONT COLOR="#0000ff">80</FONT>, <FONT COLOR="#0000ff">65</FONT>, <FONT COLOR="#800080">30.4f</FONT>); | ||
+ | weatherData.<FONT COLOR="#000080">setMeasurements</FONT>(<FONT COLOR="#0000ff">82</FONT>, <FONT COLOR="#0000ff">70</FONT>, <FONT COLOR="#800080">29.2f</FONT>); | ||
+ | weatherData.<FONT COLOR="#000080">setMeasurements</FONT>(<FONT COLOR="#0000ff">78</FONT>, <FONT COLOR="#0000ff">90</FONT>, <FONT COLOR="#800080">29.2f</FONT>); | ||
+ | } | ||
+ | } | ||
+ | |||
[[category:PO 2005/2006]] | [[category:PO 2005/2006]] |
Material correspondente à aula 28.
O padrão observer permite observar o estado de um objecto. Os observadores registam o seu interesse no estado junto do objecto; quando o estado do objecto muda, os observadores são notificados.
O padrão observer tem a seguinte estrutura de classes:
As colaborações entre os intervenientes são as que figuram no seguinte diagrama de sequência:
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); }
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 Subject weatherData; public CurrentConditionsDisplay(Subject weatherData) { this.weatherData = weatherData; weatherData.registerObserver(this); } public void update(float temperature, float humidity, float pressure) { this.temperature = temperature; this.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) { this.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"); } } }
Os dados de meteorologia correspondem ao objecto observado.
public class WeatherData implements Subject { private ArrayList observers; private float temperature; private float humidity; private float pressure; public WeatherData() { observers = new ArrayList(); } 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 = (Observer)observers.get(i); observer.update(temperature, humidity, pressure); } } public void measurementsChanged() { notifyObservers(); } public void setMeasurements(float temperature, float humidity, float pressure) { this.temperature = temperature; this.humidity = humidity; this.pressure = pressure; measurementsChanged(); } }
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); } }