Created at 288 days ago

Observer pattern

Observer pattern defines one-to-many dependencies between objects. When an object changes state, all of its dependents are notified and automatically updated.

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);
}

public interface DisplayElement {
  public void display();
}

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();
  }
}

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");
  }
}