Observer Design Pattern


Intent

It belongs to behavioral design patterns catalog.
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Also known as

Dependents, Publish-Subscribe

Structure 

Participants

1. Subject
  • knows its observers. Any number of Observer objects may observe a subject.
  • provides an interface for attaching and detaching Observer objects.
2. Observer
  • defines an updating interface for objects that should be notified of changes in a subject.
3. ConcreteSubject
  • stores state of interest to ConcreteObserver objects.
  • sends a notification to its observers when its state changes.
4. ConcreteObserver
  • maintains a reference to a ConcreteSubject object.
  • stores state that should stay consistent with the subject's.
  • implements the Observer updating interface to keep its state consistent with the subject's.

Collaborations

ConcreteSubject notifies its observers whenever a change occurs that could make its observers' state inconsistent with its own.

Source code

Step 1: Create Observer interface and DisplayElement Interface for display common method
public interface Observer {
 public void update(float temp, float humidity, float pressure);
}

public interface DisplayElement {
 public void display();
}
Step 2: Create Observable interface Subject.java
public interface Subject {
 public void registerObserver(Observer o);
 public void removeObserver(Observer o);
 public void notifyObservers();
}
Step 3: Create WeatherData class to implement Subject interface.
import java.util.*;

public class WeatherData implements Subject {
 private ArrayList<Observer> 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 float getTemperature() {
  return temperature;
 }
 
 public float getHumidity() {
  return humidity;
 }
 
 public float getPressure() {
  return pressure;
 }
}
Step 4: Create Observer ForecastDisplay class.
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");
  }
 }
}
Step 5: Create second Observer HeatIndexDisplay class.
public class HeatIndexDisplay implements Observer, DisplayElement {
 float heatIndex = 0.0f;
 private WeatherData weatherData;

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

 public void update(float t, float rh, float pressure) {
  heatIndex = computeHeatIndex(t, rh);
  display();
 }
 
 private float computeHeatIndex(float t, float rh) {
  float index = (float)((16.923 + (0.185212 * t) + (5.37941 * rh) - (0.100254 * t * rh) 
   + (0.00941695 * (t * t)) + (0.00728898 * (rh * rh)) 
   + (0.000345372 * (t * t * rh)) - (0.000814971 * (t * rh * rh)) +
   (0.0000102102 * (t * t * rh * rh)) - (0.000038646 * (t * t * t)) + (0.0000291583 * 
   (rh * rh * rh)) + (0.00000142721 * (t * t * t * rh)) + 
   (0.000000197483 * (t * rh * rh * rh)) - (0.0000000218429 * (t * t * t * rh * rh)) +
   0.000000000843296 * (t * t * rh * rh * rh)) -
   (0.0000000000481975 * (t * t * t * rh * rh * rh)));
  return index;
 }

 public void display() {
  System.out.println("Heat index is " + heatIndex);
 }
}
Step 6: Create third Observer StatisticsDisplay class.
public class StatisticsDisplay implements Observer, DisplayElement {
 private float maxTemp = 0.0f;
 private float minTemp = 200;
 private float tempSum= 0.0f;
 private int numReadings;
 private WeatherData weatherData;

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

 public void update(float temp, float humidity, float pressure) {
  tempSum += temp;
  numReadings++;

  if (temp > maxTemp) {
   maxTemp = temp;
  }
 
  if (temp < minTemp) {
   minTemp = temp;
  }

  display();
 }

 public void display() {
  System.out.println("Avg/Max/Min temperature = " + (tempSum / numReadings)
   + "/" + maxTemp + "/" + minTemp);
 }
}
Step 7: Create fourth Observer CurrentConditionsDisplay class.
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");
 }
}
Step 8: Create WeatherStation class to test observer design pattern.
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);
 }
}
Output:
Current conditions: 80.0F degrees and 65.0% humidity
Avg/Max/Min temperature = 80.0/80.0/80.0
Forecast: Improving weather on the way!
Current conditions: 82.0F degrees and 70.0% humidity
Avg/Max/Min temperature = 81.0/82.0/80.0
Forecast: Watch out for cooler, rainy weather
Current conditions: 78.0F degrees and 90.0% humidity
Avg/Max/Min temperature = 80.0/82.0/78.0
Forecast: More of the same

Applicability

Use the Observer pattern in any of the following situations
  • when an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently
  • when a change to one object requires changing others, and you don't know how many objects need to be changed
  • when an object should be able to notify other objects without making assumptions about who these objects are. In other words, you don't want these objects tightly coupled

Typical Use Case

  • changing in one object leads to a change in other objects

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