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Table of contents
- Overview
- Class Diagram
- Implementation with Example
- Conclusion (Source code on Github Repository)
- References
1. Overview
Defines a one-to-many dependency between objects so that when one object changes state,
all its dependents are notified and updated automatically.
- The Observer Pattern defines a one-to-many relationship between objects.
- When the state of one object changes, all of its dependents are notified.
- Observers are loosely coupled in that the Observable knows nothing about them, other than they implement the Observer Interface.
- Loosely coupled designs allow us to build flexible OO Systems that can handle change because they minimize the interdependency between objects.
- Java has several implementations of the Observer Pattern, including the general purpose java.util.Observable.
Strive for loosely coupled designs between objects that interact.
2. Class Diagram
3. Implementation with Example
Designing the Weather Station using Observer Pattern
- WeatherData class is the "one" and "many" is the various elements that use the weather measurements.
- WeatherData class certainly has stated, that's the temperature, humidity, and barometric pressure, those definitely changes.
- When measurements are changing, we have to notify all the display elements so they can do whatever it is they are going to with the measurements.
public interface Observer {
public void update(float temp, float humidity, float pressure);
}
public interface DisplayElement {
public void display();
}
public interface Subject {
public void registerObserver(Observer o);
public void removeObserver(Observer o);
public void notifyObservers();
}
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;
}
}
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");
}
}
}
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);
}
}
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);
}
}
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 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);
}
}
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
4. Conclusion
https://github.com/RameshMF/headfirst_design_patterns/tree/master/src/headfirst/observer
5. Reference
6. Top Java Tutorials
- Java Tutorial for Beginners
- 50 Java Keywords
- JDBC 4.2 Tutorial
- All Java/J2EE Tutorial
- Java 8 Tutorial
- Java Collections Tutorial
- Java Exceptions Tutorial
- Java Generics Tutorial
- Java 8 Stream API Tutorial
- Java Wrapper Classes
- Java Arrays Guide
- Java Multithreading Tutorial
- Java Concurrency Tutorial
- Oops Concepts Tutorial
- Java String API Guide
- Java Reflection API Tutorial
- Java I/O Tutorial
- Date and Time API Tutorial
- JUnit 5 Tutorial
- JUnit 4 Tutorial
- Java XML Tutorial
- Google GSON Tutorial
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