added observer sample

observer/pom.xml

@@ -0,0 +1,23 @@
+<?xml version="1.0"?>
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.0-SNAPSHOT</version>
+  </parent>
+  <groupId>com.iluwatar</groupId>
+  <artifactId>observer</artifactId>
+  <version>1.0-SNAPSHOT</version>
+  <name>observer</name>
+  <url>http://maven.apache.org</url>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <version>3.8.1</version>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

observer/src/main/java/com/iluwatar/App.java

@@ -0,0 +1,18 @@
+package com.iluwatar;
+
+public class App 
+{
+    public static void main( String[] args )
+    {
+
+    	Weather weather = new Weather();
+    	weather.addObserver(new Orcs());
+    	weather.addObserver(new Hobbits());
+    	
+    	weather.timePasses();
+    	weather.timePasses();
+    	weather.timePasses();
+    	weather.timePasses();
+    	
+    }
+}

observer/src/main/java/com/iluwatar/Hobbits.java

@@ -0,0 +1,25 @@
+package com.iluwatar;
+
+public class Hobbits implements WeatherObserver {
+
+	@Override
+	public void update(WeatherType currentWeather) {
+		switch (currentWeather) {
+		case COLD:
+			System.out.println("The hobbits are shivering in the cold weather.");
+			break;
+		case RAINY:
+			System.out.println("The hobbits look for cover from the rain.");
+			break;
+		case SUNNY:
+			System.out.println("The happy hobbits bade in the warm sun.");
+			break;
+		case WINDY:
+			System.out.println("The hobbits hold their hats tightly in the windy weather.");
+			break;
+		default:
+			break;
+		}
+	}
+
+}

observer/src/main/java/com/iluwatar/Orcs.java

@@ -0,0 +1,25 @@
+package com.iluwatar;
+
+public class Orcs implements WeatherObserver {
+
+	@Override
+	public void update(WeatherType currentWeather) {
+		switch (currentWeather) {
+		case COLD:
+			System.out.println("The orcs are freezing cold.");
+			break;
+		case RAINY:
+			System.out.println("The orcs are dripping wet.");
+			break;
+		case SUNNY:
+			System.out.println("The sun hurts the orcs' eyes.");
+			break;
+		case WINDY:
+			System.out.println("The orc smell almost vanishes in the wind.");
+			break;
+		default:
+			break;
+		}
+	}
+
+}

observer/src/main/java/com/iluwatar/Weather.java

@@ -0,0 +1,50 @@
+package com.iluwatar;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Weather {
+
+	private WeatherType currentWeather;
+	private List<WeatherObserver> observers;
+	
+	public Weather() {
+		observers = new ArrayList<>();
+		currentWeather = WeatherType.SUNNY;
+	}
+	
+	public void addObserver(WeatherObserver obs) {
+		observers.add(obs);
+	}
+	
+	public void removeObserver(WeatherObserver obs) {
+		observers.remove(obs);
+	}
+	
+	public void timePasses() {
+		switch (currentWeather) {
+		case COLD:
+			currentWeather = WeatherType.SUNNY;
+			break;
+		case RAINY:
+			currentWeather = WeatherType.WINDY;
+			break;
+		case SUNNY:
+			currentWeather = WeatherType.RAINY;
+			break;
+		case WINDY:
+			currentWeather = WeatherType.COLD;
+			break;
+		default:
+			break;
+		}
+		System.out.println("The weather now changes to " + currentWeather);
+		notifyObservers();
+	}
+	
+	private void notifyObservers() {
+		for (WeatherObserver obs: observers) {
+			obs.update(currentWeather);
+		}
+	}
+}

observer/src/main/java/com/iluwatar/WeatherObserver.java

@@ -0,0 +1,7 @@
+package com.iluwatar;
+
+public interface WeatherObserver {
+
+	void update(WeatherType currentWeather);
+	
+}

observer/src/main/java/com/iluwatar/WeatherType.java

@@ -0,0 +1,14 @@
+package com.iluwatar;
+
+public enum WeatherType {
+
+	SUNNY,
+	RAINY,
+	WINDY,
+	COLD;
+	
+	public String toString() {
+		return this.name().toLowerCase();
+	};
+	
+}

pom.xml

@@ -36,6 +36,7 @@
     <module>iterator</module>
     <module>mediator</module>
     <module>memento</module>
+    <module>observer</module>
   </modules>
   
 <build>