Added example of generic observer

Author: joningiwork

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

@@ -1,5 +1,9 @@
 package com.iluwatar;
 
+import com.iluwatar.generic.GHobbits;
+import com.iluwatar.generic.GOrcs;
+import com.iluwatar.generic.GWeather;
+
 /**
  * 
  * Observer pattern defines one-to-many relationship between objects. The target
@@ -19,5 +23,15 @@ public static void main(String[] args) {
 		weather.timePasses();
 		weather.timePasses();
 
+		// Generic observer inspired by Java Generics and Collection by Naftalin & Wadler
+		GWeather gWeather = new GWeather();
+		gWeather.addObserver(new GHobbits());
+		gWeather.addObserver(new GOrcs());
+
+		gWeather.timePasses();
+		gWeather.timePasses();
+		gWeather.timePasses();
+		gWeather.timePasses();
+
 	}
 }

observer/src/main/java/com/iluwatar/generic/GHobbits.java

@@ -0,0 +1,25 @@
+package com.iluwatar.generic;
+
+import com.iluwatar.WeatherType;
+
+public class GHobbits implements Race {
+    @Override
+    public void update(GWeather o, WeatherType weatherType) {
+        switch (weatherType) {
+            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/generic/GOrcs.java

@@ -0,0 +1,25 @@
+package com.iluwatar.generic;
+
+import com.iluwatar.WeatherType;
+
+public class GOrcs implements Race {
+    @Override
+    public void update(GWeather weather, WeatherType weatherType) {
+        switch (weatherType) {
+            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/generic/GWeather.java

@@ -0,0 +1,19 @@
+package com.iluwatar.generic;
+
+import com.iluwatar.WeatherType;
+
+public class GWeather extends Observable<GWeather, Race, WeatherType> {
+
+    private WeatherType currentWeather;
+
+    public GWeather() {
+        currentWeather = WeatherType.SUNNY;
+    }
+
+    public void timePasses() {
+        WeatherType[] enumValues = WeatherType.values();
+        currentWeather = enumValues[(currentWeather.ordinal() + 1) % enumValues.length];
+        System.out.println("The weather changed to " + currentWeather + ".");
+        notifyObservers(currentWeather);
+    }
+}

observer/src/main/java/com/iluwatar/generic/Observable.java

@@ -0,0 +1,31 @@
+package com.iluwatar.generic;
+
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+/**
+ * Generic observer inspired by Java Generics and Collection by Naftalin & Wadler
+ *
+ * @param <S> Subject
+ * @param <O> Observer
+ * @param <A> Argument type
+ */
+public abstract class Observable<S extends Observable<S, O, A>, O extends Observer<S, O, A>, A> {
+
+    protected List<O> observers;
+
+    public Observable() {
+        this.observers = new CopyOnWriteArrayList<>();
+    }
+
+    public void addObserver(O observer) {
+        this.observers.add(observer);
+    }
+
+    @SuppressWarnings("unchecked")
+    public void notifyObservers(A argument) {
+        for (O observer : observers) {
+            observer.update((S) this, argument);
+        }
+    }
+}

observer/src/main/java/com/iluwatar/generic/Observer.java

@@ -0,0 +1,5 @@
+package com.iluwatar.generic;
+
+public interface Observer<S extends Observable<S, O, A>, O extends Observer<S, O, A>, A> {
+    public void update(S subject, A argument);
+}

observer/src/main/java/com/iluwatar/generic/Race.java

@@ -0,0 +1,6 @@
+package com.iluwatar.generic;
+
+import com.iluwatar.WeatherType;
+
+public interface Race extends Observer<GWeather, Race, WeatherType> {
+}