added observer sample

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);
+		}
+	}
+}