add observer pattern

DesignPattern/ObserverPattern/README.md

@@ -2,3 +2,13 @@
 ==========
 
 **观察者模式**定义了对象之间的一对多依赖，这样依赖，当一个对象改变状态时，它的所有依赖这都会收到通知并自动更新。
+
+> **设计原则四：**为了交互对象之间的松耦合设计而努力。
+
+
+**Key Points:**
++ 观察者模式定义了对象之间的一对多的关系。
++ 主题用一个共同的接口来更新观察者。
++ 观察者和可观察者之间用松耦合方式结合，可观察者不知道观察者的细节，只知道观察者实现了观察者接口。
++ 使用此模式时，你可从被观察者处push或pull数据。
++ 有多个观察者的时候，不可以依赖特定的通知次序。

DesignPattern/ObserverPattern/src/CurrentConditionsDisplay.java

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+package ObserverPattern;
+
+import java.util.Observable;
+import java.util.Observer;
+
+public class CurrentConditionsDisplay implements Observer, DisplayElement {
+
+	Observable observable;
+	private float temperature;
+	private float humidity;
+
+	public CurrentConditionsDisplay(Observable observable) {
+		this.observable = observable;
+		observable.addObserver(this);
+	}
+
+	public void update(Observable obs, Object arg) {
+		if (obs instanceof WeatherData) {
+			WeatherData weatherData = (WeatherData) obs;
+			this.temperature = weatherData.getTemperature();
+			this.humidity = weatherData.getHumidity();
+			display();
+		}
+	}
+
+	public void display() {
+		System.out.println("Current conditions: " + temperature + "F degrees and " + humidity + "% humidity!");
+	}
+
+}

DesignPattern/ObserverPattern/src/DisplayElement.java

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+package ObserverPattern;
+
+public interface DisplayElement {
+
+	public void display();
+
+}

DesignPattern/ObserverPattern/src/WeatherData.java

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+package ObserverPattern;
+
+import java.util.ArrayList;
+import java.util.Observable;
+
+public class WeatherData extends Observable {
+
+	private float temperature;
+	private float humidity;
+	private float pressure;
+
+	public WeatherData() { }
+
+	public void measurementsChanged() {
+		setChanged();
+		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;
+	}
+
+}

DesignPattern/ObserverPattern/src/WeatherStation.java

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+package ObserverPattern;
+
+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);
+	}
+
+}