Merge pull request #67 from KunwarVikas/stackandqueue

Behavioral Design Pattern - Observer Pattern

com/javadevjournal/design/behavioral/observer/FirstSubscriber.java

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+package main.com.kunwar.designpatterns.behavioral.observer;
+
+public class FirstSubscriber implements IObserver {
+    @Override
+    public void update(Update m) {
+        System.out.println("MessageSubscriberOne :: " + m.getMessageContent());
+    }
+}

com/javadevjournal/design/behavioral/observer/IObserver.java

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+package main.com.kunwar.designpatterns.behavioral.observer;
+
+public interface IObserver {
+    public void update(Update m);
+}

com/javadevjournal/design/behavioral/observer/ISubject.java

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+package main.com.kunwar.designpatterns.behavioral.observer;
+
+public interface ISubject {
+    public void attach(IObserver o);
+    public void detach(IObserver o);
+    public void notifyUpdate(Update m);
+}

com/javadevjournal/design/behavioral/observer/MessagePublisher.java

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+package main.com.kunwar.designpatterns.behavioral.observer;
+
+import java.util.ArrayList;
+import java.util.List;
+ 
+public class MessagePublisher implements ISubject {
+     
+    private List<IObserver> observers = new ArrayList<>();
+
+    ////in multithreaded environment, please use thread-safe collections
+    @Override
+    public void attach(IObserver o) {
+        observers.add(o);
+    }
+
+    //in multithreaded environment, please use thread-safe collections
+    @Override
+    public void detach(IObserver o) {
+        observers.remove(o);
+    }
+ 
+    @Override
+    public void notifyUpdate(Update m) {
+        for(IObserver o: observers) {
+            o.update(m);
+        }
+    }
+}

com/javadevjournal/design/behavioral/observer/ObserverPatternDemo.java

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+package main.com.kunwar.designpatterns.behavioral.observer;
+
+public class ObserverPatternDemo {
+
+    public static void main(String[] args) {
+        FirstSubscriber firstSubscriber = new FirstSubscriber();
+        SecondSubscriber secondSubscriber = new SecondSubscriber();
+        ThirdSubscriber thirdSubscriber = new ThirdSubscriber();
+         
+        MessagePublisher p = new MessagePublisher();
+
+        p.attach(firstSubscriber);
+        p.attach(secondSubscriber);
+         
+        p.notifyUpdate(new Update("This is the first update"));   //s1 and s2 will receive the update
+         
+        p.detach(firstSubscriber);
+        p.attach(thirdSubscriber);
+         
+        p.notifyUpdate(new Update("This is the second update")); //s2 and s3 will receive the update
+    }
+}

com/javadevjournal/design/behavioral/observer/SecondSubscriber.java

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+package main.com.kunwar.designpatterns.behavioral.observer;
+
+public class SecondSubscriber implements IObserver {
+    @Override
+    public void update(Update m) {
+        System.out.println("MessageSubscriberTwo :: " + m.getMessageContent());
+    }
+}

com/javadevjournal/design/behavioral/observer/ThirdSubscriber.java

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+package main.com.kunwar.designpatterns.behavioral.observer;
+
+public class ThirdSubscriber implements IObserver {
+    @Override
+    public void update(Update m) {
+        System.out.println("MessageSubscriberThree :: " + m.getMessageContent());
+    }
+}

com/javadevjournal/design/behavioral/observer/Update.java

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+package main.com.kunwar.designpatterns.behavioral.observer;
+
+public class Update {
+    final String messageContent;
+     
+    public Update(String m) {
+        this.messageContent = m;
+    }
+ 
+    public String getMessageContent() {
+        return messageContent;
+    }
+}