Observer pattern

README.md

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-# DesignPatternsJava9
-This repo consists Gang of Four Design patterns code on Java 9. Each branch in the repository has code of 1 design pattern. Switch repository to try out different design patterns.
+# What is Observer Design Pattern 
+The observer pattern is a Behavioral design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes or updates.
+
+## Diagram
+![Diagram](https://github.com/premaseem/DesignPatternsJava9/blob/observer-pattern/diagrams/Observer-Pattern-class-diagram.png "Diagram")
+
+![Diagram](https://github.com/premaseem/DesignPatternsJava9/blob/observer-pattern/diagrams/Observer-Design-Pattern-generic.jpeg "Diagram")
+
+![Diagram](https://github.com/premaseem/DesignPatternsJava9/blob/observer-pattern/diagrams/ObserverSequenceDiagram.png "Diagram")
+
+# When to use Observer Design Pattern 
+* Heavy used in  "event driven" application
+* It is suitable for any scenario that requires push-based notification.
+
+### Learn Design Patterns with Java by Aseem Jain
+This repository contains working project code used in video Course by Packt Publication with title "Learn Design Patterns with Java " authored by "Aseem Jain".
+
+### Course link: 
+https://www.packtpub.com/application-development/learn-design-patterns-java-9-video
+
+### ![ http://in.linkedin.com/in/premaseem](https://github.com/premaseem/DesignPatternsJava9/blob/master/linkedin.png "http://in.linkedin.com/in/premaseem") Profile:  http://in.linkedin.com/in/premaseem
+
+### Authors blog on design patterns:
+https://premaseem.wordpress.com/category/computers/design-patterns/
+
+### Software Design pattern community face book page:
+https://www.facebook.com/DesignPatternGuru/
+
+### Note: 
+* This code base will work on Java 9 and above versions. 
+* `diagrams` folders carry UML diagrams.
+* `pattern` folder has code of primary example. 
+* `patternBonus` folder has code of secondary or bonus example.

pattern/src/com/premaseem/client/ClientForShareTrading.java

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+package com.premaseem.client;
+
+/*
+@author: Aseem Jain
+@title: Design Patterns with Java 9
+@link: https://premaseem.wordpress.com/category/computers/design-patterns/
+*/
+
+import java.util.Random;
+import java.util.Scanner;
+
+import com.premaseem.observable.SharePriceObservable;
+import com.premaseem.observable.impl.AppleShares;
+import com.premaseem.observable.impl.GoogleShares;
+import com.premaseem.observer.ShareTraderObserver;
+import com.premaseem.observer.impl.ShareTrader;
+
+public class ClientForShareTrading {
+
+    public static void main (String[] args) {
+        Scanner scan = new Scanner(System.in);
+        int repeatRunFlag = 1;
+        while (repeatRunFlag == 1) {
+
+            System.out.println("Share trading platform designed using Observer design pattern ");
+            System.out
+                    .println("Notification service can be turned ON / OFF for multiple company's Share \n ");
+            ShareTraderObserver shareTrader = new ShareTrader();
+
+            SharePriceObservable appleShares = new AppleShares();
+            SharePriceObservable googleShares = new GoogleShares();
+
+            System.out
+                    .println("Do you want to observe share price and get notification / updates of Apple inc. shares ? ");
+            System.out.println("Press 1 for yes and 0 or other digits for no ");
+            int appleShareWatch = scan.nextInt();
+            if (appleShareWatch == 1) {
+                appleShares.addObserver(shareTrader);
+            } else {
+                appleShares.removeObserver(shareTrader);
+            }
+
+            System.out
+                    .println("Do you want to observe share price and get notification / updates of Google inc. shares ? ");
+            System.out.println("Press 1 for yes and 0 or other digits for no ");
+            int googleShareWatch = scan.nextInt();
+            if (googleShareWatch == 1) {
+                googleShares.addObserver(shareTrader);
+            } else {
+                googleShares.removeObserver(shareTrader);
+            }
+
+            System.out.println("Simulating price change in Stock Market ^^--__-_-^^  ");
+            simulateSharePriceChange(appleShares);
+            simulateSharePriceChange(googleShares);
+            System.out.println("========================================");
+            System.out
+                    .println("Do you want to Re-run this program - Press 1 for yes and 0 or other digits to EXIT ");
+            repeatRunFlag = scan.nextInt();
+        }
+    }
+
+    public static void simulateSharePriceChange (
+            SharePriceObservable sharePriceObservable) {
+        Random rnd = new Random();
+        for (int i = 0; i < 3; i++) {
+            sharePriceObservable.setCurrentMarketPrice(rnd.nextInt());
+        }
+    }
+}
+
+/** Lesson Learnt
+ * Pooling or continuous monitoring is not required
+ * Loose coupling between Share trader (client-observer)
+ * and Share company (subject-observable)
+ * */

pattern/src/com/premaseem/observable/SharePriceObservable.java

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+package com.premaseem.observable;
+
+
+import com.premaseem.observer.ShareTraderObserver;
+
+public interface SharePriceObservable {
+
+	 void addObserver (ShareTraderObserver shareTraderObserver);
+
+	 void removeObserver (ShareTraderObserver shareTraderObserver);
+
+	 void notifyObservers ();
+
+	// This is just a work around to simulate the price from client side 
+	public void setCurrentMarketPrice (Integer currentMarketPrice);
+}

pattern/src/com/premaseem/observable/impl/AppleShares.java

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+package com.premaseem.observable.impl;
+/*
+@author: Aseem Jain
+@title: Design Patterns with Java 9
+@link: https://premaseem.wordpress.com/category/computers/design-patterns/
+*/
+
+import java.util.ArrayList;
+
+import com.premaseem.observable.SharePriceObservable;
+import com.premaseem.observer.ShareTraderObserver;
+
+public class AppleShares implements SharePriceObservable {
+
+    public ArrayList<ShareTraderObserver> shareTraders = new ArrayList<ShareTraderObserver>();
+    private Integer currentMarketPrice = 0;
+
+    @Override
+    public void addObserver (ShareTraderObserver shareTraderObserver) {
+        shareTraders.add(shareTraderObserver);
+
+    }
+
+    @Override
+    public void removeObserver (ShareTraderObserver shareTraderObserver) {
+        shareTraders.remove(shareTraderObserver);
+
+    }
+
+    @Override
+    public void notifyObservers () {
+        for (ShareTraderObserver shareBroker : shareTraders) {
+            shareBroker.notifyCurrentPrice(getCurrentMarketPrice(), " Apple Inc.  ");
+        }
+
+    }
+
+    public Integer getCurrentMarketPrice () {
+        return currentMarketPrice;
+    }
+
+    public void setCurrentMarketPrice (Integer currentMarketPrice) {
+        this.currentMarketPrice = currentMarketPrice;
+        notifyObservers();
+    }
+}

pattern/src/com/premaseem/observable/impl/GoogleShares.java

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+package com.premaseem.observable.impl;
+
+/*
+@author: Aseem Jain
+@title: Design Patterns with Java 9
+@link: https://premaseem.wordpress.com/category/computers/design-patterns/
+*/
+
+import java.util.ArrayList;
+
+import com.premaseem.observable.SharePriceObservable;
+import com.premaseem.observer.ShareTraderObserver;
+
+public class GoogleShares implements SharePriceObservable{
+
+	public ArrayList<ShareTraderObserver> shareTraders = new ArrayList<ShareTraderObserver>();
+	private Integer currentMarketPrice=0;
+
+	@Override
+	public void addObserver(ShareTraderObserver shareTraderObserver) {
+		shareTraders.add(shareTraderObserver);
+
+	}
+
+	@Override
+	public void removeObserver(ShareTraderObserver shareTraderObserver) {
+		shareTraders.remove(shareTraderObserver);
+
+	}
+
+	@Override
+	public void notifyObservers() {
+	for(ShareTraderObserver shareBroker : shareTraders){
+		shareBroker.notifyCurrentPrice(getCurrentMarketPrice()," Google Inc. ");
+	} 
+
+	}
+
+	public Integer getCurrentMarketPrice() {
+		return currentMarketPrice;
+	}
+
+	public void setCurrentMarketPrice(Integer currentMarketPrice) {
+		this.currentMarketPrice = currentMarketPrice;
+		notifyObservers();
+	}
+}

pattern/src/com/premaseem/observer/ShareTraderObserver.java

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+package com.premaseem.observer;
+
+public interface ShareTraderObserver {
+
+	public void notifyCurrentPrice (Integer shareLatestPrice, String shareCompanyName);
+
+}

pattern/src/com/premaseem/observer/impl/ShareTrader.java

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+package com.premaseem.observer.impl;
+/*
+@author: Aseem Jain
+@title: Design Patterns with Java 9
+@link: https://premaseem.wordpress.com/category/computers/design-patterns/
+*/
+
+import com.premaseem.observer.ShareTraderObserver;
+
+public class ShareTrader implements ShareTraderObserver {
+
+	@Override
+	public void notifyCurrentPrice(Integer shareLatestPrice,
+			String shareCompanyName) {
+		System.out.println();
+		System.out.println(this.getClass().getSimpleName() +  " received notification for Price change ");
+		System.out.printf("Price change notification - current price of %S Share is %d", shareCompanyName,shareLatestPrice);
+		System.out.println();
+	}
+}

pattern/src/com/premaseem/observer/impl/ShareTrader1.java

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+package com.premaseem.observer.impl;
+
+import com.premaseem.observer.ShareTraderObserver;
+
+public class ShareTrader1 implements ShareTraderObserver {
+
+	@Override
+	public void notifyCurrentPrice(Integer shareLatestPrice,
+			String shareCompanyName) {
+		System.out.println();
+		System.out.println(this.getClass().getSimpleName() +  ": Notified $$ ");
+		System.out.printf("Latest price of %S Share is %d ", shareCompanyName,shareLatestPrice);
+		System.out.println();
+	}
+}

pattern/src/com/premaseem/observer/impl/ShareTrader2.java

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+package com.premaseem.observer.impl;
+
+import com.premaseem.observer.ShareTraderObserver;
+
+public class ShareTrader2 implements ShareTraderObserver {
+	@Override
+	public void notifyCurrentPrice(Integer shareLatestPrice,
+			String shareCompanyName) {
+		System.out.println();
+		System.out.println(this.getClass().getSimpleName() +  ": Notified $$ ");
+		System.out.printf("Latest price of %S Share is %d ", shareCompanyName,shareLatestPrice);
+		System.out.println();
+	}
+}

patternBonus/src/com/premaseem/ClientJavaApi.java

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+package com.premaseem;
+/*
+@author: Aseem Jain
+@title: Design Patterns with Java 9
+@link: https://premaseem.wordpress.com/category/computers/design-patterns/
+*/
+
+import java.util.Observable;
+import java.util.Observer;
+import java.util.Scanner;
+
+public class ClientJavaApi {
+
+    public static void main(String[] args) {
+
+        // Created subject
+        Subject subject = new Subject();
+
+        // Created Observer
+        Observer observer = new Observer() {
+            public void update(Observable obj, Object arg) {
+                System.out.println("Observer notified with update: " + arg);
+            }
+        };
+
+        // registered observer to the subject
+        subject.addObserver(observer);
+
+        System.out.println("Make text updates in subject: ");
+        new Thread(subject).start();
+    }
+}
+
+class Subject extends Observable implements Runnable {
+    public void run() {
+
+        // Every time whenever there is an update in subject,
+        // observer will be notified
+        while (true) {
+            String update = new Scanner(System.in).next();
+            setChanged();
+            notifyObservers(update);
+        }
+    }
+}
+
+/** Lesson Learnt
+ *
+ *  1. Instead of creating our own observer and observable,
+ *  using (java.util) Java api's  implementation of observer and observable
+ *
+ *  2. Java 9 has deprecated above apis, WHY ?
+ *  answer is in link -https://stackoverflow.com/questions/46380073/observer-is-deprecated-in-java-9-what-should-we-use-instead-of-it
+ * */