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
+ * */