Mediator design 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 Mediator Design Pattern 
+The Mediator design pattern enables the centralised communication between related objects (Colleagues). All communication is handled by a Mediator Object and the Colleagues don't need to know anything about each other to work together.
+
+## Diagram
+![Diagram](https://github.com/premaseem/DesignPatternsJava9/blob/mediator-pattern/diagrams/Mediator%20Design%20Pattern%20communication%20diagram.png "Diagram")
+
+![Diagram](https://github.com/premaseem/DesignPatternsJava9/blob/mediator-pattern/diagrams/Mediator-Design-Pattern-general.png "Diagram")
+
+![Diagram](https://github.com/premaseem/DesignPatternsJava9/blob/mediator-pattern/diagrams/Mediator%20pattern%20sequence%20diagram.png "Diagram")
+
+![Diagram](https://github.com/premaseem/DesignPatternsJava9/blob/mediator-pattern/diagrams/aircraft.jpg "Diagram")
+
+# When to use Mediator Design Pattern 
+* when the communication logic between objects is complex, we can have a central point of communication using Mediator object. 
+* JMS- Java Message Service uses mediator pattern underneath. 
+
+### 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/atc/AtcMediator.java

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+package com.premaseem.atc;
+
+/*
+@author: Aseem Jain
+@title: Design Patterns with Java 9
+@link: https://premaseem.wordpress.com/category/computers/design-patterns/
+*/
+
+public class AtcMediator {
+
+	Flight approachingFligh;
+
+	enum FlightStatus {
+		IN_Q, IN_APPROACH, LANDED
+	}
+
+	public void grantLandingPermission(Flight flight) {
+		if (approachingFligh == null) {
+			approachingFligh = flight;
+			flight.status = FlightStatus.IN_APPROACH;
+			System.out.printf(" Landing Permission Granted to %s , START APPROACHING .... ", flight.name);
+
+		} else {
+			if (!approachingFligh.name.equalsIgnoreCase(flight.name)) {
+				System.out.printf(" Landing Permission Not Granted to %s as other filght (%s) has blocked the run way", flight.name, approachingFligh.name);
+			} else {
+				System.out.printf(" %S you already have the permission - ", flight.name);
+			}
+		}
+	}
+
+	void landFlight(Flight flight) {
+		if (flight.status.equals(FlightStatus.IN_APPROACH)) {
+			System.out.printf("Congratulation for safe Landing %s . . . ", flight.name);
+			flight.status = FlightStatus.LANDED;
+			approachingFligh = null;
+		} else if (flight.status.equals(FlightStatus.IN_Q)) {
+			System.out.printf(" Grand permission before landing - WARNING to %s ", flight.name);
+		} else {
+			System.out.printf("Flight %s is already grounded  ", flight.name);
+		}
+
+	}
+
+}

pattern/src/com/premaseem/atc/Client.java

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+/*
+@author: Aseem Jain
+@title: Design Patterns with Java 9
+@link: https://premaseem.wordpress.com/2014/10/04/mediator-design-pattern/
+*/
+
+package com.premaseem.atc;
+
+import java.util.Scanner;
+
+public class Client {
+
+	public static void main(String[] args) {
+
+		System.out.println("Welcome to Air Traffic Control program which uses Mediator design based architecture");
+		Scanner scan = new Scanner(System.in);
+
+		// Object initialization block
+		AtcMediator atcMediator = new AtcMediator();
+		Flight inOperationFlight;
+		Flight flight1 = new Flight(atcMediator, "Air India ");
+		Flight flight2 = new Flight(atcMediator, "American Air Lines ");
+		Flight flight3 = new Flight(atcMediator, "British Air ways ");
+
+		// User input block
+		String repeatRunFlag = "yes";
+		while (!repeatRunFlag.equalsIgnoreCase("no")) {
+			System.out.println("Which flight do you want to operate  ? ");
+			System.out.println("press 1 for Air India");
+			System.out.println("press 2 for American Air Lines ");
+			System.out.println("press 3 for British Air ways ");
+
+			int choice = scan.nextInt();
+			if (choice == 1) {
+				inOperationFlight = flight1;
+			} else if (choice == 2) {
+				inOperationFlight = flight2;
+			} else {
+				inOperationFlight = flight3;
+			}
+
+			// Flight should not land directly before taking grant from ATC
+            // Decision making logic / algorithm resides in Mediator class
+			System.out.println("What do you want to do with the flight " + inOperationFlight.name);
+			System.out.println(" Press 1 to Grant landing permission ");
+			System.out.println(" Press 2 to Land the Flight ");
+
+			int type = scan.nextInt();
+			try {
+				switch (type) {
+				case 1:
+					inOperationFlight.grantLandingPermission();
+					break;
+				case 2:
+					inOperationFlight.landFlight();
+					break;
+
+				}
+			} catch (Exception e1) {
+				System.out.println("################ Access is not valid ###############");
+			}
+			System.out.println("\n=============================");
+
+			System.out.println("Press No to Exit and any other character to Continue flight operations  .... ");
+			try {
+				repeatRunFlag = scan.next();
+			} catch (Exception e) {
+				repeatRunFlag = "no";
+			}
+
+		}
+
+        // Lessons Learnt
+        // Classes are loosely coupled with Client
+        // Core decision making logic is centralized in Mediator class
+        // Decision making process is fast and effective
+
+		/*** BEFORE CODE ***
+		System.out.println("Welcome to Air Traffic Control program ");
+
+		// Create flight objects
+		Flight_ flight1 = new Flight_();
+		Flight_ flight2 = new Flight_();
+		Flight_ flight3 = new Flight_();
+
+		// Set objects status and communicate to other classes
+		flight1.setFlightStatus(Flight_.FlightStatus.IN_APPROACH);
+		flight1.coordianteWithOtherFlight(flight2);
+		flight1.coordianteWithOtherFlight(flight3);
+
+		flight2.setFlightStatus(Flight_.FlightStatus.IN_Q);
+		flight2.coordianteWithOtherFlight(flight1);
+		flight2.coordianteWithOtherFlight(flight3);
+
+		flight3.setFlightStatus(Flight_.FlightStatus.IN_APPROACH);
+		flight3.coordianteWithOtherFlight(flight1);
+		flight3.coordianteWithOtherFlight(flight2);
+
+		System.out.println("Current Fight informs other flights of its current position /status " );
+		System.out.println("In case of clashes, one on one communication might delay decisions and cause accidents ");
+
+		System.out.println("\n $$$$$$$$$$$$$$$$$$$$  Thanks by Prem Aseem $$$$$$$$$$$$$$$$$$$$$$ \n ");
+		System.out.println("\n $$$$$$$$$$$$$$$$$$$$$  www.premaseem.me  $$$$$$$$$$$$$$$$$$$$$$ \n ");
+
+
+		// $$$ Lesson Learnt - BEFORE CODE $$$
+		// Tight coupling, Client needs to know about each flight.
+		// When number of objects are more, code becomes complex and confusing
+		// Communication between classes is not effective,
+*/
+
+	}
+}

pattern/src/com/premaseem/atc/Flight.java

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+package com.premaseem.atc;
+
+/*
+@author: Aseem Jain
+@title: Design Patterns with Java 9
+@link: https://premaseem.wordpress.com/category/computers/design-patterns/
+*/
+
+import com.premaseem.atc.AtcMediator.FlightStatus;
+
+public class Flight {
+
+	FlightStatus status;
+	AtcMediator atcMediator=null;
+	public String name;
+
+	public Flight(AtcMediator atcMediator, String flightName) {
+		status = FlightStatus.IN_Q;
+		name = flightName;
+		this.atcMediator = atcMediator;
+	}
+
+	void grantLandingPermission(){
+		atcMediator.grantLandingPermission(this);
+	}
+
+	void landFlight(){
+		atcMediator.landFlight(this);
+	}
+}

pattern/src/com/premaseem/atc/Flight_.java

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+package com.premaseem.atc;
+
+/*
+@author: Aseem Jain
+@title: Design Patterns with Java 9
+@link: https://premaseem.wordpress.com/category/computers/design-patterns/
+*/
+
+
+public class Flight_ {
+
+
+
+    enum FlightStatus {
+        IN_Q, IN_APPROACH, LANDED
+    }
+    FlightStatus flightStatus;
+
+    public void setFlightStatus(FlightStatus fs){
+         flightStatus = fs;
+    }
+
+    public void coordianteWithOtherFlight(Flight_ flight_){
+  }
+
+}

patternBonus/src/com/premaseem/lightSignal/Client.java

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+package com.premaseem.lightSignal;
+
+import java.util.Scanner;
+
+public class Client {
+
+	public static void main(String[] args) {
+
+		System.out.println("Welcome to Traffic light signal program which uses Mediator ");
+		System.out.println(" Out of 3 button only 1 could be ON and others has to in OFF state ");
+		Scanner scan = new Scanner(System.in);
+
+		// Object initialization block
+		LightMediator LightMediator = new LightMediator();
+		Light red = new RedLight(LightMediator);
+		Light yellow = new YellowLight(LightMediator);
+		Light green = new GreenLight(LightMediator);
+
+
+		// User input block
+		String repeatRunFlag = "yes";
+		while (!repeatRunFlag.equalsIgnoreCase("no")) {
+			System.out.println("Which light you want to turn on  ? ");
+			System.out.println("press 1 for RED");
+			System.out.println("press 2 for YELLOW ");
+			System.out.println("press 2 for GREEN  ");
+
+			int choice = scan.nextInt();
+			if (choice == 1) {
+				red.turnON();
+			} else if(choice == 2) {
+				yellow.turnON();
+			} else{
+				green.turnON();
+			}
+
+			System.out.println("\n=============================");
+
+			System.out.println("Press No to Exit and any other character to repeat  .... ");
+			try {
+				repeatRunFlag = scan.next();
+			} catch (Exception e) {
+				repeatRunFlag = "no";
+			}
+
+		}
+
+		System.out.println("\n $$$$$$$$$$$$$$$$$$$$  Thanks by Prem Aseem $$$$$$$$$$$$$$$$$$$$$$ \n ");
+		System.out.println("\n $$$$$$$$$$$$$$$$$$$$$  www.premaseem.me  $$$$$$$$$$$$$$$$$$$$$$ \n ");
+
+	}
+}
+

patternBonus/src/com/premaseem/lightSignal/Light.java

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+package com.premaseem.lightSignal;
+
+/*
+@author: Aseem Jain
+@title: Design Patterns with Java 9
+@link: https://premaseem.wordpress.com/category/computers/design-patterns/
+*/
+
+public abstract class Light {
+    Light(LightMediator LightMediator){
+        this.LightMediator = LightMediator;
+        LightMediator.registerLight(this);
+    }
+    enum state {
+        ON, OFF
+    }
+
+    state currentState;
+    LightMediator LightMediator;
+    int id;
+
+    void turnON() {
+        currentState = state.ON;
+        System.out.printf("%s is turned %s \n", this.getClass().getSimpleName(), currentState.ON);
+        LightMediator.turnON(this);
+    }
+
+    void turnOFF() {
+        currentState = state.OFF;
+        System.out.printf("%s is turned %s \n", this.getClass().getSimpleName(), currentState.OFF);
+    }
+
+    public void addId(int index) {
+        id = index;
+    }
+}
+
+class RedLight extends Light {
+    public RedLight(LightMediator LightMediator) {
+        super(LightMediator);
+    }
+}
+
+class YellowLight extends Light {
+    public YellowLight(LightMediator LightMediator) {
+        super(LightMediator);
+    }
+}
+
+class GreenLight extends Light {
+    public GreenLight(LightMediator LightMediator) {
+        super(LightMediator);
+    }
+}