Merge pull request #1 from qilingit/patch-1

Add Front-Controller explanation

Author: learning-zone

java-design-pattern-questions.md

@@ -2,6 +2,116 @@
 
 #### Q. Exaplain MVC, Front-Controller, DAO, DTO, Service-Locator, Prototype design patterns?
 
+**2. Front-Controller**
+
+The front controller design pattern is used to provide a centralized request handling mechanism so that all requests will be handled by a single handler. This handler can do the authentication/ authorization/ logging or tracking of request and then pass the requests to corresponding handlers. Following are the entities of this type of design pattern.
+
+* Front Controller - Single handler for all kinds of requests coming to the application (either web based/ desktop based).
+
+* Dispatcher - Front Controller may use a dispatcher object which can dispatch the request to corresponding specific handler.
+
+* View - Views are the object for which the requests are made.
+
+Example:
+
+We are going to create a `FrontController` and `Dispatcher` to act as Front Controller and Dispatcher correspondingly. `HomeView` and `StudentView` represent various views for which requests can come to front controller.
+
+FrontControllerPatternDemo, our demo class, will use FrontController to demonstrate Front Controller Design Pattern.
+
+Step 1
+Create Views.
+
+*HomeView.java*
+```java
+public class HomeView {
+   public void show(){
+      System.out.println("Displaying Home Page");
+   }
+}
+```
+*StudentView.java*
+```java
+public class StudentView {
+   public void show(){
+      System.out.println("Displaying Student Page");
+   }
+}
+```
+
+Step2
+Create Dispatcher.
+
+*Dispatcher.java*
+```java
+public class Dispatcher {
+   private StudentView studentView;
+   private HomeView homeView;
+   
+   public Dispatcher(){
+      studentView = new StudentView();
+      homeView = new HomeView();
+   }
+
+   public void dispatch(String request){
+      if(request.equalsIgnoreCase("STUDENT")){
+         studentView.show();
+      }
+      else{
+         homeView.show();
+      }	
+   }
+}
+```
+
+Step3
+Create FrontController.
+
+*FrontController.java*
+```java
+public class FrontController {
+	
+   private Dispatcher dispatcher;
+
+   public FrontController(){
+      dispatcher = new Dispatcher();
+   }
+
+   private boolean isAuthenticUser(){
+      System.out.println("User is authenticated successfully.");
+      return true;
+   }
+
+   private void trackRequest(String request){
+      System.out.println("Page requested: " + request);
+   }
+
+   public void dispatchRequest(String request){
+      //log each request
+      trackRequest(request);
+      
+      //authenticate the user
+      if(isAuthenticUser()){
+         dispatcher.dispatch(request);
+      }	
+   }
+}
+```
+
+Step4
+Use the FrontController to demonstrate Front Controller Design Pattern.
+
+*FrontControllerPatternDemo.java*
+```java
+public class FrontControllerPatternDemo {
+   public static void main(String[] args) {
+   
+      FrontController frontController = new FrontController();
+      frontController.dispatchRequest("HOME");
+      frontController.dispatchRequest("STUDENT");
+   }
+}
+```
+
 *ToDo*
 
 ## Q. What are the design patterns available in Java?
@@ -471,4 +581,4 @@ Output
 
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