singleton pattern & MVC pattern

Singleton Pattern

This pattern involves a single class which is responsible to create an object while making sure that only single object gets created. This class provides a way to access its only object which can be accessed directly without need to instantiate the object of the class.

public class SingleObject {

   //create an object of SingleObject
   private static SingleObject instance = new SingleObject();

   //make the constructor private so that this class cannot be
   //instantiated
   private SingleObject(){}

   //Get the only object available
   public static SingleObject getInstance(){
      return instance;
   }

   public void showMessage(){
      System.out.println("Hello World!");
   }
}

public class SingletonPatternDemo {
   public static void main(String[] args) {

      //illegal construct
      //Compile Time Error: The constructor SingleObject() is not visible
      //SingleObject object = new SingleObject();

      //Get the only object available
      SingleObject object = SingleObject.getInstance();

      //show the message
      object.showMessage();
   }
}

MVC Pattern

MVC Pattern stands for Model-View-Controller Pattern. This pattern is used to separate application's concerns.

Model - Model represents an object or JAVA POJO carrying data. It can also have logic to update controller if its data changes.

View - View represents the visualization of the data that model
contains.

Controller - Controller acts on both model and view. It controls the data flow into model object and updates the view whenever data changes. It keeps view and model separate.

We are going to create a Student object acting as a model.StudentView will be a view class which can print student details on console and StudentController is the controller class responsible to store data in Student object and update view StudentView accordingly.

Create Model.

Student.java

public class Student {
   private String rollNo;
   private String name;
   
   public String getRollNo() {
      return rollNo;
   }
   
   public void setRollNo(String rollNo) {
      this.rollNo = rollNo;
   }
   
   public String getName() {
      return name;
   }
   
   public void setName(String name) {
      this.name = name;
   }
}

Create View.

StudentView.java

public class StudentView {
   public void printStudentDetails(String studentName, String studentRollNo){
      System.out.println("Student: ");
      System.out.println("Name: " + studentName);
      System.out.println("Roll No: " + studentRollNo);
   }
}

Create Controller.

StudentController.java

public class StudentController {
   private Student model;
   private StudentView view;

   public StudentController(Student model, StudentView view){
      this.model = model;
      this.view = view;
   }

   public void setStudentName(String name){
      model.setName(name);        
   }

   public String getStudentName(){
      return model.getName();        
   }

   public void setStudentRollNo(String rollNo){
      model.setRollNo(rollNo);        
   }

   public String getStudentRollNo(){
      return model.getRollNo();        
   }

   public void updateView(){                
      view.printStudentDetails(model.getName(), model.getRollNo());
   }    
}

Use the StudentController methods to demonstrate MVC design pattern usage.

MVCPatternDemo.java

public class MVCPatternDemo {
   public static void main(String[] args) {

      //fetch student record based on his roll no from the database
      Student model  = retriveStudentFromDatabase();

      //Create a view : to write student details on console
      StudentView view = new StudentView();

      StudentController controller = new StudentController(model, view);

      controller.updateView();

      //update model data
      controller.setStudentName("John");

      controller.updateView();
   }

   private static Student retriveStudentFromDatabase(){
      Student student = new Student();
      student.setName("Robert");
      student.setRollNo("10");
      return student;
   }
}