Java Design Patterns Tutorial - Java Design Pattern - Singleton Pattern

Singleton pattern is a creational pattern.

This pattern involves only a single class which is responsible to creates its own object.

The class ensures that only single object get created.

This class provides a way to access its only object.

For example, when design a user interface, we may only have one main application windows. We can use the Singleton Pattern to ensure that there is only one instance of the MainApplicationWindow object.

Example

The following code is going to create a MainWindow class.

MainWindow class have its constructor as private and have a static instance of itself.

MainWindow class provides a static method to get its static instance to outside world.

Main, our demo class will use MainWindow class to get a MainWindow object.

class MainWindow {
   //create an object of MainWindow
   private static MainWindow instance = new MainWindow();

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

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

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

public class Main {
   public static void main(String[] args) {
      //Get the only object available
      MainWindow object = MainWindow.getInstance();

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

The code above generates the following result.