# Singleton

Paul Tomoiu

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Examples,

Design patterns

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Published 2022-12-03

This tutorial explains to you the design pattern named Singleton (which is a Creational Pattern).

# Singleton Pattern - theory

A Singleton, is a class which is instantiated once per application and exist for the lifecycle of the application.

A Singleton Design Pattern is a pattern for creating a singleton object. The pattern is used when you need an object to be instantiated per application and per container.

# Singleton Pattern - example

Here it is the UML Diagram for Singleton Pattern (Example):

When we create a singleton, we have to take care of the following:

the constructor must be private: the object cannot be created outside the class.
we will not create another object, but only the instance of the first object (created when the class is used for the first time for - see the static keyword in the code below).

Here is the code for creating a singleton object (my example) and the code for testing it :

public class SingletonClass {

     private int instanceNumber;
     
     //create an object of SingletonClass when the class is used for the first time
     private static SingletonClass instance = new SingletonClass();
 
     //the constructor is private so that this class cannot be instantiated
     private SingletonClass(){
         instanceNumber = instanceNumber + 1;
     }
 
     //Get the only object available
      public static SingletonClass getInstance(){
         return instance;
       }
       
      //Test if there are more then 1 instances
      public void getInstanceNumber(){
          System.out.println("This is instance #"+instanceNumber+" !");
      }

}

public class SingletonUsageExample {

public static void main(String[] args) {

    SingletonClass object1 = SingletonClass.getInstance();
    object1.getInstanceNumber();
     
    SingletonClass object2 = SingletonClass.getInstance();
    object2.getInstanceNumber();
                     
    }
}

And here you have the example result :