What is a Singleton Design Pattern?

Introduction

Singleton is a type of desing pattern in which the constructor is initialized only one instance of a class in the entire codebase which is used for rest of operation.

• It is used in creating applications like logger.

4 Ways to achieve this:

1. Eager
2. Lazy 
3. Synchronized Method
4. Double locking (Industry practices)

Making object private and using static we create an singleton pattern.

1. Eager Initialization:

During runtine the instance gets created.

public class DBConnection{
    private static DBConnection conObject = new DBConnection();
    private DBConnection(){
    }
    public static DBConnection getInstance(){
    	return conObject;
    }
}

public class Main{
	private static void main(String args[]){
    	DBConnection connObject = DBConnection.getInstance(); 
    }
}

2. Lazy Initialization:

In this the object gets intialized when required for user. 

Problem: 

If two threads are called at once then 2 instances are created with 2 object instances.

  public class DBConnection {  
    private static DBConnection conObject;
    private DBConnection(){}
    public static DBConnection getInstance(){
    	if(conObject == null){
            conObject = new DBConnection();
         }
         return conObject;
    }
  }
  

3. Synchronized Method:

In this the object gets intialized when required for user even various threads call the object at once. Problem: 

Its is expensive as for each threads it locks and perform operations.

  public class DBConnection {  
  	private static DBConnection conObject;
	private DBConnection(){}
        synchronized public static DBConnection getInstance(){
    	if(conObject == null){
           conObject = new DBConnection();
         }
         return conObject;
    }
  }
  

4. Double Locking:

It the most promosing mechanism of this singleton design pattern making the conditions met and called once when called without extra memory and resource usage.

  public class DBConnection {  
    private static DBConnection conObject;
    private DBConnection(){}
    public static DBConnection getInstance(){
    	if(conObject == null){
         synchronized(DBConnection.class){
         if(conObject == null){
           conObject = new DBConnection();
      	  }
    	}
        return conObject;
    }
  }