What is singleton class?

Singleton class is used to create only one instance of the class. Its preventing to create multiple Objects and it is used to create the connection pool for example to maintain single database connection object.
The singleton class created using private constructor which prevents the direct instantiation of the object by others (Other Classes). A static modifier is applied to the instance method that returns the object as it then makes this method a class level method that can be accessed without creating an object.

To implement this design pattern we need to consider the following 4 steps:

Step 1: Provide a default Private constructor

public class SingletonObjectDemo {
      // Note that the constructor is private 
     private SingletonObjectDemo()
    {
         // Optional Code 
    }
} 

Step 2: Create a Method for getting the reference to the Singleton Object

public class SingletonObjectDemo {
  
      private static SingletonObject singletonObject; // Note that the constructor is private private        

      SingletonObjectDemo() {
         // Optional Code 
     } 

      public static SingletonObjectDemo getSingletonObject() { 
               if (singletonObject == null) { 
                singletonObject = new SingletonObjectDemo();
              } 
      return singletonObject;
   }
} 

We write a public static getter or access method to get the instance of the Singleton Object at runtime. First time the object is created inside this method as it is null. Subsequent calls to this method returns the same object created as the object is globally declared (private) and the hence the same referenced object is returned.

Step 3: Make the Access method Synchronized to prevent Thread Problems.

public static synchronized SingletonObjectDemo getSingletonObject() 

It could happen that the access method may be called twice from 2 different classes at the same time and hence more than one object being created. This could violate the design patter principle. In order to prevent the simultaneous invocation of the getter method by 2 threads or classes simultaneously we add the synchronized keyword to the method declaration

Step 4: Override the Object clone method to prevent cloning

We can still be able to create a copy of the Object by cloning it using the Object’s clone method. This can be done as shown below

SingletonObjectDemo clonedObject = (SingletonObjectDemo) obj.clone(); 

This again violates the Singleton Design Pattern’s objective. So to deal with this we need to override the Object’s clone method which throws a CloneNotSupportedException exception.

public Object clone() throws CloneNotSupportedException { 
              throw new CloneNotSupportedException(); 
} 

The below program shows the final Implementation of Singleton Design Pattern in java, by using all the 4 steps mentioned above.

class SingletonClass {
      private static SingletonClass singletonObject;
    
     /** A private Constructor prevents any other class from instantiating. */ 
     private SingletonClass() { 
        // Optional Code 
      } 
   
     public static synchronized SingletonClass getSingletonObject() { 
          if (singletonObject == null) { 
                singletonObject = new SingletonClass();
          } 
       return singletonObject;
  } 

public Object clone() throws CloneNotSupportedException { 
            throw new CloneNotSupportedException(); 
  } 
}

public class SingletonObjectDemo {
       public static void main(String args[]) { 
       // SingletonClass obj = new SingletonClass();

   //Compilation error not allowed

       SingletonClass obj = SingletonClass.getSingletonObject(); // Your Business Logic   
       System.out.println("Singleton object obtained"); 
 }
}

Another approach

We don’t need to do a lazy initialization of the instance object or to check for null in the get method. We can also make the singleton class final to avoid sub classing that may cause other problems.

public class SingletonClass {
        private static SingletonClass ourInstance = new SingletonClass(); 
        public static SingletonClass getInstance() { 
                  return singletonObj;
        } 
        private SingletonClass() { }
}