In object-oriented programming, a singleton class is a class that can have only one object (an instance of the class) at a time.
After first time, if we try to instantiate the Singleton class, the new variable also points to the first instance created. So whatever modifications we do to any variable inside the class through any instance, it affects the variable of the single instance created and is visible if we access that variable through any variable of that class type defined.

To design a singleton class:

• Make constructor as private.
• Write a static method that has return type object of this singleton class. Here, the concept of Lazy initialization is used to write this static method.

Normal class vs Singleton class: Difference in normal and singleton class in terms of instantiation is that, For normal class we use constructor, whereas for singleton class we use getInstance() method (Example code:I). In general, to avoid confusion we may also use the class name as method name while defining this method (Example code:II).

Implementing Singleton class with getInstance() method

// Java program implementing Singleton class
// with getInstance() method
class Singleton
{
// static variable single_instance of type Singleton
private static Singleton single_instance = null;

// variable of type String
public String s;

// private constructor restricted to this class itself
private Singleton()
{
s = "Hello I am a string part of Singleton class";
}

// static method to create instance of Singleton class
public static Singleton getInstance()
{
if (single_instance == null)
single_instance = new Singleton();

return single_instance;
}
}

// Driver Class
class Main
{
public static void main(String args[])
{
// instantiating Singleton class with variable x
Singleton x = Singleton.getInstance();

// instantiating Singleton class with variable y
Singleton y = Singleton.getInstance();

// instantiating Singleton class with variable z
Singleton z = Singleton.getInstance();

// changing variable of instance x
x.s = (x.s).toUpperCase();

System.out.println("String from x is " + x.s);
System.out.println("String from y is " + y.s);
System.out.println("String from z is " + z.s);
System.out.println("n");

// changing variable of instance z
z.s = (z.s).toLowerCase();

System.out.println("String from x is " + x.s);
System.out.println("String from y is " + y.s);
System.out.println("String from z is " + z.s);
}
}
Output:

String from x is HELLO I AM A STRING PART OF SINGLETON CLASS
String from y is HELLO I AM A STRING PART OF SINGLETON CLASS
String from z is HELLO I AM A STRING PART OF SINGLETON CLASS

String from x is hello i am a string part of singleton class
String from y is hello i am a string part of singleton class
String from z is hello i am a string part of singleton class
Singleton class

Explanation: In the Singleton class, when we first time call getInstance() method, it creates an object of the class with name single_instance and return it to the variable. Since single_instance is static, it is changed from null to some object. Next time, if we try to call getInstance() method, since single_instance is not null, it is returned to the variable, instead of instantiating the Singleton class again. This part is done by if condition.

In the main class, we instantiate the singleton class with 3 objects x, y, z by calling static method getInstance(). But actually after creation of object x, variables y and z are pointed to object x as shown in the diagram. Hence, if we change the variables of object x, that is reflected when we access the variables of objects y and z. Also if we change the variables of object z, that is reflected when we access the variables of objects x and y.

Implementing Singleton class with method name as that of class name

// Java program implementing Singleton class
// with method name as that of class
class Singleton
{
// static variable single_instance of type Singleton
private static Singleton single_instance=null;

// variable of type String
public String s;

// private constructor restricted to this class itself
private Singleton()
{
s = "Hello I am a string part of Singleton class";
}

// static method to create instance of Singleton class
public static Singleton Singleton()
{
// To ensure only one instance is created
if (single_instance == null)
{
single_instance = new Singleton();
}
return single_instance;
}
}

// Driver Code
class Main
{
public static void main(String args[])
{
// instantiating Singleton class with variable x
Singleton x = Singleton.Singleton();

// instantiating Singleton class with variable y
Singleton y = Singleton.Singleton();

// instantiating Singleton class with variable z
Singleton z = Singleton.Singleton();

// changing variable of instance x
x.s = (x.s).toUpperCase();

System.out.println("String from x is " + x.s);
System.out.println("String from y is " + y.s);
System.out.println("String from z is " + z.s);
System.out.println("n");

// changing variable of instance x
z.s = (z.s).toLowerCase();

System.out.println("String from x is " + x.s);
System.out.println("String from y is " + y.s);
System.out.println("String from z is " + z.s);
}
}
Output:

String from x is HELLO I AM A STRING PART OF SINGLETON CLASS
String from y is HELLO I AM A STRING PART OF SINGLETON CLASS
String from z is HELLO I AM A STRING PART OF SINGLETON CLASS

String from x is hello i am a string part of singleton class
String from y is hello i am a string part of singleton class
String from z is hello i am a string part of singleton class

Explanation: In the Singleton class, when we first time call Singleton() method, it creates an object of class Singleton with name single_instance and return it to the variable. Since single_instance is static, it is changed from null to some object. Next time if we try to call Singleton() method, since single_instance is not null, it is returned to the variable, instead of instantiating the Singleton class again.