In object-oriented programming, a singleton class may be a class that will have just one object (an instance of the class) at a time.
After the first time, if we attempt to instantiate the Singleton class, the new variable also points to the primary instance created. So whatever modifications we do to any variable inside the category through any instance, it affects the variable of the only instance created and is visible if we access that variable through any variable of that class type defined.
To design a singleton class:

1. Make constructor as private.
2. Write a static method that has return type object of this singleton class. Here, the concept of Lazy initialization is employed to write down 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). generally , to avoid confusion we can also use the category name as method name while defining this method (Example code:II).

In Java, a Singleton Class is a class that allows only one instance of itself to be created and provides a global point of access to that instance. It is used when you want to restrict the instantiation of a class to one object, which can be shared across multiple classes.

The Singleton pattern is useful when you have a single resource that needs to be accessed by multiple classes. It can be used to create a unique instance of a class that can be accessed from anywhere in your code. For example, a configuration manager class could be implemented as a singleton to ensure that only one instance exists and all other classes can access it.

To create a Singleton Class in Java, you need to follow a few steps. First, make the class constructor private to prevent other classes from creating instances of the Singleton Class. Next, create a private static variable of the Singleton Class type and initialize it to null. Finally, create a public static method that returns the Singleton instance. If the Singleton instance has not been created yet, the method should create it and return it. Otherwise, it should return the existing instance.

It's important to note that Singleton Classes are not thread-safe by default. If your application is multithreaded, you need to take care of synchronizing access to the Singleton instance. One way to achieve this is by using the double-checked locking pattern.