What is a singleton?

A singleton is a design pattern used when only one instance of an object is created and shared; that is, it only allows one instance of itself to be created. Any attempt to create another instance simply returns a reference to the first one. Singleton classes are created by defining all class constructors as private. In addition, a private static member is created as the same type of the class, along with a public static member that returns an instance of the class.

Advantages

The advantages of a Singleton Pattern are:

1.    Singleton pattern can be implemented interfaces.

2.    It can be also inherit from other classes.

3.    It can be lazy loaded.

4.    It has Static Initialization.

5.    It can be extended into a factory pattern.

6.    It helps to hide dependencies.

7.    It provides a single point of access to a particular instance, so it is easy to maintain.

Disadvantages

The disadvantages of a Singleton Pattern are:

1.    Unit testing is more difficult (because it introduces a global state into an application).

2.    This pattern reduces the potential for parallelism within a program, because to access the singleton in a multi-threaded system, an object must be serialized (by locking).

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