Singleton pattern - Creational

• Create an object and make sure that only single object gets created
• One instance of a class or one value accessible globally in an application.
• Ensures that only one instance of a class is created and provides a global access point to the object.

 

         

Where to use

• When we are sure that only one instance of a class is created
• When the instance must be available through all the code
• Ensure unique instance by defining class final to prevent cloning

Use cases

Example 1 (Lazy loading)

• Class Singleton having an object of same type as the class (Singleton) and a private constructor
• A static method to check null object and create a new object.
• Client can call static method of the Singleton class to get the object.

Example 2 (Eager/Early loading)

• Class Singleton having an object of same type instantiated using new operator and a private constructor
• A static method to return the object.
• Client can call static method of the Singleton class to get the object.

Example of usage

• Logger Classes
• Configuration Classes
• Accesing resources in shared mode
• A File system 
• A Window manager
• A printer spooler
• A Test engineAn Input/Output socket and etc. 
• Implementing other patterns : 
• Factories
• Abstract Factories