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Singleton pattern Ensure a class only has one instance, and provide a global point of access to it. This is useful when exactly one object is needed to coordinate actions across the system.


  • Singleton pattern is implemented by creating a class with a method that creates a new instance of the object if one does not exist.
  • If an instance already exists, it simply returns a reference to that object.
  • To make sure that the object cannot be instantiated any other way, the constructor is made either private or protected.

UML Diagram

Singleton UML.png

Sample Code

class Singleton {

 private static Singleton instance;
 // Note: Constructor is 'protected' 
 protected Singleton() 
 public static Singleton Instance()
   // Use 'Lazy initialization' 
   if (instance == null)
     instance = new Singleton();
   return instance;