Singleton

Common uses

• The Abstract Factory, Builder, and Prototype patterns can use Singletons in their implementation.
• Facade objects are often Singletons because only one Facade object is required.
• State objects are often Singletons.
• Singletons are often preferred to global variables because:
  • They don't pollute the global namespace (or, in languages withnamespaces, their containing namespace) with unnecessary variables.
  • They permit lazy allocation and initialization, where global variables in many languages will always consume resources.

Class diagram

JAVA EXAMPLE:

public class Singleton {

   private static final Singleton INSTANCE = new Singleton();
 
   // Private constructor prevents instantiation from other classes
   private Singleton() {}
 
   public static Singleton getInstance() {
      return INSTANCE;
   }
 }