Implementation Issues

Variant 1: Subclassing not possible.

• (1) The INSTANCE constant is set to the instance of the class.
  Fields declared final are initialized once and can never be changed.
  In Java, "Final fields also allow program­mers to implement thread-safe immutable objects without synchronization." [JLS12 , 17.5 Final Field Semantics]
• (2) The constructor of the class is hidden (declared private).
  This ensures that the class can neither be instantiated (from outside the class) nor subclassed (subclasses need/call the constructor of their parent class).
• (3) The public static getInstance() operation returns the INSTANCE constant.
  Clients can access the sole instance easily by calling Singleton.getInstance().
• See also Flyweight and State / Sample Code.

Variant 2: Subclassing possible.

• (1) The instance field holds the instance of a subclass.
• (2) Singleton's default constructor is used.
  Because the class is abstract, it can't be instantiated.
• (3) The public static final getInstance() operation must decide which subclass to instantiate. This can be done in different ways: according to user input, system en­viron­ment, configuration file, etc.
  Note that operations declared final can't be redefined by subclasses.
• See also Abstract Factory / Sample Code / Example 3 / Creating families of objects.