Advantages (+)

• Can control object creation.
  • The getInstance operation can control the creation process, for example, to allow more than one instance of a class.

Disadvantages (–)

• Makes clients dependent on the concrete singleton class.
  • This stops client classes from being reusable and testable.
  • "Making a class a singleton can make it difficult to test its clients, as it's impossible to substitute a mock implementation for a singleton unless it implements an interface that serves as its type." [JB08 , p17]
  • See also Abstract Factory / Sample Code / Example 3 / Creating families of objects.
• Can cause problems in a multi-threaded environment.
  • In multi-threaded applications, a singleton that holds mutable data (i.e., data that can be changed after the object is created) must be implemented carefully (synchronized).