References
Here is a list of information you can refer to:
- GoF: The book written by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides named Design Patterns: Elements of Reusable Object-Oriented Software
- SNGMONO: An article written by Robert C. Martin, 2002, named SINGLETON and MONOSTATE: http://staff.cs.utu.fi/~jounsmed/doos_06/material/SingletonAndMonostate.pdf
- The Null Object Pattern, written by Bobby Woolf