Relation To Other Patterns
It can be regarded as a special case of the State pattern and the Strategy pattern.
It is not a pattern from Design Patterns, but is mentioned in Martin Fowler's Refactoring and Joshua Kerievsky's book on refactoring in the Insert Null Object refactoring.
Chapter 17 is dedicated to the pattern in Robert Cecil Martin's Agile Software Development: Principles, Patterns and Practices
Read more about this topic: Null Object Pattern
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