1. Null object shares interface with real object, thus they must share an ancestor class
2. Relationship is with ancestor class
3. Client class *always* has a relationship with another object (either the “real object” or the Null Object)
4. Null Object may perform default operations

Consequences

• Avoids error prone NULL checks for optional or temporal relationships, such as:

  if(myobj != 0) myobj.DoService();

• Provides a partial solution to using C++ references for optional associations

Related Patterns

• Null Object is a form of Objectifier.
• State and Strategy often provide a Null Object for safety.

Source: [PLoPD3]

Null Object Pattern, Bobby Wolf, in

Pattern Languages of Program Design 3

Pattern Languages of Program Design 3  [Barnes&Noble]