Introduce Null Object

Summarised from Refactoring Martin Fowler 1999

Adds an object that defines use and behaviour but does nothing.

Use When

If you have repeated checks for a null reference, because one can't invoke anything on a null reference.

For example your code looks like:

if (customer == null)
  plan = BillingPlan.basic();
else 
  plan = customer.getPlan();

This pattern can also be used to act as a stub for testing if a certain feature, such as a database, is not available for testing.

Structure

Replace your check for a null reference with a null object

• Create a subclass that acts as a null version of the class.
• Create an isNull() method in both classes. For the superclass it should return "false", and "true" for the subclass.
• Find all places that can give out a null value when asked for an object of the superclass and replace them to give a null object instead.
• Find all places that compare a variable of the superclass type with null and replace them with a call to isNull().

Consequences

This neglates the need to check for a null reference.

Another advantage of this approach over a working default implementation is that a null object is predictable and has no side effects: it does nothing.

Related Patterns

It can be regarded as a special case of the State pattern and the Strategy Pattern.