Introduce Null Object

Summarised from Refactoring Martin Fowler 1999

Motivation

You have repeated checks for a null value

For example your code my look like:

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

Solution

Replace your check for a null value 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().