Introduce Null Object
Summarised from Refactoring Martin Fowler 1999
Adds an object that defines use and behaviour but does nothing.
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.
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().
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.