Introduce Null Object
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| + | * 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(). | ||
Revision as of 03:11, 28 September 2008
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().
