Use Null Objects instead of NULL

Contributed by: Maximiliano Contieri

Difficulty level: Intermediate

You may have heard of null as the billion-dollar mistake. The quote is from Tony Hoare who created null in the 60s. Null can represent many different things at once. When you use it in one part of the code there’s an implicit agreement on another part of the code. When you break the agreement or miss checking it, you have a defect or a null pointer exception.

The Null Object Pattern

The Null Object Pattern replaces a generic null with a specific object polymorphic with the real one. With this strategy, you avoid making castings or checking for ifs (See the “Convert If Sentences to Hierarchies” Shortcut). Introducing a polymorphic null object reduces the need for repetitive null checks throughout your code. Instead of checking for null, you can use a null object that behaves as a default or neutral implementation.

In this C# example, you might convert arbitrary currencies to dollars:

// This is the common interface public interface ICurrencyConverter { double ConvertToDollar(double amount); } public class CurrencyToDollarConverter : ICurrencyConverter { private readonly CurrencyToDollarConverter exchangeRate; public EuroToDollarConverter(double exchangeRate) { this.exchangeRate = exchangeRate; } public double ConvertToDollar(double amount) { return amount * exchangeRate; } } // This is a null object that does nothing public class DollarToDollarConverter : ICurrencyConverter ...