Null Object Pattern

In this post I will summarise the Null Object Pattern.

The null object pattern is a means to forgo null checks. The following example will be used to explain how the null object pattern can be used to reach this goal.

public Person getPerson()
 {
  return person;
 }

 public void showName()
 {
  Person person = getPerson();
  String name = "unknown";
  if (person != null)
  {
   name = person.getName();
  }
  System.out.println("name: " + name);
 }

In my opinion a few there are thing wrong with this code.

• Different call flow. Because of the null check there are two possible paths through this code. If this is the only point at which this occurs, it would not be a big problem. But
• Caller is responsible. The caller is made responsible to check if the returned value is correct. By placing the burden at the callers side probably means a lot of duplicate boilerplate code in checking for null values.
• Error prone. You only have to forget the null check once and it will blow up in your face with a NullPointerException.

The null object pattern is a means to remedy these points. In the following example I refactored the original code.

class UnknownPerson extends Person
{
 @Override
 public String getName()
 {
  return "unknown";
 }

}

 public Person getPerson()
 {
  if (person == null)
  {
   return new UnknownPerson();
  }
  return person;
 }

 public void showName()
 {
  Person person = getPerson();
  System.out.println("name: " + person.getName());
 }

With the introduction of the UnknownPerson all the mentioned problems are solved.

• Same call flow. The calling code does not have to differentiate between a null value and a legitimate value
• Callee is responsible. The callee is now responsible to check if the returned value is defined. By placing the burden at the callee side there is a single check so no duplication of code. So
• Less error prone. Because the null check is in a single location there are less oppertunities for making a mistake.

This summarises the null object pattern.