Avoid Null

How to prevent the famous NullPointerExceptions in Java? This is one of the most important questions for every Java developer to ask independently of their level of expertise.

I call it my billion-dollar mistake. It was the invention of the null reference in 1965 - Tony Hoare

It’s a mistake, and that’s why we should know how to deal with it, get in the habit of forbidding code from using null, and then prevent NPE at all.

Here’s a Checklist of simple things to write down on a paper next to your screen:

• Avoid initializing variables to null
• Avoid returning null
• Avoid passing and receiving null parameters

Avoid initializing variables to null

I am sure you saw code in a method starting with a null initialization:

public String getFormattedDescription(String id) {
  String description = null;
  if (descriptionMap.contains(id)) {
    description = descriptionMap.get(id);
  } else {
    description = "default description";
  }
  return format(description);
}

I think we must not declare a variable until we know what value it should hold in the next lines of code. We can get rid of this problem by adapting the structure of our code:

public String getFormattedDescription(String id) {
  String description = getRawDescription(id);
  return format(description);
}

public String getRawDescription(String id) {
  if (!descriptionMap.contains(id)) {
  	return "default description";
  }
  return descriptionMap.get(id);
}

With this design, we are sure that null can not be leaked.

Avoid returning null

Never return null… never return null… never return null… sing it… but we have Optional’s API that can make it very easy to deal with the scenario where no Object was produced.

Avoid passing and receiving null parameters

The simple solution is to focus on never passing null. By doing this you will end up writing less code and avoid taking decisions about how to manage null inside of a method that doesn’t have sufficient context to decide what to do:

public string transform(string first, 
                      string second, 
                      string third){

  // are the paramters required ?
  if(first == null && second == null && third == null) {..}
  // or one of them is required ?
  if(first == null && second == null || third == null) {..}
  // but which one is optional ?
  if(first == null || second == null && third == null) {..}
  // it is diffcult to decide! 
  if(first == null || second == null || third == null) {..}
  ...
}

The problem is we don’t know what we should do? We have lots of choices, but none of them are explicit. It is more suitable to make sure that we never pass null into methods, rather than write this type full of null verification. Returning null in other methods forces us to write more code to deal with that null in the caller method!

Acceptable Nulls

In a class, the value of an attribute can be null so the logic behind this class must be aware of that absence of the value. So the access can be secured to avoid leaking this null value.

Conclusion

You have one way to choose, be with the team of the solution to the billion-dollar problem or being part of it. Keeping this in mind will help to write code without leaking null and avoid NullPointerException.