never-use-null.md

Never use null

You must not use null, prefer Option[A] instead.

null is known as The-Billion-Dollar-Mistake because it is very error prone. The compiler can't track what can be null and verify that nullable objects are checked before being used.

The only solution to be sure to not make errors is a bit extreme: DO NOT ALLOW ANY NULL IN YOUR PROGRAM!!!

Using Option[A] allows to know what can be absent and helps you avoiding mistakes.

Here is an example:

def getName(path: Option[String]): Option[String] =
  path.map(_.split("/").last)

or an other:

def getName(path: Option[String]): String =
  path.getOrElse("").split("/").last

it is not recommended to use isEmpty and get as it rely on some "knowledge" that the Option[A] is not empty in a specific context:

def getName(path: Option[String]): String =
  if(path.isEmpty) {
    ""
  } else {
    path.get.split("/").last
  }

and of course, it is forbidden to pass null argument so no need to check for nullity:

def getName(path: String): String =
  path.split("/").last

this code is considered as incorrect:

def getName(path: String): String =
  if(path == null) {
    ""
  } else {
    path.split("/").last
  }

Some tips

Combining multiple Options can be easily done use for-comprehensions:

val nameOpt: Option[String] = Some("John")
val ageOpt: Option[Int] = Some(42)

val res: Option[String] = for {
  name <- nameOpt
  age <- ageOpt
} yield s"$name is $age years old"
// if every Option is Some, the result will be Some. Otherwise it will be None.

Since Option[A] can be automatically converted to an Iterable so you can see it as a collection of 0 or 1 element.

You can use .flatMap on a collection to eliminate None elements:

val list = Seq(1, 2, 3, 4, 5)
list.flatMap(i => if(i % 2 == 0) Some(i.toString) else None)
// Seq("2", "4")

Other references

This rule is also present in other best practice guides:

• Alexandru Nedelcu best practices
• Nicolas Rinaudo best practices