Is this the proper way to initialize null references in Scala?

Question

Let's say I have a MyObject instance which is not initialized:

var a:MyObject = null

is this the proper way to initialize it to null?

Answer 1

The canonical answer is don't use null. Instead, use an option type:

var a = None : Option[MyObject]

When you want to set it:

a = Some(foo)

And when you want to read from it, test for None:

a match {
  case None => Console.println("not here")
  case Some(value) => Console.println("got: "+value)
}

Answer 2

Alternatives

Use null as a last resort. As already mentioned, Option replaces most usages of null. If you using null to implement deferred initialisation of a field with some expensive calculation, you should use a lazy val.

Canonical initialisation to null

That said, Scala does support null. I personally use it in combination with Spring Dependency Injection.

Your code is perfectly valid. However, I suggest that you use var t: T = _ to initialize t to it's default value. If T is a primitive, you get the default specific to the type. Otherwise you get null.

Not only is this more concise, but it is neccessary when you don't know in advance what T will be:

scala> class A[T] { var t: T = _ }
defined class A

scala> new A[String].t
res0: String = null

scala> new A[Object].t            
res1: java.lang.Object = null

scala> new A[Int].t   
res2: Int = 0

scala> new A[Byte].t
res3: Byte = 0

scala> new A[Boolean].t
res4: Boolean = false

scala> new A[Any].t   
res5: Any = null

Advanced

Using var t: T= null is an compile error if T is unbounded:

scala> class A[T] { var t: T = null }
<console>:5: error: type mismatch;
 found   : Null(null)
 required: T
       class A[T] { var t: T = null }

You can add an implicit parameter as evidence that T is nullable -- a subtype of AnyRef not a subtype of NotNull This isn't fully baked, even in Scala 2.8, so just consider it a curiousity for now.

scala> class A[T](implicit ev: Null <:< T) { var t: T = null }           
defined class A

Answer 3

As David and retronym have already mentioned, it's a good idea to use Option in most cases, as Option makes it more obvious that you have to handle a no-result situation. However, returning Some(x) requires an object creation, and calling .get or .getOrElse can be more expensive than an if-statement. Thus, in high-performance code, using Option is not always the best strategy (especially in collection-lookup code, where you may look up a value very many times and do not want correspondingly many object creations). Then again, if you're doing something like returning the text of an entire web page (which might not exist), there's no reason not to use Option.

Also, just to add to retronym's point on generics with null, you can do this in a fully-baked way if you really mean it should be null:

class A[T >: Null <: AnyRef] { var t: T = null }

and this works in 2.7 and 2.8. It's a little less general than the <:< method, because it doesn't obey NotNull AFAIK, but it otherwise does exactly what you'd hope it would do.