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

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 necessary 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 a 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 2

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) }