java.util.concurrent.ConcurrentMap's putIfAbsent docs say the following about the return type:
the previous value associated with the specified key, or null if there was no mapping for the key.
On Scala 2.10.4, I tried to call putIfAbsent(Int, Int), i.e. with Scala's Int type.
scala> import java.util.concurrent.ConcurrentHashMap
import java.util.concurrent.ConcurrentHashMap
scala> new ConcurrentHashMap[Int, Int]()
res0: java.util.concurrent.ConcurrentHashMap[Int,Int] = {}
scala> val x: Int = 1
x: Int = 1
Since 1 does not exist in the empty ConcurrentHashMap, I would expect null to return from a putIfAbsent(x, x) call.
scala> res0.putIfAbsent(x, x)
res1: Int = 0
But, 0 gets returned. I'm assuming that null gets converted to 0.
What exactly is going on here? It seems odd to me that it's compiling.
Int cannot be null, nor can any AnyVal. From the scaladoc:
Null is a subtype of all reference types; its only instance is the null reference. Since Null is not a subtype of value types, null is not a member of any such type. For instance, it is not possible to assign null to a variable of type scala.Int.
If we try it directly, we'll get an error:
scala> val i: AnyVal = null
<console>:9: error: type mismatch;
found : Null(null)
required: AnyVal
Note that implicit conversions are not applicable because they are ambiguous:
both method RichException in object Predef of type (self: Throwable)RichException
and method augmentString in object Predef of type (x: String)scala.collection.immutable.StringOps
are possible conversion functions from Null(null) to AnyVal
val i: AnyVal = null
The compiler instead fills it with the default value for Int, which is 0.
For example:
scala> def getNull[A](i: A): A = null.asInstanceOf[A]
getNull: [A](i: A)A
scala> getNull(1)
res6: Int = 0
Behind the scenes, we can't actually cast null to an Int, but it can be cast to a boxed type. However, once we use the boxed type in a context where it should be like the primitive it contains, it's converted to that type, which requires a default value.