If an Int can’t be null, what does null.asInstanceOf[Int] mean?

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The behaviour of casting null to an Int depends on the context in which it is done.

First of all, if you cast a null to an Int, it actually means a boxed integer, whose value is null. If you put the expression in a context where the expected type is Any (which is translated to Object behind the scene, because in the JVM bytecode, there is no way to refer to a primitive type and a reference type with the same reference), then this value is not converted further – that is why println(null.asInstanceOf[Int]) prints null.

However, if you use this same boxed integer value in a context where a primitive Int (Java int) is expected, it will be converted to a primitive, and null is (as a default value for reference types) converted to 0 (a default value for primitive types).

If a generic method does this cast, then, naturally, you get a null back.

However, if this method is specialized, then its return type is Int (which is a primitive integer in this case), so the null: Any value has to be converted to a primitive, as before.

Hence, running:

object Test extends App {
  println(null.asInstanceOf[Int])

  def printit(x: Int) = println(x)

  printit(null.asInstanceOf[Int])

  def nullint[T] = null.asInstanceOf[T]

  println(nullint[Int])

  def nullspecint[@specialized(Int) T] = null.asInstanceOf[T]

  println(nullspecint[Int])
}

produces:

null
0
null
0

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