Casting null to Int and Double in Scala

Question

Possible Duplicate:
If an Int can't be null, what does null.asInstanceOf[Int] mean?

I tried the following in REPL:

scala> null.asInstanceOf[Int]
res12: Int = 0

scala> null.asInstanceOf[Float]
res13: Float = 0.0

scala> null.asInstanceOf[Double]
res14: Double = 0.0

It would expect a runtime exception (NPE or ClassCastException) in that case.
Could anybody explain why Scala casts null to zero?

score 6 · Accepted Answer · answered Jul 05 '12 at 06:49

6

It's really strange, as it is not the behaviour expected according to the specification:

asInstanceOf[T] returns the null object itself if T conforms to scala.AnyRef, and throws a NullPointerException otherwise.

-- The Scala Language Specification, Version 2.9, p. 75.

And it's a known bug that is closed but related to this one, which is open.

answered Jul 05 '12 at 06:49

Nicolas

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Ok, complete example `val x = null.asInstanceOf[String]` – Peter Krauss Sep 27 '19 at 20:46

score 3 · Answer 2 · edited May 23 '17 at 11:52

3

The reason is that null is a reference type - casting always converts to another reference type - in this case the boxed version of Int or Double.

In the next step, the compiler converts the boxed object to a primitive value. If the boxed Int object is null, its corresponding default primitive value is 0.

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

edited May 23 '17 at 11:52

Community

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answered Jul 05 '12 at 11:11

axel22

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Thanks. Does it comply with the language specification ? – Michael Jul 05 '12 at 12:08
not sure about the part with converting between default values. – axel22 Jul 05 '12 at 12:36
Yes, it looks like this part contradicts the specification (see answer of Nicolas) – Michael Jul 05 '12 at 12:39

score 1 · Answer 3 · answered Jul 05 '12 at 06:51

1

Those types all extend AnyVal, for which a value cannot be null by assignment, the reason why it turns them into zero in response to asInstanceOf however escapes me. It appears to only be doing this in the REPL however which is a slightly special case. In real code it returns null.

answered Jul 05 '12 at 06:51

Sean Parsons

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I did not try it in "real" code by myself but this is the behaviour my colleague noticed in "real" code. – Michael Jul 05 '12 at 06:54

Casting null to Int and Double in Scala

3 Answers3