Why are primitive types such as Int erased to Object in Scala?

Question

In Scala,

{ x: Option[Int] => x }
   .getClass
   .getMethod("apply", classOf[Option[_]])
   .getGenericParameterTypes

returns Array(scala.Option<java.lang.Object>). I'd initially been expecting to see instead Array(scala.Option<scala.Int>), but I see that scala.Int is a value class (extends AnyVal) 'whose instances are not represented as objects by the underlying host system'.

I still don't understand the erasure to Object, though. Couldn't it be the much more useful java.lang.Integer?

That isn't Scala doing that -- it's Java. To get useful, as in non-Object, results from "GenericParameterTypes" (a method of a Java Class object), a Subclass has to be created. Scala *didn't* create a Subclass, say `OptionInt`, for `Option[Int]`. (See @specialized, perhaps?) — , Jun 23 '12 at 06:34
@pst; I think you're on the wrong track. Change `Int` to `Symbol` above and you get `Array(scala.Option)`. — Scott Morrison, Jun 23 '12 at 07:03
Hmm, well there goes my hypothesis :-/ I blame compiler magic. — , Jun 23 '12 at 07:36

score 12 · Accepted Answer · edited Dec 10 '20 at 15:44

12

Couldn't it be the much more useful java.lang.Integer?

Yes, and that was even the case, once. Unfortunately, that leads to broken type signatures. That is, it is impossible to generate correct bytecode in all situations if Int is erased to java.lang.Integer.

There isn't a single ticket or commit about this, but the one that changed this particular behavior is scala/bug#4214, in this commit.

edited Dec 10 '20 at 15:44

Seth Tisue

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answered Jun 23 '12 at 16:43

Daniel C. Sobral

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Thanks. I had a look at `git log --grep=Integer` but didn't see anything obvious. – Scott Morrison Jun 23 '12 at 18:22
@ScottMorrison You should have searched for "signature". Anyway, I found it for you. See revised answer. – Daniel C. Sobral Jun 23 '12 at 19:14

Why are primitive types such as Int erased to Object in Scala?

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