println(null.asInstanceOf[Int])

hepin1989 · December 7, 2025, 10:15am

It gives 0 instead of a compiling error.

Is this by design?

xuwei-k · December 7, 2025, 10:44am

github.com/scala/scala

SI-9671, SI-7397 fix null.asInstanceOf[Int] when pt erases to Object

2.12.x ← lrytz:t9671

opened 01:35PM - 18 May 16 UTC

lrytz

+155 -36

Erasure first replaces null.asInstanceOf[Int] by unbox(null). If the expected ty…pe erases to object, erasure then introduces a box operation, yielding box(unbox(null)). Note that this value is a box of zero, not null. Erasure has an optimization to replace box(unbox(x)) in case x is of primitive type. 60f1b4b extended this to the case when x is null, which is incorrect in general. The reason was to prevent creating a primitive box to be stored in the unused generic field when creating an instance of a specialized class. A special case ensures that this optimization is still performed.

github.com/scala/scala

Number files like chapters. Consolidate toc & preface.

committed 12:40AM - 29 Mar 14 UTC

adriaanm

+212 -206

Aside from the consolidation of title & preface in index.md, this commit was pro…duced as follows: ``` cd spec/ g mv 03-lexical-syntax.md 01-lexical-syntax.md g mv 04-identifiers-names-and-scopes.md 02-identifiers-names-and-scopes.md g mv 05-types.md 03-types.md g mv 06-basic-declarations-and-definitions.md 04-basic-declarations-and-definitions.md g mv 07-classes-and-objects.md 05-classes-and-objects.md g mv 08-expressions.md 06-expressions.md g mv 09-implicit-parameters-and-views.md 07-implicit-parameters-and-views.md g mv 10-pattern-matching.md 08-pattern-matching.md g mv 11-top-level-definitions.md 09-top-level-definitions.md g mv 12-xml-expressions-and-patterns.md 10-xml-expressions-and-patterns.md g mv 13-user-defined-annotations.md 11-user-defined-annotations.md g mv 14-the-scala-standard-library.md 12-the-scala-standard-library.md g mv 15-syntax-summary.md 13-syntax-summary.md g mv 16-references.md 14-references.md perl -pi -e 's/03-lexical-syntax/01-lexical-syntax/g' *.md perl -pi -e 's/04-identifiers-names-and-scopes/02-identifiers-names-and-scopes/g' *.md perl -pi -e 's/05-types/03-types/g' *.md perl -pi -e 's/06-basic-declarations-and-definitions/04-basic-declarations-and-definitions/g' *.md perl -pi -e 's/07-classes-and-objects/05-classes-and-objects/g' *.md perl -pi -e 's/08-expressions/06-expressions/g' *.md perl -pi -e 's/09-implicit-parameters-and-views/07-implicit-parameters-and-views/g' *.md perl -pi -e 's/10-pattern-matching/08-pattern-matching/g' *.md perl -pi -e 's/11-top-level-definitions/09-top-level-definitions/g' *.md perl -pi -e 's/12-xml-expressions-and-patterns/10-xml-expressions-and-patterns/g' *.md perl -pi -e 's/13-user-defined-annotations/11-user-defined-annotations/g' *.md perl -pi -e 's/14-the-scala-standard-library/12-the-scala-standard-library/g' *.md perl -pi -e 's/15-syntax-summary/13-syntax-summary/g' *.md perl -pi -e 's/16-references/14-references/g' *.md ```

devlaam · December 7, 2025, 10:51am

Why do you expect a compiling error? If you force the compiler with asInstanceOf[Int]) to interpret null as Int, the result 0 seems not so strange to me. What do I miss here?

sjrd · December 7, 2025, 10:52am

It is in the spec: Expressions

Key word: “default value”

hepin1989 · December 7, 2025, 11:33am

I was using a Source.future(Future.success(null.asInstanceOf[Int])) and then the spec failed, which supprised me, and then I change the code to `Source.future(Future.success(null.asInstanceOf[String]))`

I think Int is a sub type of the AnyVal and Null is a sub type of AnyRef, so I expect an error.

devlaam · December 7, 2025, 11:39am

Ah, i see. Thank you for taking the time to explain.