Understanding Scala AnyVal and Null

Question

Why does the following compile? I understand that AnyVal instances correspond to things in the underlying host system that cannot be constructed, and that Null is a subtype of all reference types but not of value types. I have an AnyVal type Boolean I give to safeMapDifferent, but don't see how it can satisfy this constraint of U >: Null.

object MyMainScala extends App { implicit class RichObject[T](o: T) { def safeMap[U](method: T => U)(implicit ev: Null <:< U): U = Option(o).flatMap(result => Option(method(result))).orNull def safeMapDifferent[U >: Null](method: T => U): U = Option(o).flatMap(result => Option(method(result))).orNull } override def main(args: Array[String]): Unit = { val testSubject = new Object() { def scalaBoolean: Boolean = ??? } // println(testSubject.safeMap(_.scalaBoolean)) // If not commented, this will fail to compile as I expect. println(testSubject.safeMapDifferent(_.scalaBoolean).getClass) // Why does it compile? } }

Ra Ka · Accepted Answer · 2018-03-13 06:43:35Z

Its because of Autoboxing. If you see in predef.scala, you will see bunch of implicit conversion methods that convert scala AnyVal to Java.

 /** @group conversions-anyval-to-java */ implicit def boolean2Boolean(x: Boolean): java.lang.Boolean = x.asInstanceOf[java.lang.Boolean]

When you invoke your print method, println(testSubject.safeMapDifferent(_.scalaBoolean).getClass), you are providing T => U value i.e. _.scalaBoolean which takes testSubject as parameter which satisfy T type parameter and returns Boolean which does not satisfy U >: Null. Upon getting this error, instead of throwing exception, the compiler looks for implicit methods which can convert Boolean into expected U >: Null type and it found boolean2Boolean in predef.scala which satisfy this constraint because java.land.Boolean is a reference type. Hence, compile execute the code correctly.

def foo[U >: Null](o : U) = println(o) foo(true) // compile correctly. foo[java.lang.Boolean](true) //compile correctly, because true is implicitly converted to java Boolean and java.lang.Boolean is reference type and superType of scala.Null.

To avoid this, you must statically provide type parameter: foo[Boolean](true) //won't compile.

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