Calling Java API from Scala with null argument

Question

I have some Scala code that needs to call a Java API

The Java API takes arguments that may be null. My Scala, of course, uses Option.

For example, let's say I have a Java object constructor Foo(Integer) where the Integer may be null. I want to call it given a Scala bar: Option[Int].

I tried this

import scala.collection.JavaConversions._
import scala.collection.JavaConverters._

val foo = Foo( bar.getOrElse(null) )

But got this compile error

Error:(335, 44) type mismatch;
  found   : Any
  required: Integer
  bar.getOrElse(null),

What is the correct idiom for doing this?

Chris Martin · Accepted Answer · 2014-12-23 00:12:27Z

5

You don't need a Java method to reproduce this problem:

scala> import scala.collection.JavaConversions._
import scala.collection.JavaConversions._

scala> import scala.collection.JavaConverters._
import scala.collection.JavaConverters._

scala> class Foo(a: java.lang.Integer)
defined class Foo

scala> val bar: Option[Int] = Some(5)
bar: Option[Int] = Some(5)

scala> new Foo(bar.getOrElse(null))
<console>:16: error: type mismatch;
 found   : Any
 required: Integer
              new Foo(bar.getOrElse(null))
                                   ^

The problem is that Int can't be null, so the type of bar.getOrElse(null) is Any.

scala> bar.getOrElse(null)
res0: Any = 5

scala> bar.orNull
<console>:15: error: Cannot prove that Null <:< Int.
              bar.orNull
                  ^

So you've got to convert the Option's type parameter to something that can be null before you unwrap it in a nullable way.

Quickest way I can think of immediately:

scala> new Foo(bar.map(x => x: java.lang.Integer).orNull)
res18: Foo = Foo@cdc45e

Edit: Here, I thought of a more general way to do it!

implicit class RichOption[A](o: Option[A]) {
    def toRef[B >: Null](implicit f: A => B): B = o.map(f).orNull
}

Now you can write new Foo(bar.toRef) :)

edited Dec 23, 2014 at 0:12

answered Dec 22, 2014 at 22:18

Chris Martin

30.9k12 gold badges83 silver badges142 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

opus111 Over a year ago

Thank you Chris. I understand now. I wonder if some implicit could/should be added JavaConverters to make calling Java easier, and avoid the temptation to use NULL in your Scala code ;-)

som-snytt Over a year ago

Thumbs up, but I would call it toRef. x.toRef[Integer]. The SO nanny doesn't allow increment operator in comments. It's weird being back in Kindergarten, the chairs are so small.

opus111 · Accepted Answer · 2014-12-22 22:23:04Z

3

No sooner do I post the question, than I spot the answer in the related list (sorry)

Here's a solution

val foo = Foo(bar.getOrElse(null).asInstanceOf[java.lang.Integer])

Kind of clunky. Anyone have anything better?

answered Dec 22, 2014 at 22:23

opus111

2,8945 gold badges28 silver badges45 bronze badges

1 Comment

Chris Martin Over a year ago

The complaint I have this this solution is that asInstanceOf masks type errors. If bar is an Option[String], this compiles but throws ClassCastException.

som-snytt · Accepted Answer · 2014-12-22 23:52:20Z

3

More chatter:

scala> import runtime.BoxesRunTime.boxToInteger
import runtime.BoxesRunTime.boxToInteger

scala> val x = Some(42)
x: Some[Int] = Some(42)

scala> val y: Option[Int] = None
y: Option[Int] = None

scala> x.fold(null: Integer)(boxToInteger)
res0: Integer = 42

scala> y.fold(null: Integer)(boxToInteger)
res1: Integer = null

Of course it's better to

scala> x.fold(null: Integer)(i => i: Integer)
res2: Integer = 42

and even better

scala> x.fold[Integer](null)(identity)
res3: Integer = 42

scala> y.fold[Integer](null)(identity)
res4: Integer = null

answered Dec 22, 2014 at 23:52

som-snytt

39.6k2 gold badges49 silver badges133 bronze badges

Collectives™ on Stack Overflow

Calling Java API from Scala with null argument

3 Answers 3

2 Comments

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related