Handling null and exception from Java API call in Scala

Question

I have the following code (simplified):

def getSomething(name: String): MyError \/ Option[Int] = {
    /* Returns Null if name doesn't exist, IOException if connection err. */
    val output: queryDB(name)

    // ...
}

The queryDB function is a Java call.

I would like the following to happen: IOException mapped to MyError, and null to None. How should I proceed?

I am aware that I can wrap the output in an Option since null is mapped to None. However, how do I manage the exception?

Thanks.

anything wrong with try / catch? you could use scala.util.Try too, ie Try( Option( queryDB(name) ) ) but old-school try / catch might just be simpler here. — Steve Waldman
– Steve Waldman, Commented Aug 19, 2016 at 12:15

Gabriele Petronella · Accepted Answer · 2016-08-19 13:18:49Z

1

I would use a combination of Option and Try

import scala.util.{ Try, Success, Failure }

def getSomething(name: String): MyError \/ Option[Int] = {

   Try(queryDB(name)) match {
     case Success(v) => Option(v).right // in case v is null, this returns a None
     case Failure(e) => MyError().left
   }

}

This turns all exceptions into MyError. If you need specific error handling you can match on e and have more granularity.

Also I'm using .right and .left that come from the syntax package of scalaz. If you are not using those, you can replace them with \/-(Option(v)) and -\/(MyError()) respectively.

answered Aug 19, 2016 at 13:18

Gabriele Petronella

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2 Comments

Kevin Meredith Over a year ago

"If you need specific error handling you can match on e and have more granularity." Does this mean to look at Throwable's getMessage?

Gabriele Petronella Over a year ago

No, I mean matching on the exception type

Jegan · Accepted Answer · 2016-08-19 13:18:30Z

1

I think you need a combination of Either and Try. Using "transform" method of Try, you can convert the result to an Either[Throwable, Option[Int]]. Something along the lines of,

// Change the 2nd arg of transform to MyError as per your requirement.

def getSomething(name: String): Either[Throwable, Option[Int]] = {
    Try(queryDB(name)).transform(s => Success(Right(Option(s))), f => Success(Left(f))).get
}

answered Aug 19, 2016 at 13:18

Jegan

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