I'm trying to implement a Scala trait by Java, the trait has a generic container type within another container, which couldn't be solved automatically by java import, the Scala code is below:

import cats.data.{EitherNel, Kleisli, NonEmptyList}
import cats.implicits._

package Export_To_Java {
    package object types {
        type Valid[A] = EitherNel[String, A]
        type ValidOperation[A, B] = Kleisli[Valid, A, B]
        type Amount = BigDecimal
    }

    trait InterestService[Account] {
        import types._

        type InterestOperation = ValidOperation[Account, Option[Amount]]
        type TaxOperation = ValidOperation[Option[Amount], Amount]

        def computeInterest: InterestOperation
        def computeTax: TaxOperation
    }
}

Once it has been implemented by Java, Intellij automatically generates methods:

import cats.data.Kleisli;
import cats.data.NonEmptyList;
import Export_To_Java.InterestService;
import scala.math.BigDecimal;
import scala.util.Either;


public class JavaInterestService<Account> implements InterestService<Account> {
    @Override
    public Kleisli<Either<NonEmptyList<String>, A>, Account, Option<BigDecimal>> computeInterest() {
        throw new NotImplementedException();
    }

    @Override
    public Kleisli<Either<NonEmptyList<String>, A>, Option<BigDecimal>, BigDecimal> computeTax() {
        throw new NotImplementedException();
    }
}

Now You will see the java method return type has been extended to Either<NonEmptyList<String>, A> which still has an generic parameter A inside, which is carried form Scala type Valid[A]. Error message is:

Error:(13, 82) java: computeInterest() in JavaInterestService cannot implement computeInterest() in Export_To_Java.InterestService
  return type cats.data.Kleisli<scala.util.Either<cats.data.NonEmptyList<java.lang.String>,A>,Account,scala.Option<scala.math.BigDecimal>> is not compatible with cats.data.Kleisli<scala.util.Either,Account,scala.Option<scala.math.BigDecimal>>

Remove type from Either will get error:

[error] JavaInterestService.java:13:82: scala.util.Either takes two type parameters, expected: one
[error]     public Kleisli<Either, Option<BigDecimal>, BigDecimal> computeInterest() {
[error]                    ^
[error] one error found
[error] (Compile / compileIncremental) Compilation failed

sbt configuration:

val scalaTestVersion = "3.0.5"
val catsVersion = "1.6.0"

lazy val ShapelessGuide = (project in file(".")).
        settings(
            name := "test",
            version := "0.1",
            scalaVersion := "2.12.8",
            libraryDependencies ++= Seq(
                "org.scalatest" %% "scalatest" % scalaTestVersion % Test,
                "org.typelevel" %% "cats-core" % catsVersion
            ),
            scalacOptions ++= Seq(
                "-language:higherKinds",
                "-deprecation",
                "-encoding", "UTF-8",
                "-Ypartial-unification",
                "-feature",
                "-language:_"
            )
        )

I'm wondering is it possible to implement a Scala trait in Java? or what's the correct way to do so?

1 Answers

1
Dmytro Mitin On Best Solutions

Try

public class JavaInterestService<Account> implements InterestService<Account> {
    @Override
    public Kleisli<Either, Account, Option<BigDecimal>> computeInterest() {
        throw new NotImplementedException();
    }

    @Override
    public Kleisli<Either, Option<BigDecimal>, BigDecimal> computeTax() {
        throw new NotImplementedException();
    }
}

Java compiler can see higher-kinded Scala types as raw types how match mixed type of scala higher-kinder types feature with java generic type?