Trying to implement a Java interface

[Sébastien Lorion]

Hello,

I am trying to implement the interface https://github.com/aaberg/sql2o/blob/master/core/src/main/java/org/sql2o/converters/Converter.java for Option[T] but I cannot figure out how to do it as I keep getting compiler type errors. Below is my best attempt (I am quite new at Scala). When I compile it, it gives me the error:

"Error:(29, 37) type mismatch;

 found   : x.type (with underlying type T)

 required: ?0

 valueConverter.toDatabaseParam(x)"

package foo.bar

import org.sql2o.converters.{Convert, Converter, ConverterException}

class OptionConverter[T] extends Converter[Option[T]] {

  override def convert(value: Any): Option[T] = {

    value match {

      case null | None => None

      case _ =>

        val valueConverter = Convert.getConverterIfExists(value.getClass)

        if (valueConverter == null)

          throw new ConverterException(s"Cannot find a converter for type ${value.getClass.toString}.")

        else

          Option(valueConverter.convert(value).asInstanceOf[T])

    }

  }

  override def toDatabaseParam(value: Option[T]): AnyRef =

  {

    value match {

      case None => null

      case Some(x) =>

        val valueConverter = Convert.getConverterIfExists(x.getClass)

        if (valueConverter == null)

          throw new ConverterException(s"Cannot find a converter for type ${x.getClass.toString}.")

        else

          valueConverter.toDatabaseParam(x) // compiler error here, for x

    }

  }

}

that class is then meant to be used like this:

private[this] lazy val customQuirks = new PostgresQuirks() {

  {

    converters.put(classOf[Option[java.lang.Integer]], new OptionConverter[java.lang.Integer])

    converters.put(classOf[Option[scala.Int]], new OptionConverter[scala.Int])

    // ...

  }

}

[Sonnenschein]

Hi Sébastien:

First, here is the boiled down code demonstraiting your question (please concider always supplying self-contained code otherwise you'll probably get no response):

trait Converter[T] {
  def toDatabaseParam(value: T): Object

}
class OptionConverter[T] extends Converter[Option[T]] {

  override def toDatabaseParam(value: Option[T]): AnyRef =

    value map { x =>
      // Convert.java: public static Converter getConverter(Class clazz)
      def valueConverterRaw: Converter[_] = ???
      valueConverterRaw.toDatabaseParam(x) // error
     
      def valueConverter: Converter[T] = ???
      valueConverter.toDatabaseParam(x)
   }
}

The issue is that toDatabaseParam returns a raw type that is always a sign of suboptimal design. To get rid of it you could cast by asInstanceOf respectively add something like

valueConverterRaw match {
  case c: Converter[T] => c.toDatabaseParam(x)
  case _ => ???
}

Peter

[Sébastien Lorion]

Thank you Peter, that did the trick. I think it can even be simplified to this:

override def toDatabaseParam(value: Option[T]): AnyRef = {

  value match {

    case None => null

    case Some(x) =>

      val valueConverter: Converter[T] = Convert.getConverterIfExists(x.getClass)

      if (valueConverter == null)

        throw new ConverterException(s"Cannot find a converter for type ${x.getClass.toString}.")

      else

        valueConverter.toDatabaseParam(x)

  }

}

That said, I do not understand why the compiler cannot infer that valueConverter is a Converter[T] without me telling it explicitly.

Sébastien