scala convert String to generic type

Question

I am am parsing a json. I would like to convert it's values to other types . i.e

//json = JSON String 
val seq = net.liftweb.json.parse(json).\\("seq").values.toString.toLong
val userName = net.liftweb.json.parse(json).\\("name").values.toString
val intNum = net.liftweb.json.parse(json).\\("intId").values.toInt

I would like to cast it using generic method more "scala" way, I tried something like this:

object Converter{
  def JSONCaster[T](json:String,s:String):T={
    net.liftweb.json.parse(json).\\(s).values.toString.asInstanceOf[T]
  }
}

but got casting error :

java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long at scala.runtime.BoxesRunTime.unboxToLong(Unknown Source)

There is a confusing array of competing solutions (competing libraries offering solutions) to this... — Seth Tisue
– Seth Tisue, Commented Jul 18, 2013 at 15:31

alav · Accepted Answer · 2018-04-09 18:44:02Z

I had the same problem and remembered that Play framework had some functionality that did something similar. After digging through the source I found this Class.

Basically, we can do something like this:

object JSONCaster {

  def fromJson[T](json: String, s: String)(implicit converter: Converter[T]): T = {
    converter.convert(net.liftweb.json.parse(json).\\(s).values.toString)
  }

  trait Converter[T] { self =>
    def convert(v: String): T
  }

  object Converter{
    implicit val longLoader: Converter[Long] = new Converter[Long] {
      def convert(v: String): Long = v.toLong
    }

    implicit val stringLoader: Converter[String] = new Converter[String] {
      def convert(v: String): String = v
    }

    implicit val intLoader: Converter[Int] = new Converter[Int] {
      def convert(v: String): Long = v.toInt
    }

    // Add any other types you want to convert to, even custom types!
  }
}

Can be called like:

JSONCaster.fromJson[Long](json, "seq")

The downside is we have to define implicit converters for all the types we want to cast to. The upside being this keeps the interface really clean and reusable.

cfeduke · Accepted Answer · 2013-07-18 12:37:06Z

1

Take a look at marshalling/unmarshalling as implemented in Spray. You may not need to reinvent the solution, and if you do, you can take a look at their source to see how they've implemented it.

Spray's marshalling/unmarshalling is similar to object graph serialization and works with more than just JSON so there's some additional inherent complexity within the implementation.

You could also get around manually parsing JSON and try lift-json.

lift-json is closer to JSON though through extract it can operate similar to Spray's marshaller/unmarshaller.

answered Jul 18, 2013 at 12:37

cfeduke

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1 Comment

igx Over a year ago

The json is just an example. Wanted to do a nice casting to string to generic type in scalaish [email protected]

igx · Accepted Answer · 2013-07-21 06:55:44Z

0

The best and nice solution I found is at Derek Wyatt's Blog -heres one of the reasons why monads are awesome

answered Jul 21, 2013 at 6:55

igx

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Comments

Radu Linu · Accepted Answer · 2020-12-09 16:11:53Z

0

Here I have a generic method in scala, working with liftweb / lift-json. As an idea, you need to provide the implicit Manifest.

import net.liftweb

private def jsonToObjectsSeq[T](jsonAsString: String)(implicit man: Manifest[T]): Seq[T] = {
  parse(jsonAsString)
    .children
    .map(_.extract[T])
}

answered Dec 9, 2020 at 16:11

Radu Linu

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