Using a scala generic class in java

Question

I have this in scala:

object Tester {
  def apply[T : Manifest](description: String, initValue: T) = new Tester[T](description, initValue)
  implicit def TesterToValue[T](j: Tester[T]): T = j.value
}

class Tester[T : Manifest](someData: String, initValue: T) {
  private var value = initValue
  def getValue : T = value
}

which allows me to do

val t1 = JmxValue("some data", 4)

I'm trying to create an instance of this is java, so far I've had no luck I've tried:

Tester t1 = Tester<Integer>("data", 0);
Tester<Integer> t1 = Tester<Integer>("data", 0);
Tester t1 = Tester("data", 0);
Tester t1 = new Tester<Integer>("data", 0);
Tester<Integer> t1 = new Tester<Integer>("data", 0);

Is there some limitation in using scala generic classes in java? Or am I just doing something horribly wrong

For starters, you can strike all of the attempts that don't use new. Tester t1 = Tester<Integer>("data", 0); isn't syntactically valid Java under any circumstance (that doesn't even have anything to do with Scala). — Chris Martin
– Chris Martin, Commented Feb 13, 2014 at 23:18

Dylan · Accepted Answer · 2014-02-13 23:25:03Z

6

Your Tester class actually has an implicit parameter (because of the [T : Manifest] type boundary. The syntax you use is sugar for

// Scala Class
class Tester[T](someData: String, initValue: T)(implicit man: Manifest[T]){...}

When that gets compiled, the two argument lists are condensed to one, so you end up with the java equivalent of

//Java Constructor
public Tester(String someData, T initValue, Manifest<T> man){...}

You can see the type signature by running the javap command on the Tester.class file that gets generated by the scala compiler.

So if you are trying to use the Tester class from Java, you have to explicitly figure out the parameter that scala's compiler would normally figure out for you.

Looking at the scaladocs, it looks like ManifestFactory is where you need to go to create Manifest instances. So your java code would look something like

Manifest<Integer> man = ManifestFactory$.MODULE$.classType(Integer.class);
Tester<Integer> tester = new Tester<Integer>("data", 123, man);

edited Feb 13, 2014 at 23:25

answered Feb 13, 2014 at 23:18

Dylan

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3 Comments

Dylan Over a year ago

I edited my answer a little while ago. The example I added should answer that question.

James Lee Over a year ago

I will trust that this will work, but upon further investigation this requires latest version of scala?

Dylan Over a year ago

I think in older versions of Scala there was a Manifest companion object that had factory methods for Manifests. Poke around in the docs to find what you need.

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