Java classes reflection in scala

Question

I'm trying to use java's reflection API on scala. I have a KDTree class loaded from bytecode using a ClassLoader. Here's it's methods:

public class KDTree
{
public KDTree(int k)
public void insert(double[] key, Object value) throws Exception
public Object[] range(double[] lowk, double[] uppk) throws Exception
}

And here's my wrapper scala class:

class KDTree( dimentions: Int )
    //wrapper!
    {
    private val kd= Loader.loadClass("KDTree")
    private val constructor= kd.getConstructor(java.lang.Class.forName("java.lang.Integer"))
    val wrapped= constructor.newInstance("1")
    def insert( key:Array[Double], element:Object)=
        kd.getDeclaredMethod("insert", classOf[Array[Double]])
            .invoke(key, element)
    def range( lowkey:Array[Double], highkey:Array[Double])=
        kd.getDeclaredMethod("range", classOf[Array[Double]])
            .invoke(lowkey, highkey)
    }

When I try to initialize I get an error:

java.lang.NoSuchMethodException: KDTree.<init>(java.lang.Integer)

However, the constructor's only argument is indeed a integer!

Also, I can't simply do java.lang.Integer.class, since scala complains of the syntax: error: identifier expected but 'class' found.

Does anyone have any tips?

EDIT Here is my finished code, in case someone has a use for it:

class KDTreeWrapper[T]( dimentions: Int )
{
private val kd= Loader.loadClass("KDTree")
private val constructor= kd.getConstructor(classOf[Int])
private val wrapped= constructor.newInstance(dimentions:java.lang.Integer)
    .asInstanceOf[Object]
private val insert_method= kd.
    getMethod("insert", classOf[Array[Double]], classOf[Object])
private val range_method= 
    kd.getMethod("range", classOf[Array[Double]], classOf[Array[Double]])
def insert( key:Iterable[Double], element:T)=
    insert_method.invoke(wrapped, key.toArray, element.
        asInstanceOf[Object])
def range( lowkey:Iterable[Double], highkey:Iterable[Double]):Array[T]=
    range_method.invoke(wrapped, lowkey.toArray, highkey.toArray).
        asInstanceOf[Array[T]]
}

@Arjan because I need to have a single scala file with all my code, and KDTree is part of a Java library — scala_newbie
– scala_newbie, Commented Nov 26, 2012 at 13:57

Uwe Plonus · Accepted Answer · 2012-11-26 13:21:14Z

3

Your problem is that you try to load a constructor with a type parameter java.lang.Integer. Try it with int.class.

Also it is shorter to write kd.getConstructor(int.class).

answered Nov 26, 2012 at 13:21

Uwe Plonus

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2 Comments

scala_newbie Over a year ago

as I mentioned, in scala anything.class complains

Uwe Plonus Over a year ago

Use classOf[Integer] or classOf[int] ans described here.

WamBamBoozle · Accepted Answer · 2014-04-29 21:30:25Z

1

I think my example is a lot simpler, but that could be because I wrote it:

class Config {
  val c = "some config"
}

class Moo(c: Config) {
  val x = "yow!"
}

class Loo(c: Config)  extends Moo(c) {
  override val x = c.c + " yodel!"
}

object CallMe {
  def main(args: Array[String]) {
    val cn = new Config

    // val m: Moo = new Loo(cn)

    val c = Class.forName("Loo")
    val ars = c.getConstructor(classOf[Config])
    val m: Moo = ars.newInstance(cn).asInstanceOf[Moo]

    println(m.x)
  }
}

prints out

some config yodel!

answered Apr 29, 2014 at 21:30

WamBamBoozle

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