Override a java method that expects a double array

Question

Say I define the following java interface:

public interface A
{
  public Double[] x();
}

And then try to implement it in scala as follows:

class B extends A {
  val v: Array[Double] = Array(2.3, 6.7)
  override def x() = v
}

The compiler gives me the following error:

type mismatch;
[error]  found   : Array[scala.Double]
[error]  required: Array[java.lang.Double]
[error]     override def x() = v

Can someone tell me the recommended way to automatically convert this array?

Thanks Des

Answer 1

You can't convert it automatically. The issue is that Double in Java means the class java.lang.Double (while in Scala it means scala.Double which is mostly the same as double in Java), and so the overriding method has to return Array[java.lang.Double]. If you have an Array[Double], you can convert it using map:

val v: Array[Double] = ...
val v1 = v.map(java.lang.Double.valueOf(_)) // valueOf converts Double into java.lang.Double

You could make this conversion implicit:

implicit def wrapDoubleArray(arr: Array[Double]): Array[java.lang.Double] =
  arr.map(java.lang.Double.valueOf(_))

but this is a bad idea in most circumstances.

Answer 2

If the type of v must be Array[scala.Double], then perhaps you should consider performing the conversion yourself in the overridden method:

class B extends A {
  val v: Array[Double] = Array(2.3, 6.7)

  override def x(): Array[java.lang.Double] = {
    v map { java.lang.Double.valueOf(_) }
  }
}

Answer 3

Will this help?

class B extends A {
  val v: Array[java.lang.Double] = Array(2.3D, 6.7D)
  override def x() = v
}