使用反射与 java.math.BigInteger 交互时，我会遇到哪些问题？

Question

I'm implementing a faster BigInt implementation and I'm not sure on how far I should go to provide interop with the underlying platform.

Today BigInt just wraps a BigInteger and the value bigInteger just returns the wrapped value:

class BigInt(val bigInteger: BigInteger) ...

Because I'm not wrapping a Java type, I would have to do something like

final class BigInt private(final val signum: Int,
                           final private[math] val arr: Array[Int])
  def bigInteger: java.math.BigInteger = {
    // Avoid copying of potentially large arrays.
    val ctor = classOf[java.math.BigInteger]
                 .getDeclaredConstructor(classOf[Array[Int]], classOf[Int])
    ctor setAccessible true
    ctor.newInstance(arr, signum.asInstanceOf[Object])
  }
...
}

Can this cause trouble or is there a better way to do it?

Answer 1

In general when I've seen people use private (or otherwise undocumented) constructors or methods like this, they catch NoSuchMethodException and provide an alternative:

object BigInt {
  import java.math.BigInteger

  private val toBigInteger: (Array[Int], Int) => BigInteger = try {
    val ctor = classOf[BigInteger].getDeclaredConstructor(
      classOf[Array[Int]], classOf[Int]
    )
    ctor.setAccessible(true)

    (arr, signum) => ctor.newInstance(arr, signum.asInstanceOf[Object])
  } catch { case _: NoSuchMethodException =>
    (arr, signum) =>
      val buffer = java.nio.ByteBuffer.allocate(arr.length * 4)
      buffer.asIntBuffer.put(arr)
      new BigInteger(signum, buffer.array)
  }
}

final class BigInt(final val signum: Int, final val arr: Array[Int]) {
  def bigInteger = BigInt.toBigInteger(arr, signum)
}

I've also moved the reflection business off to a companion object in order to avoid paying for most of it every time you call bigInteger .