Avoiding Scala memory leaks - Scala constructors

Question

I was working through the "Programming in Scala" book, and was struck by a bit of a problem in the implementation of the class Rational in Chapter 6.

This is my initial version of the Rational class (based on the book)

class Rational(numerator: Int, denominator: Int) {
  require(denominator != 0)

  private val g = gcd(numerator.abs, denominator.abs)

  val numer = numerator / g
  val denom = denominator / g

  override def toString  = numer + "/" + denom

  private def gcd(a: Int, b: Int): Int =
    if(b == 0) a else gcd(b, a % b)

  // other methods go here, neither access g
}

The problem here is that the field g remains for the lifetime of the class, even if never again accessed. This problem can be seen by running the following mock program:

object Test extends Application {

  val a = new Rational(1, 2)
  val fields = a.getClass.getDeclaredFields

  for(field <- fields) {
    println("Field name: " + field.getName)
    field.setAccessible(true)
    println(field.get(a) + "\n")
  }  

}

Its output is going to be:

Field: denom
2

Field: numer
1

Field: g
1

A solution I found at the Scala Wiki involves the following:

class Rational(numerator: Int, denominator: Int) {
  require(denominator != 0)

  val (numer, denom) = { 
    val g = gcd(numerator.abs, denominator.abs)
    (numerator / g, denominator / g)
  }

  override def toString  = numer + "/" + denom

  private def gcd(a: Int, b: Int): Int =
    if(b == 0) a else gcd(b, a % b)

  // other methods go here
}

Here, the field g is only local to its block, but, running the small test application, I found another field x$1 which keeps a copy of the tuple consisting of (numer, denom)!

Field: denom
2

Field: numer
1

Field: x$1
(1,2)

Is there any way to construct a rational in Scala with the above algorithm, without causing any memory leaks?

Thanks,

Flaviu Cipcigan

Answer 1

You could do this:

object Rational {
    def gcd(a: Int, b: Int): Int =
        if(b == 0) a else gcd(b, a % b)
}

class Rational private (n: Int, d: Int, g: Int) {
    require(d != 0)

    def this(n: Int, d: Int) = this(n, d, Rational.gcd(n.abs, d.abs))

    val numer = n / g

    val denom = d / g

    override def toString = numer + "/" + denom

}