Pre-initialization and lazy vals.scala

trait RationalNumberTrait {
  val numerator: Int
  val denominator: Int
 
  protected def gcd(a: Int, b: Int): Int =
    if (a == 0) b
    else if (a == 1) 1
    else gcd(b % a, a)
 
  println("Superclass constructor: " + this.numerator + "/" + this.denominator)
}
trait LazyRationalNumber extends RationalNumberTrait {
  lazy val n = numerator / g
  lazy val d = denominator / g
  lazy val g = {
    require(denominator != 0);
    println("Fraction is reduced.");
    gcd(numerator, denominator);
  }
  override def toString =
    if (n == 0) "0" else if (d == 1) n.toString else n + "/" + d
 
  println("Subclass constructor")
}
 
trait HastyRationalNumber extends RationalNumberTrait {
  val n = numerator / g // !! Will cause divided-by-zero exception !!
  val d = denominator / g
  // Use "lazy val g" to avoid above error
  val g = {
    require(denominator != 0);
    println("Fraction is reduced.");
    gcd(numerator, denominator);
  }
  override def toString =
    if (n == 0) "0" else if (d == 1) n.toString else n + "/" + d
 
  println("Subclass constructor")
}
 
class RationalNumber(numerator0: => Int, denominator0: => Int) extends RationalNumberTrait {
  val numerator = numerator0 // You have to pass parameters to abstract fields in RationalNumberTrait
  val denominator = denominator0 // Otherwise this class would become a abstract class
  val g = {
    require(denominator != 0);
    println("Fraction is reduced.");
    gcd(numerator, denominator);
  }
  val n = numerator / g
  val d = denominator / g
 
  override def toString =
    if (n == 0) "0" else if (d == 1) n.toString else n + "/" + d
 
  println("Subclass constructor")
}
 
class RationalNumberV2(numerator0: => Int, denominator0: => Int) extends {
  val numerator = numerator0
  val denominator = denominator0
  val msg = { println("Pre-Initialization: " + numerator + "/" + denominator) }
} with RationalNumberTrait {
  val g = {
    require(denominator != 0);
    println("Fraction is reduced.");
    gcd(numerator, denominator);
  }
  val n = numerator / g
  val d = denominator / g
  override def toString =
    if (n == 0) "0" else if (d == 1) n.toString else n + "/" + d
  println("Subclass constructor")
}
 
object Main {
  def main(args: Array[String]) {
    println("======= Creating object from trait -- Lazy vals")
    /**
     * Initialization order: Superclass -> Class -> Initialization
     */
    val a = new LazyRationalNumber {
      val numerator = 2
      val denominator = 10
      println("Initialization: " + this.numerator + "/" + this.denominator)
    }
    println("Evaluation: " + a)
 
    println("\n======= Creating object from trait -- Pre-initialization")
    /**
     * Initialization order: Pre-Initialization -> Superclass -> Class
     */
    val c = new {
      val numerator = 10
      val denominator = 2
      val msg = { println("Pre-Initialization: " + numerator + "/" + denominator) }
    } with LazyRationalNumber
    println("Evaluation: " + c)
 
    println("\n======= Creating object from trait -- Naive initialization")
    try {
      /**
       * Initialization order: Superclass -> Class -> Initialization
       */
      val c = new HastyRationalNumber {
        val numerator = 10
        val denominator = 2
        println("Initialization: " + this.numerator + "/" + this.denominator)
      }
    } catch {
      case ex: Exception => println("Error: " + ex)
    }
    println("\n======= Creating object from class -- normal")
    /**
     * Initialization order: Superclass -> Parameter -> Class
     */
    val b = new RationalNumber({ println("Numerator=21"); 21 }, { println("Denominator=49"); 49 })
    println("Evaluation: " + b)
 
    println("\n======= Creating object from class -- with pre-initialization")
    /**
     * Initialization order: Parameter -> Pre-Initialization -> Superclass -> Class
     */
    val d = new RationalNumberV2({ println("Numerator=21"); 21 }, { println("Denominator=49"); 49 })
    println("Evaluation: " + d)
  }
}

Results:

======= Creating object from trait -- Lazy vals
Superclass constructor: 0/0
Subclass constructor
Initialization: 2/10
Fraction is reduced.
Evaluation: 1/5

======= Creating object from trait -- Pre-initialization
Pre-Initialization: 10/2
Superclass constructor: 10/2
Subclass constructor
Fraction is reduced.
Evaluation: 5

======= Creating object from trait -- Without lazy vals
Superclass constructor: 0/0
Error: java.lang.ArithmeticException: / by zero

======= Creating object from class -- normal
Superclass constructor: 0/0
Numerator=21
Denominator=49
Fraction is reduced.
Subclass constructor
Evaluation: 3/7

======= Creating object from class -- Pre-initialization
Numerator=21
Denominator=49
Pre-Initialization: 21/49
Superclass constructor: 21/49
Fraction is reduced.
Subclass constructor
Evaluation: 3/7