Scala Constructors and Auxiliary Constructors

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Constructors

The constructor is the code that "constructs" a new object. The constructor is the combined effect of the class argument list - initialized before entering the class body - and the class body, whose statements execute from top to bottom.

The simplest form of a constructor is a single line class definition, with no class arguments and no executable lines of code, such as:

class Person

In Fields, the constructor initializes the fields to the values specified, or to defaults if no values were specified. In Class Arguments, the constructor quietly initializes the arguments and makes them accessible to other objects; it also unravels a variable argument list.

In those cases, we didn’t write constructor code – Scala did it for us. For more customization, add your own constructor code. For example: 

package org.fool.scala.constructors

class Coffee(val shots: Int = 2,
             val decaf: Boolean = false,
             val milk: Boolean = false,
             val toGo: Boolean = false,
             val syrup: String = "") {
  var result = ""
  println(shots, decaf, milk, toGo, syrup)

  def getCup(): Unit = {
    if (toGo)
      result += "ToGoCup "
    else
      result += "HereCup "
  }

  def pourShots(): Unit = {
    for (s <- 0 until shots)
      if (decaf)
        result += "decaf shot "
      else
        result += "shot "
  }

  def addMilk(): Unit = {
    if (milk)
      result += "milk "
  }

  def addSyrup(): Unit = {
    result += syrup
  }

  getCup()
  pourShots()
  addMilk()
  addSyrup()
}

object ConstructorsTest extends App {
  val usual = new Coffee()
  println(usual.result)

  val mocha = new Coffee(decaf = true, toGo = true, syrup = "Chocolate")
  println(mocha.result)
}

Console Output

Auxiliary Constructors

Named and default arguments in the class argument list can construct objects in multiple ways. We can also use constructor overloading by creating multiple constructors. The name is overloaded here because you’re making different ways to create objects of the same class. To create an overloaded constructor you define a method (with a distinct argument list) called this (a keyword). Overloaded constructors have a special name in Scala: auxiliary constructors.

Because constructors are responsible for the important act of initialization, constructor overloading has an additional constraint: all auxiliary constructors must first call the primary constructor. This is the constructor produced by the class argument list together with the class body. To call the primary constructor within an auxiliary constructor, you don’t use the class name, but instead the this keyword:

package org.fool.scala.constructors

class GardenGnome(val height: Double,
                  val weight: Double,
                  val happy: Boolean) {
  println("Inside primary constructor...")
  var painted = true

  def magic(level: Int): String = "Poof! " + level

  def this(height: Double) {
    this(height, 100.0, true)
  }

  def this(name: String) = {
    this(15.0)
    painted = false
  }

  def show(): String = height + " " + weight + " " + happy + " " + painted
}

object AuxiliaryConstructorsTest extends App {
  val g1 = new GardenGnome(20.0, 110.0, false)
  println(g1.show())
  println(g1.magic(1))

  println(new GardenGnome("MyGarden").show())
  println(new GardenGnome("MyGarden").magic(2))
}

Console Output

Reference

Atomic Scala 2nd Edition - Construtors