Scala Cookbook

4.1. Creating a Primary Constructor

Problem

You want to create a primary constructor for a class, and you quickly find that the approach is different than Java.

Solution

The primary constructor of a Scala class is a combination of:

• The constructor parameters

• Methods that are called in the body of the class

• Statements and expressions that are executed in the body of the class

Fields declared in the body of a Scala class are handled in a manner similar to Java; they are assigned when the class is first instantiated.

The following class demonstrates constructor parameters, class fields, and statements in the body of a class:

class Person(var firstName: String, var lastName: String) {

  println("the constructor begins")

  // some class fields
  private val HOME = System.getProperty("user.home")
  var age = 0

  // some methods
  override def toString = s"$firstName $lastName is $age years old"
  def printHome { println(s"HOME = $HOME") }
  def printFullName { println(this) }  // uses toString

  printHome
  printFullName
  println("still in the constructor")

}

Because the methods in the body of the class are part of the constructor, when an instance of a Person class is created, you’ll see the output from the println statements at the beginning and end of the class declaration, along with the call to the printHome and printFullName methods near the bottom of the class:

scala> val p = new Person("Adam", "Meyer")
the constructor begins
HOME = /Users/Al
Adam Meyer is 0 years old
still in the constructor

Discussion

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