singleton-objects.md

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An object is a class that has exactly one instance. It is created lazily when it is referenced, like a lazy val.

As a top-level value, an object is a singleton.

As a member of an enclosing class or as a local value, it behaves exactly like a lazy val.

Defining a singleton object

An object is a value. The definition of an object looks like a class, but uses the keyword object:

object Box

Here's an example of an object with a method:

package logging

object Logger {
  def info(message: String): Unit = println(s"INFO: $message")
}

The method info can be imported from anywhere in the program. Creating utility methods like this is a common use case for singleton objects.

Let's see how to use info in another package:

import logging.Logger.info

class Project(name: String, daysToComplete: Int)

class Test {
  val project1 = new Project("TPS Reports", 1)
  val project2 = new Project("Website redesign", 5)
  info("Created projects")  // Prints "INFO: Created projects"
}

The info method is visible because of the import statement, import logging.Logger.info.

Imports require a "stable path" to the imported symbol, and an object is a stable path.

Note: If an object is not top-level but is nested in another class or object, then the object is "path-dependent" like any other member. This means that given two kinds of beverages, class Milk and class OrangeJuice, a class member object NutritionInfo "depends" on the enclosing instance, either milk or orange juice. milk.NutritionInfo is entirely distinct from oj.NutritionInfo.

Companion objects

An object with the same name as a class is called a companion object. Conversely, the class is the object's companion class. A companion class or object can access the private members of its companion. Use a companion object for methods and values which are not specific to instances of the companion class.

import scala.math._

case class Circle(radius: Double) {
  import Circle._
  def area: Double = calculateArea(radius)
}

object Circle {
  private def calculateArea(radius: Double): Double = Pi * pow(radius, 2.0)
}

val circle1 = new Circle(5.0)

circle1.area

The class Circle has a member area which is specific to each instance, and the singleton object Circle has a method calculateArea which is available to every instance.

The companion object can also contain factory methods:

class Email(val username: String, val domainName: String)

object Email {
  def fromString(emailString: String): Option[Email] = {
    emailString.split('@') match {
      case Array(a, b) => Some(new Email(a, b))
      case _ => None
    }
  }
}

val scalaCenterEmail = Email.fromString("[email protected]")
scalaCenterEmail match {
  case Some(email) => println(
    s"""Registered an email
       |Username: ${email.username}
       |Domain name: ${email.domainName}
     """)
  case None => println("Error: could not parse email")
}

The object Email contains a factory fromString which creates an Email instance from a String. We return it as an Option[Email] in case of parsing errors.

Note: If a class or object has a companion, both must be defined in the same file. To define companions in the REPL, either define them on the same line or enter :paste mode.

Notes for Java programmers

static members in Java are modeled as ordinary members of a companion object in Scala.

When using a companion object from Java code, the members will be defined in a companion class with a static modifier. This is called static forwarding. It occurs even if you haven't defined a companion class yourself.