A class can have a companion object. It must be declared in the same file as the class, using the keyword object followed by the name of the class it is accompanying. A companion object is a singleton – there is only one instance of this object in the JVM. It has its own type and is not an instance of the accompanied class.

This object defines static functions or values that are closely related to the class it is accompanying. If you are familiar with Java, it replaces the keyword static: in Scala, all static members of a class are declared inside the companion object.

Some functions in the companion object have a special meaning. Functions named apply are constructors of the class. The name apply can be omitted when we call them:

case class City(name: String, urbanArea: Int)
object City {
  val London = City("London", 1738)
  val Lausanne = City("Lausanne", 41)
}
 
case class Person(firstName: String, lastName: String, city: City)
object Person {
  def apply(fullName: String, city: City): Person = {
    val splitted = fullName.split(" ")
    new Person(firstName = splitted(0), lastName = splitted(1), city = city)
  }
}

// Uses the default apply method
val m1 = Person("Mikael", "Valot", City.London)
// Call apply with fullName
val m2 = Person("Mikael Valot", City.London)
// We can omit 'apply'
val n = Person.apply("Nicolas Jorand", City.Lausanne)

In the preceding code, we defined a companion object for the class City, which defines some constants. The convention for constants is to have the first letter in uppercase.

The companion object for the class Person defines an additional apply function that acts as a constructor. Its implementation calls the method split(" "), which splits a string separated by spaces to produce an array of type string. It allows us to construct a Person instance using a single string where the first name and last name are separated by a space. We then demonstrated that we can either call the default apply function that comes with the case class, or the one we implemented.