apply
This is the companion object method which is used to create instance of the concrete class. Using this, object can be created without new keyword.
class Person(val name: String)
val person = Person("Nitin") // works
// Or
val person = new Person("Nitin") // worksPerson("Nitin") works because a companion object for Person is generated implicitly with apply method.
class Person(val name: String)
// companion object
object Person:
def apply(name: String): Map[String, String] =
Map("name" -> name)
val person = Person("Nitin")
// val res1: Map[String, String] = Map(name -> Nitin)
new Person("Nitin")
// val res2: Person = Person@1e592ef2Calling Person like function call, calls apply method in companion object.
Using new keyword doesn’t do that.
unapply
This is opposite of apply method. This extracts the values from structure. Pattern matching uses this method extract the values from class objects.
class Person(val name: String)
object Person:
def unapply(person: Person): Option[String] =
Some(person.name)
val person = Person("Nitin")
person match
case Person(name) => println("Name : " + name)So, pattern matching must be using Person.unapply to destruct person instance.
val name = Person.unapply(person)