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scalacompanion-object

Why fields declaration with var/val not allowed in apply method of companion object

I am using scala 2.12 and IntelliJ IDE. I tried to auto-generate apply method in Scala companion object using readily available template. It generated apply method with fields having val because I have created class having fields declared as val in primary constructor.

Please refer below code snippet for mode details or check it here :

class Name(val firstName : String,val lastName : String)

object Name {
  def apply(val firstName: String,val lastName: String): Name =
    new Name(firstName,lastName)
}

I am getting compilation error : identifier expected but val found

Why Scala compiler generates error here while using var or val on fields of apply method ? What is the important concept behind this restriction ?

Solution

  • Because that's the way methods are defined in Scala, there is no need for a val or var in front of a method parameter. As Luis said, a method parameter is always immutable.

    It should just be:

    def apply(firstName: String, lastName: String) = ???

    I'm a bit surprised but IntelliJ IDEA is fooling you on this one and generating wrong code.