How to initialize function type parameter in Scala class?

Question

For example I have a code like this:

class Foo(name: String = "", callback: String => String) {

  def doCallback(arg: String): String = {
    callback(arg)
  }
}

Questions:

1. In Foo class constructor, how to give callback parameter a default value? Is it okay to set it to null ? Or is there better alternative?

class Foo(name: String = "", callback: String => String = null) {
  ...
}

2. If answer of question no. 1 is not a null , how to do a check of callback before calling it inside the doCallback method? For example, suppose null is acceptable, I might do it like this:

class Foo(name: String = "", callback: String => String = null) {

  def doCallback(arg: String): String = {
    if (callback != null) callback(arg) else ""  // the checking of not null
  }
}

Answer 1

class Foo(name: String = "", callback: Option[String => String] = None) {

def doCallback(arg: String): String = {
  val call: Option[String] = callback.map(c => c(arg)) // which will either be None or Some(String)

  call match {
      case Some(s: String) => s
      case None => null
    }
  }
}

Of course if you change the signature of doCallback to Option[String] you won't have to do pattern matching.

In general avoiding having null s in Scala is a good practice. One benefit of which is not getting NullPointerException s.

This approach however does force you to pass your callback on class instantiation within Some() .

Or, more in line with your original request:

class Foo(name: String = "", callback: String => Option[String] = {(s) => None}) {

  def doCallback(arg: String): String = {
    val call: Option[String] = callback(arg)

    call match {
      case Some(s: String) => s
      case None => null
    }
  }
}

Answer 2

As others have said, it's good to wrap everything with Option .
But if you just want the default result to be "" , things can be a bit easier:

val alwaysEmpty = { given: String => "" }

class Foo(val doCallback: String => String = alwaysEmpty)