CODEWITHSUNDEEP
scalaoop
Vijayan P
Vijayan P

Reputation: 3

Understanding companion object in scala

While learning Scala, I came across interesting concept of companion object. Companion object can used to define static methods in Scala. Need few clarifications in the below Spark Scala code in regard of companion object.

 class BballStatCounter extends Serializable {
  val stats: StatCounter = new StatCounter()
  var missing: Long = 0

  def add(x: Double): BballStatCounter = {
    if (x.isNaN) {
      missing += 1
    } else {
      stats.merge(x)
    }
    this
  }
}
object BballStatCounter extends Serializable {
  def apply(x: Double) = new BballStatCounter().add(x)
}

Above code is invoked using val stat3 = stats1.map(b=>BballStatCounter(b)).

  1. What is nature of variables stats and missing declared in the class? Is it similar to class attributes of Python?
  2. What is the significance of apply method in here?

Upvotes: 0

Views: 1223

Answers (2)

Tim
Tim

Reputation: 27421

  1. stats and missing are members of the class BcStatCounter. stats is a val so it cannot be changed once it has been defined. missing is a var so it is more like a traditional variable and can be updated, as it is in the add method. Every instance of BcStatCounter will have these members. (Unlike Python, you can't add or remove members from a Scala object)

  2. The apply method is a shortcut that makes objects look like functions. If you have an object x with an apply method, you write x(...) and the compiler will automatically convert this to x.apply(...). In this case it means that you can call BballStatCounter(1.0) and this will call the apply method on the BballStatCounter object.

Neither of these questions is really about companion objects, this is just the normal Scala class framework.

Please note the remarks in the comments about asking multiple questions.

Upvotes: 0

raober
raober

Reputation: 56

Here stats and missing are class attributes and each instance of BballStatCounter will have their own copy of them just like in Python.

In Scala the method apply serves a special purpose, if any object has a method apply and if that object is used as function calling notation like Obj() then the compiler replaces that with its apply method calling, like Obj.apply() .

The apply method is generally used as a constructor in a Class Companion object. All the collection Classes in Scala has a Companion Object with apply method, thus you are able to create a list like : List(1,2,3,4)

Thus in your above code BballStatCounter(b) will get compiled to BballStatCounter.apply(b)

Upvotes: 2

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