CODEWITHSUNDEEP
scalacase-classcompanion-object
Alejandro Alcalde
Alejandro Alcalde

Reputation: 6220

Using New Keyword inside apply method in a companion object

I am a little bit confused using companion objects in scala. When you want to provide multiple constructors, usually you declare a companion object and overload the apply method. But what is the difference between this two ways of doing it?:

case class Node(....)

object Node {
   def apply(...) = new Node(....) // 1 way
   def apply(...) = Node(...) // second way
}

Almost all examples I've seen use the first form:

But my code seems to work the same using both forms. Does using new keyword only have sense when we have a normal class? (Not a case class)?

Upvotes: 2

Views: 287

Answers (2)

Yuval Itzchakov
Yuval Itzchakov

Reputation: 149538

When you call

val n = Node(..)

The compiler will expand the code to a Node.apply call. Now, one of these apply methods will internally have to call new in order to create an instance of the type. Case classes provide companion objects with an apply method for you out of the box to allow the shorter syntax.

When you want to provide multiple constructors, usually you declare a companion object and overload the apply method

This is the case for case classes. You can also provide additional auxiliary constructors using this():

class Foo(i: Int) {
  def this() {
    this(0)
  }
}

Note this will not provide the syntax sugar apply does, you'll need to use new.

Upvotes: 2

Nagarjuna Pamu
Nagarjuna Pamu

Reputation: 14825

When you declare a case class. A companion object is generated by the compiler with apply method in it whose implementation creates the object of the case class using new keyword.

So you need not create a companion object again with apply method creating object of the case class using new keyword. This work will be done by the compiler

Upvotes: 1

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