When to use companion object factory versus the new keyword

Many classes in the Scala standard library use apply() of their companion object as factory. This is often convenient when chaining calls like List(List(1)). On the other hand, it's still possible to create objects directly with new (new HashMap[Int, Int]()).

That's standard library. Now, in my own code, which approach is better to use: companion factory or creating objects with new?

Is there any convention on when to create companion object factory and when to do with the new keyword?

What are the advantages of using one over the other?

Solution

In most cases I use the companion object's apply method, because the code looks less cluttered. However, there is at least one benefit of using a static factory. Consider the unimaginative type MyInt which just wraps an Int:

class MyInt(val i: Int)

I can obtain instances of MyInt calling the constructor which will instantiate a new object each time the constructor is called. If my program relies heavy on MyInt this results in a lot of instances created. Assuming most of the MyInt I use are -1, 0, and 1, since MyInt is immutable I can reuse the same instances:

class MyInt(val i: Int) 

object MyInt {
  val one = new MyInt(1)
  val zero = new MyInt(0)
  val minusOne = new MyInt(-1)

  def apply(i: Int) = i match {
    case -1 => minusOne
    case 0 => zero
    case 1 => one
    case _ => new MyInt(i)
  }
}

So at least for immutable values there can be a technical advantage of using the static factory over calling the constructor. As an implication, if you want to express in code that a new instance is created, then use the new keyword. Personally, I use the new-keyword when creating objects, and the apply-method when creating values, though I don't know if there is an official convention.