Preventing a class instantiation in Scala using Fa

问题:

This question already has an answer here:

• How to check constructor arguments and throw an exception or make an assertion in a default constructor in Scala? 2 answers

Suppose that I have the following class defined in Scala:

class ClassA(val n: Int) {
   ...
}

I want to limit this class instances to only those that have n between 5 to 10 using Factory Pattern. For example, In case I write something like:

val a = new ClassA(11)

This raises an exception with an appropriate error message, or at least it returns null or something. How can I achieve this behaviour?

Update:

It is possible to achieve this in java with Factory pattern.

Update2:

This questions seems to be answered here, notably with a verbose title though. I tweak the title and content to save the question being deleted, because of two reasons: 1) The example in this one is concise, 2) the provided answer by @Chris Martin explains briefly the way Factory pattern can be reached in Scala by Using Companion Objects.

回答1:

The conventional way to write a factory in Scala is to define an apply method on the companion object.

Here's an example using Either (because null is never/rarely used in Scala, and exceptions are ugly):

class A private (n: Int) {
  override def toString = s"A($n)"
}

object A {
  def apply(n: Int): Either[String, A] =
    if (n < 5) Left("Too small")
    else if (n > 10) Left("Too large")
    else Right(new A(n))
}

A(4)  // Left(Too small)
A(5)  // Right(A(5))
A(11) // Left(Too large)

This is the essentially the same as the Java example you referenced. The A constructor is private, so the class can only be instantiated via the factory method.