How to implement a single factory pattern?

Question

I'd like to have some type 'ValidatedVals' that can be instantiated exclusively by a single class (i.e. it is the single factory for ValidatedVals).

I'd then like to reference this ValidatedVals type in many other classes.

Private constructors seems like one answer... but ...

The code below fails because (apparently) an inner class with a private constructor, declared within

object X{  ... }

makes that constructor not visible within object X

So, in the code below, how can I make it so that:

ONLY ValidatedValsFactory can construct a ValidatedVals object
AND I can reference a ValidatedVals type in other classes?

_

class ValidatedValsFactory {}

object ValidatedValsFactory {

  class ValidatedVals private (val a: Int, val b: Int) {}

  def validate(a: Int, b: Int): Unit = { /* .... */ }

  def makeValidated(a: Int, b: Int): ValidatedVals = {
    validate(a, b);
    // **** Nope. Fail. Not visible. *****
    return new ValidatedVals(a, b)
  }
}

class ValidatedValsConsumer {
  def f(param: ValidatedValsFactory.ValidatedVals): Unit = { /* ... */ }
}

I don't want f's parameter's type to be an interface (because its easy to circumvent the validate by implementing the interface).
I can't move class ValidatedVals into the class definition of ValidatedValsFactory because ValidatedValsConsumer needs to be able to reference the inner class typename (and therefore ValidatedVals must be a static declaration).

garrghh!

In Scala you place factory methods for class `X` in the companion object of `X` instead of `XFactory`. — ziggystar, Feb 12 '15 at 21:15
... well that really depends on whether your factory will contain state. — user48956, Feb 17 '15 at 17:30

score 2 · Accepted Answer · answered Feb 12 '15 at 21:04

2

Looks like the compilation error goes away if you make it private to the object:

class ValidatedVals private[ValidatedValsFactory] (val a: Int, val b: Int) {}

answered Feb 12 '15 at 21:04

dhg

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Nice! Can also share the class with Java. – user48956 Feb 12 '15 at 21:06

score 1 · Answer 2 · edited May 23 '17 at 11:43

1

Consider declaring your ValidatedVals type as a sealed class or trait. More details here: What is a sealed trait?

edited May 23 '17 at 11:43

Community

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answered Feb 12 '15 at 20:50

Daryl Odnert

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This only prevents extension, but does not control instantiation. – ziggystar Feb 12 '15 at 21:15
1

I agree. The "other" answer is a better match for the goal that was stated in the original question. – Daryl Odnert Feb 12 '15 at 21:21

How to implement a single factory pattern?

2 Answers2