Hi

# getting-started

[Tower Guidev2]

Hi

[Tower Guidev2]

Im investigating the use of when

I have a sealed class of values and want to identify each case as shown below

*when*(x) {
*is* SealedInitial -> doNothing()
*is* SealedOne -> something(x.value)
*is* SealedTwo -> somethingElse(x.value)
*else* -> doNothing()
}

I have to perform this test "All Over" my application, however the only constant check I will always need is
the first one, e.g. *is* SealedInitial -> doNothing()
I would like to have thi scase handled by a super class so that all my Sub Classes do not need to keep coding that line

Is this possible? To have a when statement "extend" a super when?

[hho]

You can't extend when statements, but you can of course create a method in the superclass to call from any subclass.

[Tower Guidev2]

yes, I was just "Painting a picture with words" 🎨 😄

[Tower Guidev2]

however I was looking for an approach where I achived the "standard" is case fro free,
e.g. if I have to write a line to always call the super class method, I might as well stick to always coding the *is* SealedInitial -> doNothing()

[Rob Elliot]

This would be one way:

sealed class SealedSuper {
   open fun doWork(work: () -> Unit) = work()
}

class SealedInitial : SealedSuper() {
   override fun doWork(work: () -> Unit) {
      doNothing()
   }
}

x.doWork {
  when (x) {
    is SealedOne -> something(x.value)
    is SealedTwo -> somethingElse(x.value)
    else -> doNothing()
  }
}

[Rob Elliot]

But you might want to think about whether you should just make a separate abstract method on the superclass for each time you want to do this when.

[Jilles Soeters]

I thought it was bad practice to have an else clause in a when when going over sealed classes

[Rob Elliot]

Probably better not to - just list all the remaining as the final case. Then the compiler will force you to think about it if you add a new subtype, rather than it just going in the else bucket.