Given this code ```data class Flow val ids List lt MyId gt v

Question

Given this code ```data class Flow val ids List lt MyId gt val steps MySteps constructor ids List lt MyId gt this ids MySteps ids toSteps ids toSteps first data class MySteps val steps List lt MyStep gt val currentStep MyStep ``` How can I throw an error in the convenience constructor if `ids` is not empty such that it crashes before `ds toSteps first `

wbertan · Answer

```constructor ids List lt MyId gt this ids ifEmpty error Not possible MySteps ids toSteps ids toSteps first ```

iex · Answer

< wbertan> awesome thanks

streetsofboston · Answer

Avoid throwing exceptions during construction of an object Consider creating a factory method function instead that will either throw that exception or return an instance of that class if all input is correct For a factory method to look like a constructor add the `operator fun invoke Flow ` to the `Flow` s companion object

wbertan · Answer

< streetsofboston> OMG That is amazing I wasn t aware of that < iex> ``` companion object operator fun invoke ids List lt MyId gt Flow if ids isEmpty error Not possible return Flow ids MySteps ids toSteps ids toSteps first ```

iex · Answer

< streetsofboston> thanks aware of factory methods But I don t see how this helps in my particular case

streetsofboston · Answer

```data class Flow private constructor val ids List lt MyId gt val steps MySteps companion object operator fun invoke ids List lt MyId gt Flow if ids isEmpty error return Flow ids MySteps ids topSteps ids toSteps first ```

Mike · Answer

Are there particular downsides to throwing an exception from the constructor Or is it more of a style expectations thing

streetsofboston · Answer

It is more like a style but you also avoid the construction of an object allocation of memory followed by throwing an exception which will cause the object to be GCed Also in the future you may want to return something else not a `Flow` or throwing an exception but maybe something more like a `Either lt Error Flow gt ` A factory method can be changed to make that happen a constructor can only return a Flow never something else

iex · Answer

avoid memory allocation + flexible return types white check mark

Kroppeb · Answer

Or `Flow `

Mike · Answer

Thank you for the follow up Firms up my suspicions I ve been adopting `Either` more and more as I like the separation of business errors from truly exceptional occurrences