why not a factory method to return null when neces...

# android

[menegatti]

why not a factory method to return null when necessary instead?

[jasonlow]

menegatti: Can you elaborate on what you mean by that?

[menegatti]

well, have a companion object with a method that takes the json and returns null in case it's not valid

[menegatti]

I don't think data classes have a really good way of achieving what you're trying to do by themselves

[jasonlow]

To extend on your point for a companion object, why not a function instead that massages the json and return null if not valid?

[jasonlow]

If not a data class, then what?

[menegatti]

I mean, you would still use a data class

[menegatti]

but a method will work on the instance itself and validating the json should be a step before, don't you agree?

[menegatti]

meaning... you verify whether or not it is a valid json and then decide on whether you should create an instance of the data class or not

[menegatti]

so it seems like a case where I would use a static (companion object) factory method that takes a json and returns your class in case the json is valid, null otherwise

[jasonlow]

I see where you’re getting at now. Interesting, never thought of that. Will give it a shot.

[jasonlow]

Have you personally never encountered such cases before though?

[jasonlow]

OR, alternatively we could make the constructors throw an exception, no?

[menegatti]

yes, definitely

[jasonlow]

Noted, thanks Edson!

[menegatti]

also you could let the json do its magic

[menegatti]

and after parsing you just filter out the instances that don't pass a certain validation, does that make sense?

[jasonlow]

Haha, I thought of that too actually

[menegatti]

if you wanna go very clean about it, have an interface like Validator with a method validate or something, and filter out by the return of validate, but then it's a matter of taste

[kirillrakhman]

constructors throwing exceptions are bad style. I support the suggestion for a factory method.

[kirillrakhman]

Kotlin's nullability language features lend themselves very well for early returns:

fun create(json: JosnObject): MyClass? {
    val id = json.opt("id") ?: return null
    ...
    return MyClass(id, foo, bar, ...)
}