Kotlin. Vararg how to filter null values

Question

I have an API that has method which takes vararg of some object (fox example Param). I need to filter null params and not put it to vararg. Is it possible ?

I know there is a method in kotlin listOfNotNull but api accepts vararg(

This is example of calling API method (apiMethod(vararg params: Param)):

someFun() {
   apiMethod( 
      Param("first"),
      Param("second"),
      null // need to filter it
    )
}

P.S. I can't change apiMethod()

You can pass a `List` to a varargs. See https://stackoverflow.com/q/51161558/5133585 — Sweeper, Sep 15 '22 at 09:12
Does this answer your question? [Kotlin convert List to vararg](https://stackoverflow.com/questions/51161558/kotlin-convert-list-to-vararg) — Sweeper, Sep 15 '22 at 09:13

score 3 · Accepted Answer · answered Sep 15 '22 at 13:17

If I understand your question correctly, you need to filter your arguments before passing them to the function since you cannot modify the function. To do this, you can filter to a List, and then convert that list to a typed array and pass it using the * spread operator:

fun someFun() {
   apiMethod( 
      *listOfNotNull(
          Param("first"),
          Param("second"),
          null // need to filter it
      ).toTypedArray()
    )
}

It's a shame you have to use toTypedArray(). IMO, it should be supported for all Iterables, since Lists are far more common than Arrays. This feature request has not had a lot of attention from JetBrains.

Nikola Despotoski · Answer 2 · 2022-09-15T10:30:33.373

0

Your apiMethod() should fail either filter out nulls or just fail if provided list that contains null

fun apiMethod(a : Int, b : String, vararg params : List<Any?>){
   require(params.size == params.mapNotNull { it }.size) {
        "Params must not contain null values"
   }
   // or 
   val filtered = params.mapNotNull { it } 
  
}

edited Sep 15 '22 at 10:30

answered Sep 15 '22 at 09:00

Nikola Despotoski

49,966
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Wouldn't `filterNotNull()` be simpler than `mapNotNull{ it }`? (Note: `…Not…`, not `…Non…`.) – gidds Sep 15 '22 at 10:25
The final result is the same. (I made a mistake about not and non) – Nikola Despotoski Sep 15 '22 at 10:32
Same result, less readable code, though. – Tenfour04 Sep 15 '22 at 13:19

score 0 · Answer 3 · answered Sep 15 '22 at 09:02

0

Try mapNotNull method

 val notNullArgs = args.mapNotNull { it }

If there will be any null it it will be filtered

answered Sep 15 '22 at 09:02

Michael Babozhko

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Kotlin. Vararg how to filter null values

3 Answers3