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Kotlin / Null and Type System

In variance

Learn in variance in Kotlin through a focused explanation, a dedicated topic-specific example, an engineering boundary, and a mastery check.

12 min read • Intermediate

in variance is an atomic Kotlin topic inside Null and Type System. The example uses the named construct directly so you can see its syntax, data flow, and observable behavior.

Read the code by locating the exact feature named in variance, then trace the value before and after that feature runs. This lesson is intentionally isolated from neighboring topics: changing identifiers alone would not reproduce the same behavior because the construct itself is different.

Topic-specific example

kotlin-0056-6778025987.kt
kotlin Try It Live 
interface Sink<in T>{fun put(v:T)}; class AnySink:Sink<Any>{override fun put(v:Any){println(v)}}
fun main(){val s:Sink<String>=AnySink();s.put("ok")}
Edge case / engineering note

For in variance, test the boundary that makes the feature meaningful: invalid input, an empty value, a missing branch, lifecycle/cleanup, or a type/runtime constraint. Do not infer correctness only from the happy path.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

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