Use-site variance

Use-site variance is similar to Java wildcards:

Out-projection:

val takeList : MutableList<out SomeType> = ... // Java: List<? extends SomeType>

val takenValue : SomeType = takeList[0] // OK, since upper bound is SomeType

takeList.add(takenValue) // Error, lower bound for generic is not specified

In-projection:

val putList : MutableList<in SomeType> = ... // Java: List<? super SomeType>

val valueToPut : SomeType = ...
putList.add(valueToPut) // OK, since lower bound is SomeType

putList[0] // This expression has type Any, since no upper bound is specified

Star-projection

val starList : MutableList<*> = ... // Java: List<?>

starList[0] // This expression has type Any, since no upper bound is specified
starList.add(someValue) // Error, lower bound for generic is not specified

See also:

Variant Generics interoperability when calling Kotlin from Java.

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