filterIsInstance

FilterIsInstance is a Kotlin standard library function that filters elements of a collection by their runtime type and returns them cast to that type. It is defined as an inline function with a reified type parameter, which allows type checks to run at runtime without passing a separate Class object.

For Iterable collections, the signature is roughly: filterIsInstance<T>(): List<T>. For Sequences, there is a corresponding inline function returning a Sequence<T>, enabling lazy evaluation. The function uses the reified type parameter T to test each element with a type check (element is T) and casts matching elements to T.

Usage examples:

- val mixed = listOf(1, "two", 3.0, "four")

val strings = mixed.filterIsInstance<String>() // ["two", "four"]

- val items = sequenceOf(1, "a", 2.5, "b")

val numbers = items.filterIsInstance<Number>() // yields 1 and 2.5 when iterated

Semantics and behavior:

- Returns a collection containing only elements that are instances of T, cast to T.

- On Iterable, the result is a new List<T>. On Sequence, the result is a Sequence<T> and can be processed lazily.

- Useful when a heterogeneous collection must be narrowed to elements of a specific type without explicit casting or manual checks.

Notes:

- Requires a reified type parameter, so it must be called in an inline function context or at top level where reified generics are supported.

- Works with reference types; primitive types are boxed as needed since Kotlin uses runtime type checks.