Use cases of removeall and removeif

Question

I found this:

fun main() {
    val list: MutableList<Int> = mutableListOf(1, 2, 3, 4, 5)

    list.removeAll { x -> x in 1..3 } // [4, 5]
    list.removeIf { x -> x in 1..3 } // [4, 5]
}

Both of them yield the same result.

Though I understand that removeAll is Kotlin and removeIf is Java but I don't understand why removeAll is there when removeIf was already there?

And for the fact that we could use removeIf in Kotlin without any hassle. Or is there any use case that might need one over another?

Answer 1

Java's removeIf() is there since Java 1.8.

Kotlin started at 2011 (wikipedia). Java 1.8 appeared in 2014.

I'm not sure when the Kotlin's removeAll(predicate) was specified and implemented, however it probably predates Java's removeIf().

Answer 2

There is one more important difference:

Calling removeIf on a CopyOnWriteArrayList is thread-safe, but removeAll is not.

Looking at the code, removeIf has a custom implementation for CopyOnWriteArrayList, but removeAll iterates over the indices and will end up throwing ArrayIndexOutOfBoundsException or even worse, removing the wrong element, if called concurrently.