In Java I can remove items from a list with this code:
private void filterList(List<Item> items) {
Iterator<Item> iterator = items.iterator();
while (iterator.hasNext()) {
if (checkItem(iterator.next())) {
iterator.remove();
}
}
}
How to make the same in Kotlin (i.e. remove some items in a List
without re-creation)?
Just use .retainAll { ... }
or .removeAll { ... }
, both accepting a predicate, to filter it in-place:
items.retainAll { shouldRetain(it) }
items.removeAll { shouldRemove(it) }
Note that items
should be a MutableList<T>
for that, not just List<T>
, which is a read-only list in Kotlin and thus does not expose any mutating functions (see: Collections in the language reference).
By the way, these two function are implemented efficiently for lists that support random access: then the list is not compacted after each item is removed (O(n2) time worst-case), and instead the items are moved within the list as it is processed, giving O(n) time.
And if you don't want to modify the original list, you can produce a separate collection with only the items you want to retain using .filter { ... }
or .filterNot { ... }
, this will work for read-only List<T>
as well:
val filtered = items.filter { shouldRetain(it) }