Map and/or Filter

Map

You can create a new collection resulting of the mapping of another.

List<int> list = [1, 2, 3, 4]
List<int> list2 = [for int i in list -> i + 1] // [2, 3, 4, 5]

Filter

Filtering is also possible using a similar syntax.

List<int> list = [1, 2, 3, 4]
List<int> list2 = [for int i in list if i <= 2] // [1, 2]

Map and Filter

You can do both in one operation.

List<int> list = [1, 2, 3, 4]
List<float> list2 = [for int i in list -> i + 0.1f if i <= 2] // [1.1f, 2.2f]

Casting

All the above operations can return Lists, primitive Lists, Sets, or primitive Sets. The type is usually guessed by the compiler when possible (e.g. looking at the type of the variable you're trying to set), but you can explicitly specify the wanted type using the as operator.

List<int> list = [1, 2, 3, 4]
Set<int> list2 = [for int i in list -> i % 2] // [0, 1]
println([for int i in list -> Optional.of(i % 2)] as Set) // [Optional(0), Optional(1)]