What Java 8 Stream.collect equivalents are available in the standard Kotlin library?

Question

In Java 8, there is Stream.collect which allows aggregations on collections. In Kotlin, this does not exist in the same way, other than maybe as a collection of extension f

Answer 1

More on laziness

Let's take the example solution for "Compute sum of salaries by department" given by Jayson:

val totalByDept = employees.groupBy { it.dept }.mapValues { it.value.sumBy { it.salary }}

In order to make this lazy (i.e. avoid creating an intermediate map in the groupBy step), it is not possible to use asSequence(). Instead, we must use groupingBy and fold operation:

val totalByDept = employees.groupingBy { it.dept }.fold(0) { acc, e -> acc + e.salary }

To some people this may even be more readable, since you're not dealing with map entries: the it.value part in the solution was confusing for me too at first.

Since this is a common case and we'd prefer not to write out the fold each time, it may be better to just provide a generic sumBy function on Grouping:

public inline fun  Grouping.sumBy(
        selector: (T) -> Int
): Map = 
        fold(0) { acc, element -> acc + selector(element) }

so that we can simply write:

val totalByDept = employees.groupingBy { it.dept }.sumBy { it.salary }