sum-groupingby-aggregate.kt

employees.groupingBy { it.department }
        .aggregate { key, acc: Int?, element, isFirstElement ->
            return@aggregate if (isFirstElement) {
                0 + element.salary
            } else {
                acc!! + element.salary
            }
        }
        .map {
            println("Total salary for department ${it.key} is ${it.value}")
        }