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 functions in the stdlib. But it isn't clear what the equivalences are for different use cases.

For example, at the top of the JavaDoc for Collectors are examples written for Java 8, and when porting them to Kolin you can't use the Java 8 classes when on a different JDK version, so likely they should be written differently.

In terms of resources online showing examples of Kotlin collections, they are typically trivial and don't really compare to the same use cases. What are good examples that really match the cases such as documented for Java 8 Stream.collect? The list there is:

• Accumulate names into a List
• Accumulate names into a TreeSet
• Convert elements to strings and concatenate them, separated by commas
• Compute sum of salaries of employee
• Group employees by department
• Compute sum of salaries by department
• Partition students into passing and failing

With details in the JavaDoc linked above.

Note: this question is intentionally written and answered by the author (Self-Answered Questions), so that the idiomatic answers to commonly asked Kotlin topics are present in SO. Also to clarify some really old answers written for alphas of Kotlin that are not accurate for current-day Kotlin.

Answer 1

There are functions in the Kotlin stdlib for average, count, distinct,filtering, finding, grouping, joining, mapping, min, max, partitioning, slicing, sorting, summing, to/from arrays, to/from lists, to/from maps, union, co-iteration, all the functional paradigms, and more. So you can use those to create little 1-liners and there is no need to use the more complicated syntax of Java 8.

I think the only thing missing from the built-in Java 8 Collectors class is summarization (but in another answer to this question is a simple solution).

One thing missing from both is batching by count, which is seen in another Stack Overflow answer and has a simple answer as well. Another interesting case is this one also from Stack Overflow: Idiomatic way to spilt sequence into three lists using Kotlin. And if you want to create something like Stream.collect for another purpose, see Custom Stream.collect in Kotlin