Eclipse Collections (formerly GS-Collections) a really high performance, JDK compatible, top performer in parallel processing with immutable and mutable variations (home: and Github: )Īlso, the Kotlin team is working on Immutable Collections natively for Kotlin, that effort can be seen here:.Dexx a port of the Scala collections to Java ( ) with Kotlin helpers ( ).So if you can't avoid wanting immutable collections, Kotlin easily works with anything from the JVM space: Kotlin gives a good mix of interop and lack of conversion, with readonly protection by contract. In the JVM world it is hard to avoid interop with libraries that want standard Java collections, and converting to/from these collections adds a lot of pain and overhead for libraries that do not support the common interfaces. And only use them in those cases, not in all general cases. In those cases, it might be best to look at really high performance collections designed just for those purposes. Some people want them for other reasons, such as parallel processing. If that type is lazy, then you will lazily apply the change until you demand the collection in its entirety. In other words, most operators return you the same type as you started with, and if that type is 'readonly' then you will receive a copy. By naming convention almost anything that starts with to is making a copy. Although for a Sequence, calling anything such as toList, toSet, toMap will result in the final copy being made. When used on a Sequence they modify the values as the collection as it is accessed and continue to be lazy (resulting in another Sequence). So to answer part of your questions, things like filter, map, flatmap, operators + - all create copies when used against non lazy collections. The Kotlin collections allow both copy-on-change mutations, as well as lazy mutations.

But in cooperative Kotlin code that is fine, most uses do not need truly immutable collections and if your team avoids casts to the mutable form of the collection then maybe you don't need fully immutable collections. But the collection can be bypassed via casting or manipulated from Java. As you see in other answers, Kotlin has readonly interfaces to mutable collections that let you view a collection through a readonly lens.