How to create an immutable list in Kotlin that is also an immutable list in Java?

Question

I have a Java/Kotlin interop problem. A Kotlin immutable list is compiled into a normal java.util.ArrayList that is mutable!

Kotlin (library):

class A {
  val items: List<Item> = ArrayList()
}

Java (consumer):

A a = new A();
a.getItems().add(new Item());   // Compiles and runs but I wish to fail or throw

How to make my Kotlin class fully immutable from Java perspective too?

Answer 1

All non- Mutable____ collections in Kotlin are compile time read-only types by default, but not immutable . See the following code snippet:

fun main(args: Array<String>) {
  // Explanation for ArrayList(listOf()) later down the post
  val list: List<Int> = ArrayList(listOf(1, 2, 3))
  println(list) // [1, 2, 3]

  // Fails at compile time
  // list.add(4)

  // Uh oh! This works at runtime!
  (list as MutableList<Int>).add(4)
  println(list) // [1, 2, 3, 4]
}

To truly have an immutable list, consider Guava's Immutable____ collections:

import com.google.common.collect.ImmutableList

fun main(args: Array<String>) {
  val list: List<Int> = ImmutableList.of(1, 2, 3)
  println(list) // [1, 2, 3]

  // Fails at compile time
  // list.add(4)

  // Fails at runtime, as expected
  (list as MutableList<Int>).add(4)
  println(list) // [1, 2, 3, 4]
}

Be aware that some of Kotlin's standard runtime function may return collections that are either unmodifiable, not resizable, etc., so all bets are off when you directly cast a read-only collection to a mutable one.

For example, listOf() currently ( this may change in the future! ) returns a java.util.Arrays.ArrayList around the array of vararg parameters via Arrays.asList(T...) . This "list" can be modified, but elements can never be added or removed, as you cannot resize an array. See Arrays.asList(T...) javadoc for more information.

If you really want a mutable collection from any given collection, consider making a copy using .toMutableList() . This will work on any collection:

import com.google.common.collect.ImmutableList

fun main(args: Array<String>) {
  val list: List<Int> = ImmutableList.of(1, 2, 3)

  val copy = list.toMutableList()
  copy.add(4)

  println(copy) // [1, 2, 3, 4]
  println(list) // [1, 2, 3]
}

Answer 2

There is ImmutableList.copyOf(list) that returns an ImmutableList . This is Kotlin API, and you won't be able to use it in Java.

Another option is Collections.unmodifiableList from java.util , when used in Kotlin, it returned a mutable list type, but just like in Java, you will get an exception if you attempt to modify it.