I have a Java/Kotlin interop problem. A Kotlin immutable list is compiled into a normal java.util.ArrayList that is mutable!
Kotlin (图书馆):
class A {
val items: List<Item> = ArrayList()
}
Java(消费者):
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?
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]
}
要想有一个不变的列表，请考虑番石榴的Immutable____个集合:
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]
}
请注意，Kotlin的一些标准运行时函数可能返回不可修改、不可调整大小等的集合，因此当您将只读集合直接强制转换为可变集合时，所有赌注都会失效.
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]
}