Is there a Kotlin equivalent of Java's Collections.synchronizedList ? Or is this not needed in Kotlin

Question

Coding in Kotlin, want a thread-safe List as described here: java concurrent Array List access

It seems Collections.kt does not have this function. Are Kotlin's mutable lists already threadsafe ? If not, how do I accomplish this ?

Thanks.

Solution

If I try to use the Java List a warning message comes up "This class shouldn't be used in Kotlin..."

Java lists (and other collections) are mapped types in Kotlin. So you can use Collections.synchronizedList, and it takes and returns a Kotlin List or MutableList.

OTOH, synchronizedList is rarely what you actually want: it works for single method calls, but anything else must be synchronized manually.

Comments

[Shadov]

Why not just use the Java one? It will work fine since Kotlin is completely interoperable with Java. Kotlin's mutable lists are not thread-safe.

[shmosel]

@Shadov I don't think Kotlin's collection library is interoperable with the JDK's.

[A.Sanchez.SD]

If I try to use the Java List a warning message comes up "This class shouldn't be used in Kotlin..." Not sure if that is merely a suggestion or serious direction due to some issue...

[Alexey Romanov]

@shmosel Yes, it is: kotlinlang.org/docs/reference/java-interop.html#mapped-types‌​.

[shmosel]

@AlexeyRomanov Interesting. So does that mean you can actually use the Collections wrappers?

[Alexey Romanov]

@shmosel Yes (in a JVM-only project, of course).

[milosmns]

Right, it doesn't really prevent ConcurrentModificationException, for example. You can still get it while accessing the list from two different threads using two methods/functions at the same time.

[IgorGanapolsky]

So there is no completely synchronized collection in Kotlin right now?

[Alexey Romanov]

As far as I know it's impossible to create one, in Kotlin or in Java. It has to be specific to your project because different projects need different operations to be atomic.

[user924]

@milosmns because you didn't read description of this method It is imperative that the user manually synchronize on the returned list when iterating over it. So Collections.synchronizedList should work fine docs.oracle.com/javase/8/docs/api/java/util/…

[milosmns]

@user924 Nah, I waited 3 years for you to read it for me and point out that I didn't do it :)