class Foo {
    private val someConcurrentCollection = //...
    
    fun addToCollection(val s: String) {
        if (!checkS(s)) {
            return
        }

        synchronized(this) {
            val t = doSomethingToS(s)
            someConcurrentCollection.add(t)
        }
    }
}

fun checkS(s: String): Boolean = //...

I have something that kind of looks like this. does the fact that a thread is currently in the

synchronized

block, completely prevents other threads from entering the

addToCollection()

function and running

checkS()

? same for the other way around, do multiple threads with

s

values such that

checkS(s) = false

stop 'legitimate' threads from entering

addToCollection()

?

Only the synchronized block is non-concurrent. Other threads can call

checkS

. They will only stop at the beginning of the

synchronized

block if one thread is already there.

great! thank you

Beware of check-then-act race conditions if the check isn’t protected by the same guard as the action. Two threads could both see checkS as true, and both would then call doSomethingToS, just one after the other. Perhaps that’s ok.

Ah yeah of course it depends on the semantics you're trying to achieve. It seemed to me that the OP just wanted to protect the list interaction so no 2 threads would do it at the same time, but your point is quite important if I misunderstood the purpose

@Rob Elliot fortunately @Joffrey is correct about my actual use case, but that is a great point