```class Foo { private val someConcurrentColle...

# getting-started

[y]

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()?

[Joffrey]

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.

[y]

great! thank you

[Rob Elliot]

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.

[Joffrey]

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

[y]

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