Understanding Kotlin’s yield function

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You can think of yield() as “return, and next time start from where you stopped“:

val sequence = sequence {
    val start = 0
    yield(start) // first return
    yieldAll(1..5 step 2) // if called again, will start from here
    yieldAll(generateSequence(8) { it * 3 }) // if called more that six times, start from here
}

It creates state machine and stuff, but you can translate it to something like following in Java:

class Seq {
    private AtomicInteger count = new AtomicInteger(0);
    private int state = 0;
    
    public int nextValue() {
        if (count.get() == 0) {
           return state;
        }
        else if (count.get() >= 1 && count.get() <= 5) {
           state += 2;
           return state;
        }
        else {
           state *= 3;
           return state;
        }
    }
}

In Java class we maintain explicit state by having two variables: count and state. Combination of sequence and yield allow this state to be maintained implicitly.

Note that yield() is a suspending function, so it may be invoked only from another suspend function or from within a coroutine.

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