adding numbers one to ten can easily lead to random results when done in parallel

RNG.java

import java.util.Optional;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Supplier;
import java.util.stream.Stream;

public class RNG {
    static Supplier<Integer> countGen(AtomicInteger i) {
        return (()-> i.getAndIncrement());
    }

    public static void main(String[] args) {
        final Supplier<Integer> count = countGen(new AtomicInteger(1));
        final Optional<Integer> sum = Stream.generate(count).limit(10).parallel().reduce((a, b) -> a + b);
        System.out.println("sum = " + sum);
    }
}

The randomness does not come from limit() but from generate() - and I very much agree: this is, just from the method name, quite unexpected. iterate(T seed, UnaryOperator f) would probably be the right choice - at least I then get always the same result (55).

With iterate() there is also no need for AtomicInteger anymore.