what is the difference between a stateful and a stateless lambda expression?

Question

According to the OCP book one must avoid stateful operations otherwise known as stateful lambda expression. The definition provided in the book is \'a stateful lambda expres

Answer 1

Here is an example where a stateful operation returns a different result each time:

public static void main(String[] args) {

Set seen = new HashSet<>();

IntStream stream = IntStream.of(1, 2, 3, 1, 2, 3);

// Stateful lambda expression
IntUnaryOperator mapUniqueLambda = (int i) -> {
    if (!seen.contains(i)) {
        seen.add(i);
        return i;
    }
    else {
        return 0;
    }
};

int sum = stream.parallel().map(mapUniqueLambda).peek(i ->   System.out.println("Stream member: " + i)).sum();

System.out.println("Sum: " + sum);
}

In my case when I ran the code I got the following output:

Stream member: 1
Stream member: 0
Stream member: 2
Stream member: 3
Stream member: 1
Stream member: 2
Sum: 9

Why did I get 9 as the sum if I'm inserting into a hashset?
The answer: Different threads took different parts of the IntStream For example values 1 & 2 managed to end up on different threads.