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 expression is one whose result depends on any state that might change during the execution of a pipeline.'
They provide an example where a parallel stream is used to add a fixed collection of numbers to a synchronized ArrayList using the .map() function.
The order in the arraylist is completely random and this should make one see that a stateful lambda expression produces unpredictable results in runtime. That's why its strongly recommended to avoid stateful operations when using parallel streams so as to remove any potential data side effects.
They don't show a stateless lambda expression that provides a solution to the same problem (adding numbers to a synchronized arraylist) and I still don't get what the problem is with using a map function to populate an empty synchronized arraylist with data... What is exactly the state that might change during the execution of a pipeline? Are they referring to the Arraylist itself? Like when another thread decides to add other data to the ArrayList when the parallelstream is still in the process adding the numbers and thus altering the eventual result?
Maybe someone can provide me with a better example that shows what a stateful lambda expression is and why it should be avoided. That would be very much appreciated.
Thank you
To try to give an example, let's consider the following Consumer (note : the usefulness of such a function is not of the matter here) :
public static class StatefulConsumer implements IntConsumer {
private static final Integer ARBITRARY_THRESHOLD = 10;
private boolean flag = false;
private final List<Integer> list = new ArrayList<>();
@Override
public void accept(int value) {
if(flag){ // exit condition
return;
}
if(value >= ARBITRARY_THRESHOLD){
flag = true;
}
list.add(value);
}
}
It's a consumer that will add items to a List (let's not consider how to get back the list nor the thread safety) and has a flag (to represent the statefulness).
The logic behind this would be that once the threshold has been reached, the consumer should stop adding items.
What your book was trying to say was that because there is no guaranteed order in which the function will have to consume the elements of the Stream, the output is non-deterministic.
Thus, they advise you to only use stateless functions, meaning they will always produce the same result with the same input.