Java stream operation invocations

Question

Can anyone point to a official Java documentation which describes how many times Stream will invoke each "non-interfering and stateless" intermediate operation for each element.

For example:

Arrays.asList("1", "2", "3", "4").stream()
        .filter(s -> check(s))
        .forEach(s -> System.out.println(s));

public boolean check(Object o) {
    return true;
} 

The above currently will invoke check method 4 times.

Is it possible that in the current or future versions of JDKs the check method gets executed more or less times than the number of elements in the stream created from List or any other standard Java API?

Answer 1

This does not have to do with the source of the stream, but rather the terminal operation and optimization done in the stream implementation itself. For example:

Stream.of(1,2,3,4)
      .map(x -> x + 1)
      .count();

Since java-9, map will not get executed a single time.

Or:

 someTreeSet.stream()
            .sorted()
            .findFirst();

sorted might not get executed at all, since the source is a TreeSet and getting the first element is trivial, but if this is implemented inside stream API or not, is a different question.

So the real answer here - it depends, but I can't imagine one operation that would get executed more that the numbers of elements in the source.