Java 8/9: Can a character in a String be mapped to its indices (using streams)?

Question

Given a String s and a char c, I'm curious if there exists some method of producing a List<Integer> list from s (where the elements within list represent the indices of c within s).

A close, but incorrect approach would be:

public static List<Integer> getIndexList(String s, char c) {
    return s.chars()
            .mapToObj(i -> (char) i)
            .filter(ch -> ch == c)
            .map(s::indexOf) // Will obviously return the first index every time.
            .collect(Collectors.toList());
}

The following inputs should yield the following output:

getIndexList("Hello world!", 'l') -> [2, 3, 9]

Answer 1

Can be done with IntStream

public static List<Integer> getIndexList(String s, char c) {
    return IntStream.range(0, s.length())
                    .filter(index -> s.charAt(index) == c)
                    .boxed()
                    .collect(Collectors.toList());
}