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());
}

Answer 2

Using Java 9, you can iteratively search using the last index as the starting point, and stop when no match is found:

public static List<Integer> getIndexList(String s, char c) {
    return IntStream.iterate(s.indexOf(c), i -> s.indexOf(c, i + 1))
            .takeWhile(i -> i > -1)
            .boxed()
            .collect(Collectors.toList());
}

Disclaimer: I didn't test this.

Answer 3

An alternate in Java9 could be to make use of the iterate(int seed, IntPredicate hasNext,IntUnaryOperator next) as follows:-

private static List<Integer> getIndexList(String word, char c) {
  return IntStream
          .iterate(word.indexOf(c), index -> index >= 0, index -> word.indexOf(c, index + 1))
          .boxed()
          .collect(Collectors.toList());
}

Answer 4

...Or in java-9:

Stream.of("Hello world!")
            .map(Scanner::new)
            .flatMap(s -> s.findAll("l"))
            .map(mr -> mr.start())
            .forEach(System.out::println);