Collect String to LinkedHashMap <Character, Integer> , with frequency of characters

I need to collect a sentence to a map using stream, where the key is a character (the order should be like this: the first letter in the sentence is the first in the map, so I choose LinkedHashMap ), and the value is counter which shows the frequency of repeating the same character.
> Example input: hello world

> Example output: h=1 , e=1, l=3, o=2, w=1 , r=1, d=1

There is my code, and I can`t understand what is the problem.

private Map<Character, Integer> countSymbols(String sentence) {
    return sentence.chars()
            .collect(Collectors.toMap(key -> (Character) key, 
                    counter -> 1, 
                    Integer::sum, 
                    LinkedHashMap::new));
}

String.chars() returns an IntStream (i.e., a stream of primitieve ints), that can't be collected with Collectors.toMap.

You could map it to a Character object, and once you've done so, you could use Collectors.grouping to do the heavy lifting instead of implementing its logic:

private Map<Character, Long> countSymbols(String sentence) {
    return sentence.chars()
                    .mapToObj(c -> (char) c)
                    .collect(Collectors.groupingBy(Function.identity(), 
                                                   Collectors.counting()));
}

EDIT:<br/>
Collectors.grouping does not retain the order of the stream, so you'll really have to implement it manually as you've done.
the mapToObj call is still required, and if you want to avoid counting the space between "hello" and "world" in the example, you need to add a filter call:

private Map<Character, Integer> countSymbols(String sentence) {
    return sentence.chars()
                   .filter(Character::isAlphabetic)
                   .mapToObj(c -> (char) c)
                   .collect(Collectors.toMap(Function.identity(),
                            counter -> 1,
                            Integer::sum,
                            LinkedHashMap::new));
}