Count and print char frequency in text using streams, no maps, no key-value pairs

Question

I'm trying to count and print the frequency of all characters in a text. I want to do this using an Array or ArrayList and no maps with key-value pairs.

The code below gives the preferred result. I want to get rid of the for loop of main And do all the work in method countLetters().

To be clear, I wish to do this in a functional way not using any for loops or if statements. Can this be done? And if so, how?

public class LetterCounter4 {


    public static void main(String[] a) {

    System.out.print("Input text > ");

    int[] res = countLetters();

    for (int i = 0; i < res.length; i++) {
        if(res[i] != 0){
            System.out.println((char) ('a' + i) + " appears "
                + res[i] + ((res[i] == 1 ? " time" : " times")));
        }
    }
}

private static int[] countLetters() {


    return Arrays.stream(new Scanner(System.in).nextLine().toLowerCase()
            .split(""))
            .map(s -> s.charAt(0))
            .filter(Character::isLetter)
            .collect(Collector.of(
                    () -> new ArrayList<Integer>(Collections.nCopies(26, 0)),
                    (li, el) -> {
                        Integer oInt = li.get(el - 'a');
                        li.set(el - 'a', ++oInt);
                    },
                    (result1, result2) -> {
                        for (int i = 0; i < result1.size(); i++) {
                            Integer temp = result1.get(i);
                            result1.set(i, temp + result2.get(i));
                        }
                        return result1;
                    }))
            .stream()
            .mapToInt(Integer::intValue)
            .toArray();    
    }
}

Answer 1

The straightforward way is to use .groupingBy(c -> c, Collectors.counting()). The code would be simpler and won't blow up when somebody enters À or some other letter outside [a-zA-Z] range. But that would create a Map, which you say you don't want.

If you specifically wish to stick with arrays and lists, here's a way to do it:

public static void main(String[] args) {
    String input = new Scanner(System.in).nextLine();

    int[] counts = countLetters(input);

    IntStream.range(0, counts.length)
            .filter(i -> counts[i] > 0)
            .forEachOrdered(i -> System.out.printf("%c appears %s %s%n", 
                    'a' + i, 
                    counts[i], 
                    counts[i] > 1 ? "times" : "time"
            ));
}

public static int[] countLetters(String s) {
    return s.chars() // this is better than stream(split(""))
            .filter(Character::isLetter)  // WRONG to assume that all letters are [a-zA-Z]
            .map(chr -> Character.toLowerCase(chr) - 'a')
            .collect(
                    () -> new int[26], 
                    (ary, i) -> ary[i]++, 
                    (a,b) -> Arrays.setAll(a, i -> a[i] + b[i])
            );
}