CountingLetters.java

import org.assertj.core.api.Assertions;
import org.eclipse.collections.api.bag.primitive.CharBag;
import org.eclipse.collections.impl.factory.Strings;
import org.junit.Test;

import java.util.HashSet;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

public class CountingLetters {

    @Test
    public void the_quick_brown_fow_jumps_over_the_lazy_dog_with_this_awesome_framework_called_Eclipse_Collections() {

        String sentence = "The quick brown fox jumps over the lazy dog.";
        CharBag bag =
                Strings.asChars(sentence)
                        .select(Character::isLetter)
                        .collectChar(Character::toLowerCase)
                        .toBag();
        int sizeDistinct = bag.sizeDistinct();
        int size = bag.size();
        Assertions.assertThat(sizeDistinct).isEqualTo(26);
        Assertions.assertThat(size).isEqualTo(35);
    }

    @Test
    public void the_quick_brown_fow_jumps_over_the_lazy_dog_with_this_with_this_amazing_Stream_and_Collectors_API_V1() {
        String sentence = "The quick brown fox jumps over the lazy dog.";

        Map<Character, Long> counts = sentence.chars()
                .filter(Character::isLetter)
                .map(Character::toLowerCase)
                .mapToObj(i -> Character.valueOf((char) i))
                .collect(Collectors.groupingBy(letter -> letter, Collectors.counting()));

        int sizeDistinct = counts.keySet().size();
        int size = counts.values().stream().mapToInt(Long::intValue).sum();
        Assertions.assertThat(sizeDistinct).isEqualTo(26);
        Assertions.assertThat(size).isEqualTo(35);
    }

    @Test
    public void the_quick_brown_fow_jumps_over_the_lazy_dog_with_this_with_this_amazing_Stream_and_Collectors_API_V2() {
        String sentence = "The quick brown fox jumps over the lazy dog.";

        LetterCollector letterCollector = sentence.chars()
                .filter(Character::isLetter)
                .mapToObj(Character::toLowerCase)
                .reduce(new LetterCollector(), LetterCollector::accumulate, LetterCollector::combine);

        int sizeDistinct = letterCollector.numberOfDifferentLetters();
        int size = letterCollector.numberOfLetters();
        Assertions.assertThat(sizeDistinct).isEqualTo(26);
        Assertions.assertThat(size).isEqualTo(35);
    }

    private class LetterCollector {

        private int count = 0;
        private Set<Integer> differentLetters = new HashSet<>();

        public LetterCollector() {
        }

        public LetterCollector accumulate(int letter) {
            count++;
            differentLetters.add(letter);
            return this;
        }

        public LetterCollector combine(LetterCollector other) {
            count += other.count;
            differentLetters.addAll(other.differentLetters);
            return this;
        }

        public int numberOfDifferentLetters() {
            return differentLetters.size();
        }

        public int numberOfLetters() {
            return count;
        }
    }
}