Fork/Join

import java.util.Map;
import java.util.List;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;

class ParallelLetterFrequency {

    List<String> texts;
    ConcurrentMap<Character, Integer> letterCount; 

    ParallelLetterFrequency(String[] texts) {
        this.texts = List.of(texts);
        letterCount = new ConcurrentHashMap<>();
    }

    Map<Character, Integer> countLetters() {
        if (texts.isEmpty()) {
            return letterCount;
        }

        ForkJoinPool forkJoinPool = new ForkJoinPool(); 
        forkJoinPool.invoke(new LetterCountTask(texts, 0, texts.size(), letterCount));
        forkJoinPool.shutdown();

        return letterCount;
    }

    private static class LetterCountTask extends RecursiveTask<Void> {
        private static final int THRESHOLD = 10; 
        private final List<String> texts;
        private final int start;
        private final int end;
        private final ConcurrentMap<Character, Integer> letterCount;

        LetterCountTask(List<String> texts, int start, int end, ConcurrentMap<Character, Integer> letterCount) {
            this.texts = texts;
            this.start = start;
            this.end = end;
            this.letterCount = letterCount;
        }

        @Override
        protected Void compute() {
            if (end - start <= THRESHOLD) {
                for (int i = start; i < end; i++) {
                    for (char c : texts.get(i).toLowerCase().toCharArray()) {
                        if (Character.isAlphabetic(c)) {
                            letterCount.merge(c, 1, Integer::sum);
                        }
                    }
                }
            } else {
                int mid = (start + end) / 2;
                LetterCountTask leftTask = new LetterCountTask(texts, start, mid, letterCount);
                LetterCountTask rightTask = new LetterCountTask(texts, mid, end, letterCount);
                invokeAll(leftTask, rightTask); 
            }
            return null;
        }
    }
}

Using ConcurrentHashMap ensures that frequency counting and updates are safely handled in a parallel environment.

If there are no strings, a validation step prevents unnecessary processing.

A ForkJoinPool is then created. The core of ForkJoinPool is the Fork/Join mechanism, which divides tasks into smaller units and processes them in parallel.

THRESHOLD is the criterion for task division. If the range of texts exceeds the THRESHOLD, the task is divided into two subtasks, and invokeAll(leftTask, rightTask) is called to execute both tasks in parallel. Each subtask in LetterCountTask will continue calling compute() (via invokeAll(leftTask, rightTask)) to divide itself further until the range is smaller than or equal to the THRESHOLD. For tasks that are within the THRESHOLD, letter frequency is calculated.

The Character.isAlphabetic method identifies all characters classified as alphabetic in Unicode, covering characters from various languages like English, Korean, Japanese, Chinese, etc., returning true. Non-alphabetic characters, including numbers, special characters, and spaces, return false.

Additionally, since uppercase and lowercase letters are treated as the same character (e.g., A and a), each character is converted to lowercase.

After updating letter frequencies, the final map is returned.