Example: FindFirstNonRepeatingStreamCounting in Java

// FindFirstNonRepeatingStreamCounting
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Optional;
import java.util.function.Function;
import java.util.stream.Collectors;

public class Main {
    public static Optional<Character> findFirstNonRepeatingStreamCounting(String s) {
        if (s == null || s.isEmpty()) {
            return Optional.empty();
        }

        // Step 1: Stream characters and collect into a LinkedHashMap with counts
        // LinkedHashMap ensures iteration order matches character first appearance
        Map<Character, Long> charCounts = s.chars()
            .mapToObj(c -> (char) c)
            .collect(Collectors.groupingBy(
                Function.identity(),      // Key: the character itself
                LinkedHashMap::new,       // Map factory: ensures LinkedHashMap is used
                Collectors.counting()     // Value: count of occurrences
            ));

        // Step 2: Stream the entries of the map, filter for counts of 1, and find the first key
        return charCounts.entrySet().stream()
            .filter(entry -> entry.getValue() == 1)
            .map(Map.Entry::getKey)
            .findFirst();
    }

    public static void main(String[] args) {
        System.out.println("\n--- Stream API with Counting Approach ---");
        System.out.println("stress -> " + findFirstNonRepeatingStreamCounting("stress").orElse(null)); // t
        System.out.println("aabbc -> " + findFirstNonRepeatingStreamCounting("aabbc").orElse(null));   // c
        System.out.println("aabb -> " + findFirstNonRepeatingStreamCounting("aabb").orElse(null));     // null
        System.out.println("java -> " + findFirstNonRepeatingStreamCounting("java").orElse(null));     // j
        System.out.println(" -> " + findFirstNonRepeatingStreamCounting("").orElse(null));           // null
        System.out.println("zzzaac -> " + findFirstNonRepeatingStreamCounting("zzzaac").orElse(null)); // c
    }
}