Example: First Non-Repeating Character using Java 8 Stream API in Java

// First Non-Repeating Character using Java 8 Stream API
import java.util.LinkedHashMap;
import java.util.Optional;
import java.util.function.Function;
import java.util.stream.Collectors;

// Main class containing the entry point of the program
public class Main {
    public static void main(String[] args) {
        String input1 = "swiss";
        String input2 = "teeter";
        String input3 = "aabbcc";
        String input4 = "java";
        String input5 = "";

        System.out.println("Input: \"" + input1 + "\" -> First non-repeating character: " + findFirstNonRepeatingCharStream(input1).orElse(' '));
        System.out.println("Input: \"" + input2 + "\" -> First non-repeating character: " + findFirstNonRepeatingCharStream(input2).orElse(' '));
        System.out.println("Input: \"" + input3 + "\" -> First non-repeating character: " + findFirstNonRepeatingCharStream(input3).orElse(' '));
        System.out.println("Input: \"" + input4 + "\" -> First non-repeating character: " + findFirstNonRepeatingCharStream(input4).orElse(' '));
        System.out.println("Input: \"" + input5 + "\" -> First non-repeating character: " + findFirstNonRepeatingCharStream(input5).orElse(' '));
    }

    public static Optional<Character> findFirstNonRepeatingCharStream(String str) {
        // Step 1: Handle null or empty string
        if (str == null || str.isEmpty()) {
            return Optional.empty();
        }

        // Step 2: Stream characters, group them by identity, count occurrences,
        // and collect into a LinkedHashMap to preserve order.
        Map<Character, Long> charCounts = str.chars()
                .mapToObj(c -> (char) c)
                .collect(Collectors.groupingBy(
                        Function.identity(),
                        LinkedHashMap::new, // This ensures insertion order is preserved
                        Collectors.counting()
                ));

        // Step 3: Stream the entries of the charCounts map, filter for count 1, and find the first character
        return charCounts.entrySet().stream()
                .filter(entry -> entry.getValue() == 1)
                .map(Map.Entry::getKey)
                .findFirst();
    }
}