import java.util.LinkedHashMap;

import java.util.Map;

import java.util.Optional;

/**

* Inspired by a not-so-elegant example solution that I saw online.

*

* This implementation only needs to pass through the input String once, as LinkedHashMap preserves ordering.

*/

public class FindFirstNonDuplicate {

public static void main(String args[]) {

String input = "Some text with some letters in it. So how did it go?";

Optional<Character> firstNonDup = firstNonDuplicate(input);

if (firstNonDup.isPresent()) {

System.out.println("Found first non-duplicate character '" + firstNonDup.get() + "'");

} else {

System.out.println("Sorry, there were no non-duplicate characters.");

}

}

// Returns Optional of first non duplicate character if one exists

private static Optional<Character> firstNonDuplicate(String input) {

LinkedHashMap<Character, Integer> characterOccurences = new LinkedHashMap<>();

for (Character c : input.toCharArray()) {

characterOccurences.put(c, characterOccurences.getOrDefault(c, 0) + 1);

}

return characterOccurences.entrySet().stream()

.filter(e -> e.getValue() == 1)

.findFirst()

.map(Map.Entry::getKey);

}

}