first repeated char

Application.java

public class Application {

  static String solve(String word) {
    Objects.requireNonNull(word);

    Map<String, Integer> chars = new LinkedHashMap<String, Integer>();
    for (int i = 0; i < word.length(); i++) {
      var w = word.charAt(i);
      chars.compute(String.valueOf(w), (k, v) -> {
        if (v != null) {
          return v + 1;
        }
        return 1;
      });

    }

    return chars.entrySet().stream().filter(entry -> entry.getValue().equals(1)).findFirst()
        .map(e -> e.getKey()).orElse("");

  }

  public static void main(String[] args) {
    System.out.println(solve("swiss"));
  }