First repeated character in string (2)

public class Main {

    public static void main(String[] args) {

        System.out.println(firstRepeatedChar("dictionary"));    // i
        System.out.println(firstRepeatedChar("abcdefgh"));      // null
    }

    public static char firstRepeatedChar(String str){
        if(str.length() < 2){
            return '\0';
        }
        String s = str.toLowerCase();  // Optional

        boolean[] dictionary = new boolean[256];
        for (int i = 0; i < s.length(); i++) {
            char current = s.charAt(i);
            if(!dictionary[current]){
                dictionary[current] = true;
            } else {
                return current;
            }
        }
        return '\0';
    }

}