如何在Java中找出第一个不重复的字符？

public class FindNonRepeatingChar {     public static void main(String[] args) {         System.out.println(printFirstNonRepeatingChar("沉默王沉沉默二"));         System.out.println(printFirstNonRepeatingChar("沉默王沉"));         System.out.println(printFirstNonRepeatingChar("沉沉沉"));     }      private static Character printFirstNonRepeatingChar(String string) {         char[] chars = string.toCharArray();          List<Character> discardedChars = new ArrayList<>();          for (int i = 0; i < chars.length; i++) {             char c = chars[i];              if (discardedChars.contains(c))                 continue;              for (int j = i + 1; j < chars.length; j++) {                 if (c == chars[j]) {                     discardedChars.add(c);                     break;                 } else if (j == chars.length - 1) {                     return c;                 }             }         }         return null;     } }

王 默 null