docs: public String getChinese(String str) {
          if (str == null) return null;
  
          StringBuilder sb = new StringBuilder();
          for (int i = 0; i < str.length(); i++) {
              char ch = str.charAt(i);
              if (getPinyinFromChar(ch) != null) {
                  sb.append(ch);
              }
          }
          return sb.toString();
      }