code: public static String toHalfWidthString(CharSequence str) {
          StringBuilder sb = new StringBuilder();
          for (int i = 0; i < str.length(); i++) {
              sb.append(toHalfWidthCharacter(str.charAt(i)));
          }
          return sb.toString();
      }
docs: 转换为半角字符串
  
  @param str
  待转换字符串
  @return 转换后的字符串