code: public static String camelhumpToUnderline(String str) {
          final int size;
          final char[] chars;
          final StringBuilder sb = new StringBuilder(
                  (size = (chars = str.toCharArray()).length) * 3 / 2 + 1);
          char c;
          for (int i = 0; i < size; i++) {
              c = chars[i];
              if (isUppercaseAlpha(c)) {
                  sb.append('_').append(toLowerAscii(c));
              } else {
                  sb.append(c);
              }
          }
          return sb.charAt(0) == '_' ? sb.substring(1) : sb.toString();
      }
docs: 将驼峰风格替换为下划线风格