code: public static String sqrt(String value) {
          try {
              BigDecimal bd = new BigDecimal(value);
              return Math.sqrt(bd.doubleValue()) + "";
          } catch (Exception ex) {
              return null;
          }
      }
docs: Returns the correctly rounded positive square root of a {@code double}
  value.
  
  Any numeric string recognized by {@code BigDecimal} is supported.
  
  @param value A valid number
  @return the positive square root of {@code value}.