保留有效位数的算法

/**
     * 保留有效位数
     * @param s 原始数据
     * @param significantDigit 需要保留的有效位数
     * @return 保留significantDigit位后的数
     */
    private String handleSignificantDigit(String s, int significantDigit) {
        int length = s.length();
        int i = 0;
        while (i < length) {
            if (s.charAt(i) == '.') {
                break;
            }
            i++;
        }
        if ((length - i - 1) <= significantDigit) {
            StringBuilder sb = new StringBuilder(s);
            for (int j = 0; j < significantDigit - (length - i - 1); j++) {
                sb.append('0');
            }
            return sb.toString();
        }
        else {
            if (s.charAt(i + significantDigit + 1) < '4') {
                return s.substring(i + significantDigit + 1);
            }
            else {
                double num = Double.parseDouble(s);
                StringBuilder pattern = new StringBuilder("0.");
                for (int j = 0; j < significantDigit; j++) {
                    pattern.append('0');
                }
                DecimalFormat decimalFormat = new DecimalFormat(pattern.toString());
                String tmp = decimalFormat.format(num);
                return tmp;
            }
        }
    }