JAVA保留小数点后N位，不足N位的用0补

    private String roundByScale(double v, int scale) {
        if (scale < 0) {
            throw new IllegalArgumentException("The scale must be a positive integer or zero");
        }
        if (scale == 0) {
            return new DecimalFormat("0").format(v);
        }
        String formatStr = "0.";
        for (int i = 0; i < scale; i++) {
            formatStr = formatStr + "0";
        }
        return new DecimalFormat(formatStr).format(v);
    }