public static void main(String[] args) {
        List doubles = Arrays.asList(new Double(3.3), new Double(3.3), new Double(3.3));
        double sum = doubles.stream().mapToDouble(Double::doubleValue).sum();
        System.out.println(sum);// 9.899999999999999
        System.out.println(formatDigit(sum,2));// 9.9
        System.out.println(formatDigit_down(sum, 2)); // 9.89
        List doubles1 = Arrays.asList(new Double(1.1), new Double(1.1), new Double(1.1));
        double sum1 = doubles1.stream().mapToDouble(Double::doubleValue).sum();
        System.out.println(sum1);// 3.3000000000000003
        System.out.println(formatDigit(sum1,2));// 3.3
        System.out.println(formatDigit_down(sum1, 2));// 3.3
    }
    /**
     * 将数据转换为保留指定小数位数（0，1，2）格式的数，四舍五入
     *
     */
    public static double formatDigit(double num, int decimalPlace) {
        DecimalFormat fm = null;
        switch (decimalPlace) {
            case 0:
                fm = new DecimalFormat("##");
                break;
            case 1:
                fm = new DecimalFormat("##.#");
                break;
            case 2:
                fm = new DecimalFormat("##.##");
                break;
            case 3:
                fm = new DecimalFormat("##.###");
                break;
            case 4:
                fm = new DecimalFormat("##.####");
                break;
            default:
                break;
        }

        if (fm == null) {
            return num;
        }

        StringBuffer sbf = new StringBuffer();
        fm.format(num, sbf, new FieldPosition(java.text.NumberFormat.FRACTION_FIELD));
        return Double.parseDouble(sbf.toString());
    }

    /**
     * 将数据转换为保留指定小数位数（0，1，2）格式的数。向下取值：如 5.567 -> 5.56 -5.567 -> -5.56
     */
    public static double formatDigit_down(double num, int decimalPlace) {
        DecimalFormat fm = null;
        switch (decimalPlace) {
            case 0:
                fm = new DecimalFormat("##");
                break;
            case 1:
                fm = new DecimalFormat("##.#");
                break;
            case 2:
                fm = new DecimalFormat("##.##");
                break;
            case 3:
                fm = new DecimalFormat("##.###");
                break;
            case 4:
                fm = new DecimalFormat("##.####");
                break;
            default:
                break;
        }

        if (fm == null) {
            return num;
        }

        StringBuffer sbf = new StringBuffer();
        fm.setRoundingMode(RoundingMode.DOWN);// 向下取值设置、
        fm.format(num, sbf, new FieldPosition(java.text.NumberFormat.FRACTION_FIELD));
        return Double.parseDouble(sbf.toString());
    }

文章题目：Lambda表达式mapToDouble.sum精度问题
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