/**
     * 前提，开始时间<结束时间
     * 天数：计算1.1-1.1 ，1.1-1.2 ，1.3-1.4 一共有2个区间1.1-1.2 ，1.3-1.4 共4 天
     * @param list 时间区间列表
     */
    public static void distinctDate(List<TestModel> list) {
        System.out.println(Arrays.toString(list.toArray()));
        //开始时间升序排序,保证数组下标越大，开始时间越大
        list.sort((o1, o2) -> o1.getStartDate().compareTo(o2.getEndDate()));

        System.out.println(Arrays.toString(list.toArray()));
        //时间区间结果，数组下标越大，开始时间越大
        List<TestModel> newestModelList = new ArrayList<>();

        for (TestModel model : list) {
            //时间区间结果为空，直接加入数组
            if (newestModelList.size() == 0) {
                newestModelList.add(model);
            } else {
                //最新时间区间
                TestModel newestModel = newestModelList.get(newestModelList.size() - 1);
                //如果开始时间小于等于最新时间区间的结束时间，
                //而且结束时间大于最新时间区间的结束时间，
                //将当前结束时间赋值给最新时间区间的结束时间
                if (model.getStartDate().getTime() <= newestModel.getEndDate().getTime()) {
                    if (model.getEndDate().getTime() > newestModel.getEndDate().getTime()) {
                        newestModel.setEndDate(model.getEndDate());
                    }
                } else {
                    //如果开始时间大于最新结束时间，加入新的时间区间
                    newestModelList.add(model);
                }
            }
        }
        System.out.println(Arrays.toString(newestModelList.toArray()));
        int daySum = 0;
        for (TestModel model : newestModelList) {
            daySum += (int) ((model.getEndDate().getTime() - model.getStartDate().getTime()) / 86400000) + 1;
        }
        System.out.println(daySum);
    }

实体模型

static class TestModel {
        private Timestamp startDate;
        private Timestamp endDate;

        public TestModel(Timestamp startDate, Timestamp endDate) {
            this.startDate = startDate;
            this.endDate = endDate;
        }

        public Timestamp getStartDate() {
            return startDate;
        }

        public void setStartDate(Timestamp startDate) {
            this.startDate = startDate;
        }

        public Timestamp getEndDate() {
            return endDate;
        }

        public void setEndDate(Timestamp endDate) {
            this.endDate = endDate;
        }
    }

字符串转Timestamp

    public static Timestamp convertToTimestampByString(String source) {
        if (StringUtils.isEmpty(source)) {
            return null;
        }
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        Date date;
        try {
            date = sdf.parse(source);
            return new Timestamp(date.getTime());
        } catch (ParseException e) {

        }
        return null;
    }

调用

public static void main(String[] args) {
        Timestamp startDate3 = convertToTimestampByString("2021-12-20");
        Timestamp endDate3 = convertToTimestampByString("2021-12-21");
        Timestamp startDate2 = convertToTimestampByString("2021-12-21");
        Timestamp endDate2 = convertToTimestampByString("2021-12-22");
        Timestamp startDate1 = convertToTimestampByString("2021-12-21");
        Timestamp endDate1 = convertToTimestampByString("2021-12-23");

        TestModel testModel = new TestModel(startDate1, endDate1);
        TestModel testMode2 = new TestModel(startDate2, endDate2);
        TestModel testMode3 = new TestModel(startDate3, endDate3);

        List<TestModel> list = new ArrayList<>();
        list.add(testModel);
        list.add(testMode2);
        list.add(testMode3);
        distinctDate(list);
    }

结果：

[Test.TestModel(startDate=2021-12-21 00:00:00.0, endDate=2021-12-23 00:00:00.0), Test.TestModel(startDate=2021-12-21 00:00:00.0, endDate=2021-12-22 00:00:00.0), Test.TestModel(startDate=2021-12-20 00:00:00.0, endDate=2021-12-21 00:00:00.0)]
[Test.TestModel(startDate=2021-12-20 00:00:00.0, endDate=2021-12-21 00:00:00.0), Test.TestModel(startDate=2021-12-21 00:00:00.0, endDate=2021-12-22 00:00:00.0), Test.TestModel(startDate=2021-12-21 00:00:00.0, endDate=2021-12-23 00:00:00.0)]
[Test.TestModel(startDate=2021-12-20 00:00:00.0, endDate=2021-12-23 00:00:00.0)]
4