Java中常用的获取从当前月开始的前第i个月、取结束时间与开始时间相差多少个月份等的方法

@RunWith(SpringRunner.class)
@SpringBootTest
public class DateTest {
    @Test
    public void test(){
        DateFormat dfs = new SimpleDateFormat("yyyy-MM");
        String date1 = dfs.format(new Date(Long.valueOf("1546327530000")));
        String date2 = dfs.format(new Date(Long.valueOf("1559373930000")));
      //  getLastMonths(1);
      //  getDifference(date1,date2);
      //  getMouths(new Date(),1);

        getEveryDay(new Date(),7L);
    }

    /**
     * 获取从当前月开始的前第i个月
     * @param i
     * @return
     */
    public String getLastMonths(int i) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM");
        Calendar c = Calendar.getInstance();
        c.setTime(new Date());
        c.add(Calendar.MONTH, -i);
        Date m = c.getTime();
        System.out.println(sdf.format(m));
        return sdf.format(m);
    }

    /**
     * 获取结束时间与开始时间相差多少个月份
     * @param start
     * @param end
     * @return
     */
    public int getDifference(String start, String end) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM");
        String str1 = start.substring(0, 7);
        String str2 = end.substring(0, 7);
        Calendar bef = Calendar.getInstance();
        Calendar aft = Calendar.getInstance();
        try {
            bef.setTime(sdf.parse(str1));
            aft.setTime(sdf.parse(str2));
        } catch (ParseException e) {
            e.printStackTrace();
        }
        int result = aft.get(Calendar.MONTH) - bef.get(Calendar.MONTH);
        int month = (aft.get(Calendar.YEAR) - bef.get(Calendar.YEAR)) * 12;
        System.out.println(Math.abs(month + result));
        return Math.abs(month + result);
    }

    /**
     * 获取某个月份在i个月之前的月份
     * @param date
     * @param i
     * @return
     */

    public String getMouths(Date date,int i){
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM");
        Calendar c = Calendar.getInstance();
        c.setTime(date);
        c.add(Calendar.MONTH, Integer.valueOf("-"+i));
        Date m = c.getTime();
        String mon = format.format(m);
        System.out.println("过去一个月："+mon);
        return mon;
    }

    /**
     * 计算两个时间点相差多少天、多少小时、多少分钟
     * @param date1
     * @param date2
     * @return
     * @throws ParseException
     */
    public Long getDay(Date date1,Date date2) throws ParseException {
        long nd = 1000 * 24 * 60 * 60;//每天毫秒数
        long nh = 1000 * 60 * 60;//每小时毫秒数
        long nm = 1000 * 60;//每分钟毫秒数
        long diff = date2.getTime() - date1.getTime(); // 获得两个时间的毫秒时间差异
        long day = diff / nd;   // 计算差多少天
        long hour = diff % nd / nh; // 计算差多少小时
        long min = diff % nd % nh / nm;  // 计算差多少分钟
        return  day;
    }

    /**
     *  计算某个时间点在i天之前的时间点
     *
     * @param date
     * @param i
     * @return
     */
    public String getEveryDay(Date date,Long i){
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
        Calendar c = Calendar.getInstance();
        c.setTime(date);
        c.add(Calendar.DATE, Integer.valueOf("-"+i));
        Date d = c.getTime();
        String day = format.format(d);
        System.out.println("过去七天："+day);
        return day;
    }
}