Java常见帮助类（9）日期帮助类（一）

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public class DateUtils{        /**     * 获取时间     *     * @param num +今天后某天  -今天前某几天     * @return     */    public static String getNextDateStr(int num)    {        Date date = new Date();        Calendar cal = Calendar.getInstance();        cal.add(Calendar.DATE, num);        date = cal.getTime();        SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd");        String dateString = formatter.format(date);        return dateString;    }        /**     * 获取时间     *     * @param num +今天后某天  -今天前某几天     * @return     */    public static String getNextDateStr(String day, int num) throws Exception    {        SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd");        Date date = formatter.parse(day);        Calendar cal = Calendar.getInstance();        cal.setTime(date);        cal.add(Calendar.DATE, num);        date = cal.getTime();        String dateString = formatter.format(date);        return dateString;    }        /**     * 获取本年第一天     *     * @return     */    public static String getYear()    {        Date date = new Date();        SimpleDateFormat formatter = new SimpleDateFormat("yyyy");        String dateString = formatter.format(date) + "-01-01";        return dateString;    }        /**     * 获取本月第一天     *     * @return     */    public static String getMonth()    {        Date date = new Date();        SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM");        String dateString = formatter.format(date) + "-01";        return dateString;    }        /**     * 获取两个时间相差的天数     *     * @param BT     * @param ET     * @return     * @throws ParseException     */    public static long getDiffDay(String BT, String ET) throws ParseException    {        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");        Date startTime = sdf.parse(BT);        Date endTime = sdf.parse(ET);        long l = endTime.getTime() - startTime.getTime();        long day = l / (24 * 60 * 60 * 1000);        return day;    }        public static void main(String[] args) throws Exception    {        String today = DateUtils.getNextDateStr(0);        String Yesterday = DateUtils.getNextDateStr( -1);        String last7day = DateUtils.getNextDateStr( -6);        String last30day = DateUtils.getNextDateStr( -29);        String beginyear = DateUtils.getYear();                System.out.println(today);        System.out.println(Yesterday);        System.out.println(last7day);        System.out.println(last30day);        System.out.println(beginyear);                System.out.println(getDiffDay("2016-02-01", "2017-02-11"));        System.out.println(getNextDateStr("2016-02-01", 1));    }}
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