JAVA给定日期,计算前几天后几天的日期，计算两个日期之间的天数

/**
* 指定日期后几天
*
* @param startDay 指定日期
* @param count    天数
* @return
*/
String getAfterDay(String startDay, int count) {
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
try {
Date date = sdf.parse(startDay);
Calendar cl = Calendar.getInstance();
cl.setTime(date);
cl.add(Calendar.DATE, count);
return sdf.format(cl.getTime());
} catch (ParseException e) {
e.printStackTrace();
}
return "";
}

/**
* 指定日期前几天
*
* @param startDay 指定日期
* @param count    天数
* @return
*/
String getBeforeDay(String startDay, int count) {
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
try {
Date date = sdf.parse(startDay);
Calendar cl = Calendar.getInstance();
cl.setTime(date);
cl.set(Calendar.DATE, cl.get(Calendar.DATE) - count);
return sdf.format(cl.getTime());
} catch (ParseException e) {
e.printStackTrace();
}
return "";
}

/**
*
* @param startDay 开始日期
* @param endDay 截止日期
* @return
*/
public static int daysBetween(String startDay, String endDay) {
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Date date1 = null;
Date date2 = null;
try {
date1 = sdf.parse(startDay);
date2 = sdf.parse(endDay);
} catch (ParseException e) {
e.printStackTrace();
}
Calendar cal = Calendar.getInstance();
cal.setTime(date1);
cal.set(Calendar.HOUR_OF_DAY, 0);
cal.set(Calendar.MINUTE, 0);
cal.set(Calendar.SECOND, 0);
long time1 = cal.getTimeInMillis();
cal.setTime(date2);
cal.set(Calendar.HOUR_OF_DAY, 0);
cal.set(Calendar.MINUTE, 0);
cal.set(Calendar.SECOND, 0);
long time2 = cal.getTimeInMillis();
long between_days = (time2 - time1) / (1000 * 3600 * 24);
return Integer.parseInt(String.valueOf(between_days));
}