Java获取指定几天前的或几天后的时间

负数为当前时间往前-1为昨天的当前时间
Calendar c = Calendar.getInstance();
c.add(Calendar.DAY_OF_MONTH, -1);
System.out.println(formatter.format(c.getTime()));

获取次日凌晨
Calendar calendar = Calendar.getInstance();
calendar.add(Calendar.DAY_OF_MONTH, -1);
calendar.set(Calendar.HOUR_OF_DAY, 0);
calendar.set(Calendar.MINUTE, 0);
calendar.set(Calendar.SECOND, 0);
calendar.set(Calendar.MILLISECOND, 0);
System.out.println(formatter.format(calendar.getTimeInMillis()));

根据当前年月日获取是周几
SimpleDateFormat sdf = new SimpleDateFormat("EEEE");
String week = sdf.format(date);