java得到当前日期的前一天或后一天

public String getNextDay(String startdate) throws ParseException{
Date date = (new SimpleDateFormat("yyyy-MM-dd")).parse(startdate);
Calendar cal = Calendar.getInstance();
cal.setTime(date);
cal.add(Calendar.DATE,1);
date =cal.getTime();
startdate=(new SimpleDateFormat("yyyy-MM-dd")).format(date);
return startdate;
}

public String getLastDay(String startdate) throws ParseException{
Date date = (new SimpleDateFormat("yyyy-MM-dd")).parse(startdate);
Calendar cal = Calendar.getInstance();
cal.setTime(date);
cal.add(Calendar.DATE,-1);
date =cal.getTime();
startdate=(new SimpleDateFormat("yyyy-MM-dd")).format(date);
return startdate;
}