怎么获取当前的前一天和后一天的日期啊?

//type 是判断输出格式，i为正就是之后，为负表示之前
public static String dateAdd(String type, int i) {
SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String str = formatDateTimeNow("yyyy-MM-dd HH:mm:ss");
Calendar c = Calendar.getInstance();
if (type.equals("s")) {
int s = c.get(Calendar.SECOND);
s = s + i;
c.set(Calendar.SECOND, s);
str = df.format(c.getTime());
} else if (type.equals("d")) {
int d = c.get(Calendar.DAY_OF_MONTH);
d = d + i;
c.set(Calendar.DAY_OF_MONTH, d);
str = df.format(c.getTime());
}
return str;
}