JAVA得到某个日期的前几天或者后几天

 SimpleDateFormat   sdf=new   SimpleDateFormat( "yyyyMMdd ");
String   str= "20011230 ";
Date   dt=sdf.parse(str,new   ParsePosition(0));
Calendar   rightNow   =   Calendar.getInstance();
rightNow.setTime(dt);
rightNow.add(Calendar.DATE,2);//你要加减的日期  
Date   dt1=rightNow.getTime();
String   reStr=sdf.format(dt1, " ",new   FieldPosition(0));
System.out.println(reStr);