java 关于时间

①String与Date的相互转换

//String 转换为 Date
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
ParsePosition pos = new ParsePosition(0);
Date d =sdf.parse("2018-10-1 12:11:22",pos);

//Date 转换为 String 
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String str = sdf.format(new Date());

SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy-MM-dd");
        ParsePosition pos = new ParsePosition(0);
        Calendar c = Calendar.getInstance();
        Date today =sdf1.parse(sdf2.format(new Date()) + "21:00:00",pos);       //当天 21:00:00
        c.setTime(today);
        c.add(Calendar.DATE,-1);
        Date yesterday = c.getTime();
        c.setTime(today);
        c.add(Calendar.DATE,-2);
        Date beforeYesterday = c.getTime();