String->Date || Date->String

1.Date turn String

　　　　Date date = new Date();
       SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
       String dateTime = sdf.format(date);

2.String turn Date

　　　　String dateTime = "2019-01-01 01:01:01";
       SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
       try {
           Date date = sdf.parse(dateTime);
       } catch (ParseException e) {
           e.printStackTrace ();
       }