DateTimeFormatter dateTimeFormatter1 = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
DateTimeFormatter dateTimeFormatter2 = DateTimeFormatter.ofPattern("yyyy-MM-dd");

LocalDateTime localDateTime = LocalDateTime.parse("2019-07-31 00:00:00",dateTimeFormatter1);
LocalDate localDate = LocalDate.parse("2019-07-31",dateTimeFormatter2);
Date date = Date.from(LocalDateTime.parse("2019-07-31 00:00:00",dateTimeFormatter1).atZone(ZoneId.systemDefault()).toInstant());


String strDateTime = "2019-07-31 00:00:00";
String strDate = "2019-07-31";
Long timestamp=1564502400000l;

/** LocalDateTime 转 LocalDate */
System.out.println("LocalDateTime 转 LocalDate: "+localDateTime.toLocalDate());
/** LocalDateTime 转 Long */
System.out.println("LocalDateTime 转 Long: "+localDateTime.atZone(ZoneId.systemDefault()).toInstant().toEpochMilli());
/** LocalDateTime 转 Date */
System.out.println("LocalDateTime 转 Date: "+Date.from(localDateTime.atZone(ZoneId.systemDefault()).toInstant()));
/** LocalDateTime 转 String */
System.out.println("LocalDateTime 转 String: "+localDateTime.format(dateTimeFormatter1));

System.out.println("-------------------------------");

/** LocalDate 转 LocalDateTime */
System.out.println("LocalDate 转 LocalDateTime: "+LocalDateTime.of(localDate,LocalTime.parse("00:00:00")));
/** LocalDate 转 Long */
System.out.println("LocalDate 转 Long: "+localDate.atStartOfDay(ZoneId.systemDefault()).toInstant().toEpochMilli());
/** LocalDate 转 Date */
System.out.println("LocalDate 转 Date: "+Date.from(localDate.atStartOfDay().atZone(ZoneId.systemDefault()).toInstant()));
/** LocalDate 转 String */
System.out.println("LocalDateTime 转 String: "+localDateTime.format(dateTimeFormatter2));

System.out.println("-------------------------------");

/** Date 转 LocalDateTime */
System.out.println("Date 转 LocalDateTime: "+LocalDateTime.ofInstant(date.toInstant(), ZoneId.systemDefault()));
/** Date 转 Long */
System.out.println("Date 转 Long: "+date.getTime());
/** Date 转 LocalDate */
System.out.println("Date 转 LocalDateTime: "+LocalDateTime.ofInstant(date.toInstant(), ZoneId.systemDefault()).toLocalDate());
/** Date 转 String */
SimpleDateFormat sdf =new SimpleDateFormat("yyyy-MM-dd HH:mm:ss SSS" );
System.out.println("Date 转 String: "+sdf.format(date));

System.out.println("-------------------------------");

/** String 转 LocalDateTime */
System.out.println("String 转 LocalDateTime: "+LocalDateTime.parse(strDateTime,dateTimeFormatter1));
/** String 转 LocalDate */
System.out.println("String 转 LocalDate: "+LocalDateTime.parse(strDateTime,dateTimeFormatter1).toLocalDate());
System.out.println("String 转 LocalDate: "+LocalDate.parse(strDate,dateTimeFormatter2));
/** String 转 Date */
System.out.println("String 转 Date: "+Date.from(LocalDateTime.parse(strDateTime,dateTimeFormatter1).atZone(ZoneId.systemDefault()).toInstant()));

System.out.println("-------------------------------");

/** Long 转 LocalDateTime */
System.out.println("Long 转 LocalDateTime:"+LocalDateTime.ofInstant(Instant.ofEpochMilli(timestamp), ZoneId.systemDefault()));
/** Long 转 LocalDate */
System.out.println("Long 转 LocalDate:"+LocalDateTime.ofInstant(Instant.ofEpochMilli(timestamp), ZoneId.systemDefault()).toLocalDate());

日期比较       
 LocalDateTime now = LocalDateTime.now();//2020-09-11T09:41:01.547
        LocalDateTime DiscountStartTime = now.minusDays(3);//减法 2020-09-08T09:41:01.547
        LocalDateTime DiscountEndTime = now.plusDays(3);//加法 2020-09-14T09:41:01.547

//        now>DiscountStartTime && DiscountEndTime>now
//        DiscountEndTime>now>DiscountStartTime
        if(now.isAfter(DiscountStartTime) && DiscountEndTime.isAfter(now)){
            true;//输出true
        }