Java 8 LocalDateTime type conversion stuff (String, java.util.Date)

Introduction

Date and Calendar classes have been used as Java date types, but it is recommended to use LocalDateTime / ZonedDateTime classes etc. from Java 8 onwards.

This time, I had the opportunity to use the LocalDateTime class, so I studied it. It turned out that there is no method that can be converted in one shot, and it is not possible to easily convert the type.

This time, I organized String⇔LocalDateTime and java.util.Date⇔LocalDateTime in my own way.

String → LocalDateTime

When using the SimpleDateFormat class

Convert in the flow of String → SimpleDateFormat → Date → LocalDateTime.

    public static LocalDateTime toLocalDateTime(String date, String format) throws ParseException {
        SimpleDateFormat sdf = new SimpleDateFormat(format);
        Date formatDate = sdf.parse(date);
        return LocalDateTime.ofInstant(formatDate.toInstant(), ZoneId.systemDefault());
    }

Usage example

    public static void main(String args[]) {
        //Execution result: 2018-05-07T00:00
        System.out.println(toLocalDateTime("2018/05/07", "yyyy/MM/dd"));
    }

When using the DateTimeFormatter class

Unlike SimpleDateFormat, the DateTimeFormatter class does not have to be new. It is necessary to determine the date pattern with the ofPattern method at the time of generation.

Convert in the flow of String → DateTimeFormatter → LocalDateTime. Can be shorter than using SimpleDateFormat.

    public static LocalDateTime toLocalDateTime(String date, String format) {

        DateTimeFormatter dtf = DateTimeFormatter.ofPattern(format);
        return LocalDateTime.parse(date, dtf);
    }

Usage example

    public static void main(String args[]) {
        //Execution result: 2018-05-07T10:00
        System.out.println(toLocalDateTime("2018/05/07 10:00:00","yyyy/MM/dd HH:mm:ss"));
    }

However, since it is a LocalDateTime that has a date and time, an error will occur if you do not specify the time.

    public static void main(String args[]) {
        //Execution result: DateTimeParseException
        System.out.println(toLocalDateTime("2018/05/07","yyyy/MM/dd"));
    }

So, if you only need the date, try returning LocalDate, or

    public static LocalDate toLocalDate(String date, String format) {

        DateTimeFormatter dtf = DateTimeFormatter.ofPattern(format);
        return LocalDate.parse(date, dtf);

    }

After converting to LocalDate, add 0 o'clock to make it LocalDateTime.

I told you in the comments.

    public static LocalDateTime toLocalDateTime(String date, String format) {

        DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern(format);
        return LocalDate.parse(date, dateTimeFormatter).atTime(LocalTime.MIN);

    }

LocalDateTime → String ~~ This can be converted in one shot by using the parse method. ~~ I'm sorry, it's a lie. It can be converted with format method + DateTimeFormatter.

    public static String toStr(LocalDateTime localDateTime, String format) {

        DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern(format);
        return localDateTime.format(dateTimeFormatter);

    }

Usage example

    public static void main(String args[]) {
        //Execution result: 2018/05/07
        System.out.println(toStr(LocalDateTime.now(), "yyyy/MM/dd"));
    }

LocalDateTime → java.util.Date Convert in the flow of LocalDateTime → ZonedDateTime → Instant → java.util.Date.

    public static Date toDate(LocalDateTime localDateTime) {
        ZoneId zone = ZoneId.systemDefault();
        ZonedDateTime zonedDateTime = ZonedDateTime.of(localDateTime, zone);
        Instant instant = zonedDateTime.toInstant();
        return Date.from(instant);
    }

Usage example

    public static void main(String args[]) {
        //Execution result: Mon May 07 19:59:32 JST 2018
        System.out.println(toDate(LocalDateTime.now()));
    }

java.util.Date → LocalDateTime

Convert in the flow of Date → Instant → Local Date Time.

    public static LocalDateTime toLocalDateTime(Date date) {
        Instant instant = date.toInstant();
        return LocalDateTime.ofInstant(instant, ZoneId.systemDefault());
    }

Usage example

    public static void main(String args[]) {
        // 2018-05-07T19:59:32.139
        System.out.println(toLocalDateTime(new Date()));
    }

Finally

LocalDateTime is difficult to handle. (Small feeling) It may be better to use LocalDate and LocalTime properly. Variables are declared line by line for ease of understanding (for me), but of course it is better to do everything in one line as told in the comments.

public static void main(String args[]) {
    // String→LocalDateTime
    System.out.println(LocalDate.parse("2018/05/07", 
                           DateTimeFormatter.ofPattern("uuuu/MM/dd"))
                       .atTime(LocalTime.MIN));
}

Question

I haven't fully grasped the difference between SimpleDateFormat and DateTimeFormatter, so I will investigate it later.