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16 Aug 2018
java8 Date/Time Formatting/Parsing

网上搜索出的结果大部分都是通过java.text.DateFormat和java.text.SimpleDateFormat来format和parse java date and time。

DateFormat/SimpleDateFormat与DateTimeFormatter的比较

DateTimeFormatter is threadsafe and immutable.

传统实现方式：

private final String dateTimeFormatPattern = "yyyy/MM/dd HH:mm:ss z";
private final Date now = new Date();

private final String dateTimeString = "2014/09/03 13:59:50 MDT";

public void demonstrateSimpleDateFormatFormatting()
{
   final DateFormat format = new SimpleDateFormat(dateTimeFormatPattern);
   final String nowString = format.format(now);
   out.println(
        "Date '" + now + "' formatted with SimpleDateFormat and '"
      + dateTimeFormatPattern + "': " + nowString);
}


public void demonstrateSimpleDateFormatParsing()
{
   final DateFormat format = new SimpleDateFormat(dateTimeFormatPattern);
   try
   {
      final Date parsedDate = format.parse(dateTimeString);
      out.println("'" + dateTimeString + "' is parsed with SimpleDateFormat as " + parsedDate);
   }
   // DateFormat.parse(String) throws a checked exception
   catch (ParseException parseException)
   {
      out.println(
           "ERROR: Unable to parse date/time String '"
         + dateTimeString + "' with pattern '"
         + dateTimeFormatPattern + "'.");
   }
}

java8 的实现方式：
date/time类不再使用java.util包，而是java.time包，同样的，date/time formatting/parsing类不再是java.text包，而是来自java.time.format包。

The java.time package offers numerous classes for modeling dates and/or times. These include classes :

  1. model dates only (no time information),
  2. model times only (no date information),
  3. model date and time information,
  4. use timezone information,
  5. do not incorporate time zone information.
private final ZonedDateTime now8 = ZonedDateTime.now();
private final String dateTimeFormatPattern = "yyyy/MM/dd HH:mm:ss z";

public void demonstrateDateTimeFormatFormatting()
{
   final DateTimeFormatter formatter =
      DateTimeFormatter.ofPattern(dateTimeFormatPattern);
   final String nowString = formatter.format(now8);
   out.println(
        now8 + " formatted with DateTimeFormatter and '"
      + dateTimeFormatPattern + "': " + nowString);
}


public void demonstrateDateTimeFormatParsingTemporalStaticMethod()
{
   final DateTimeFormatter formatter =
      DateTimeFormatter.ofPattern(dateTimeFormatPattern);
   final ZonedDateTime zonedDateTime = ZonedDateTime.parse(dateTimeString, formatter);
   out.println(
        "'" + dateTimeString
      + "' is parsed with DateTimeFormatter and ZonedDateTime.parse as "
      + zonedDateTime);
}


public void demonstrateDateTimeFormatFormattingAndParsingInstant()
{
   // Instant instances don't have timezone information
   final Instant instant = now.toInstant();
   final DateTimeFormatter formatter =
      DateTimeFormatter.ofPattern(
         dateTimeFormatPattern).withZone(ZoneId.systemDefault());
   final String formattedInstance = formatter.format(instant);
   out.println(
        "Instant " + instant + " formatted with DateTimeFormatter and '"
      + dateTimeFormatPattern + "' to '" + formattedInstance + "'");
   final Instant instant2 =
      formatter.parse(formattedInstance, ZonedDateTime::from).toInstant();
      out.println(formattedInstance + " parsed back to " + instant2);
}

Til next time,
at 10:42

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