如何使用“ DateFormat”将String转换为Date对象？

Question

I am trying to convert the date/Time of my machine into GMT time zone by using the following code:

DateFormat gmtFormat = new SimpleDateFormat("E MMM dd HH:mm:ss 'GMT' yyyy");
TimeZone gmtTime = TimeZone.getTimeZone("GMT");
gmtFormat.setTimeZone(gmtTime);
System.out.println("Orginal Date : " + new Date());
String s= gmtFormat.format(new Date());
System.out.println("Converted Date As String: "+ s);
System.out.println("Converted Date As date object : " + gmtFormat.parse(s));

Above code output :

Orginal Date : Mon Apr 21 21:04:06 AST 2014
Converted Date As String: Mon Apr 21 18:04:06 GMT 2014
Converted Date As date object : Mon Apr 21 21:04:06 AST 2014

However my problem when I parse the " Converted Date As String " to " Converted Date As date object " by using the parse function, but as you can notice that the date changes to the original date

Why is that happen ?

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The problem is solve, when I do the following :

DateFormat gmtFormat = new SimpleDateFormat("E MMM dd HH:mm:ss 'GMT' yyyy");
TimeZone gmtTime = TimeZone.getTimeZone("GMT");
gmtFormat.setTimeZone(gmtTime);
System.out.println("Orginal Date : " + new Date());
String s= gmtFormat.format(new Date());
System.out.println("Converted Date As String: "+ s);
// Just I added Local Time Zone Formatand I use it to call parse instead of gmtFormat
DateFormat LocalFormat = new SimpleDateFormat("E MMM dd HH:mm:ss 'GMT' yyyy");
System.out.println("Converted Date As date object : " + LocalFormat.parse(s));

I am wondering why thats happen >> How the Local format related to parse Function ? Does any one know why?

Answer 1

The return value from DateFormat.parse(String) is a java.util.Date , which does not retain any information about time zone or locale. A Date is essentially a wrapper object around a long value representing a number of milliseconds since January 1, 1970 at 00:00 GMT. When its toString() method is invoked, the default implementation renders the date in your local system's default timezone.

If you want to create a date/time representation which stores both a timestamp and an associated time zone / locale, you should create a java.util.Calendar object (using Calendar.getInstance(...) and provide it with a TimeZone and/or a Locale , then set the time associated with the calendar with the Date object representing the date/time you want.