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Java Calendar always shows the same time

Below is my code.
public class TestCalendar {
public static void main(String[] args){
int unique_id = Integer.parseInt("" + Calendar.HOUR + Calendar.MINUTE
+ Calendar.SECOND);
System.out.println(unique_id);
}
}
Calendar.HOUR is supposed to give me
public static final int HOUR Field number for get and set indicating the hour of the morning or
afternoon. HOUR is used for the 12-hour clock (0 - 11). Noon and midnight are represented by 0, not
by 12. E.g., at 10:04:15.250 PM the HOUR is 10.
It doesnt matter how many times I run this code, it always gives me the same unique_id. (101213) and my local time on my machine is 1:30pm. What am I doing wrong here?
Thanks.

Your code is just concatenating constants, that the Calendar defines to identify some of it's fields. To get values of these fields, call Calendar.get() and pass the constant identifier as an argument:
public class TestCalendar {
public static void main(String[] args){
Calendar c = Calendar.getInstance();
int unique_id = Integer.parseInt("" + c.get(Calendar.HOUR) + c.get(Calendar.MINUTE)
+ c.get(Calendar.SECOND));
System.out.println(unique_id);
}
}
The above would work, but the result will be far from unique ID.
To get an ID uniquely identifying a point in time (with the precision of milliseconds), consider Calendar.getTimeInMillis().

Calendar.HOUR, Calendar.MINUTE and Calendar.SECOND are public static int field of the Calendar class. Their value is
CALENDAR.HOUR: 10
CALENDAR.MINUTE: 12 and
CALENDAR.SECOND: 13.
Your String concatenation is just appending this values. To read from a Calendar you could so something similar to
calendar.get(Calendar.HOUR);
calendar.get(Calendar.MINUTE);

I am not sure what is requirement but if you want system time then may be you can use this "System.currentTimeMillis()"

Calendar.HOUR (or MINUTE, SECOND) is just an indicator that indicates which field we want to extract from the Calendar instance, not the value, i.e. we want to 'extract HOUR from the calendar object' like below:
Calendar c = Calendar.getInstance();
int hour = c.get(Calendar.HOUR_OF_DAY); // in 24-hours,
// or c.get(Calendar.HOUR) in 12-hours.
int minute = c.get(Calendar.MINUTE);
int second = c.get(Calendar.SECOND);
int weekday = c.get(Calendar.DAY_OF_WEEK);
int weekOfYear = c.get(Calendar.WEEK_OF_YEAR);

For those who are still having some problems with this, like I did, especially when you want to print multiple times you may need to to use new like follows:
System.out.println(""+new GregorianCalendar().get(Calendar.HOUR_OF_DAY)+":"+new GregorianCalendar().get(Calendar.MINUTE)+":"+new GregorianCalendar().get(Calendar.SECOND)+":"+new GregorianCalendar().get(Calendar.MILLISECOND));

Getting a correct date by adding substracting a number of milliseconds

Today is 18/12/2013.
I was testing this program where I wanted get another date by adding/substracting a number of milliseconds from System.currentMillis(). It works well when I shift the date in decembre but it doesn't work correctly for some similar values. For example the following code gives me 2013 12 20 as the result! I wonder, no I still wonder how it could be possible! or I'm making a mistake?
public class CreateDirFiles {
public static final String PLOG_DIR = "ProductLog";
private SimpleDateFormat dateFormatter = new SimpleDateFormat();
public CreateDirFiles() {
}
public void createDayDir(Date date){
dateFormatter.applyPattern("YYYY");
String year = dateFormatter.format(date);
dateFormatter.applyPattern("MM");
String month = dateFormatter.format(date);
dateFormatter.applyPattern("dd");
String day = dateFormatter.format(date);
System.out.printf("%s %s %s\n", year, month, day);
}
public static void main(String... args){
CreateDirFiles dfc = new CreateDirFiles();
dfc.createDayDir(new Date(System.currentTimeMillis() -
((long)( 48 * 24 * 3600 * 1000)) ));
}
}

This is the problem:
((long)( 48 * 24 * 3600 * 1000))
That's doing all the arithmetic in 32 bits, and then converting the (now truncated, as the result is too large for an int) result to a long. You want:
48L * 24 * 3600 * 1000
where the L suffix means that it'll use a long for the value 48.
However, you really don't want to do this at all - you want to use Joda Time which is a much nicer API for date/time work. You really don't want to have to mess around with the low level stuff at all.
LocalDate date = ...;
LocalDate twoDaysLater = date.minusDays(48);
If you really want to stick with the built-in API, then use Calendar. At the very least use the TimeUnit enum, which will allow:
long millisFor48Days = TimeUnit.DAYS.toMillis(48);
You also need to consider the time zone - while "today" may be the 18th of December for you, it isn't elsewhere in the world.

How to write a method that returns number (int) of days from provided day to the todays date?

Please help me to write a method that returns number (int) of days from a provided day to the todays date.
So let's say, I am providing into a method an int 110515 (for May 15, 2011). It should return 9 (inclusive or exclusive is not important to me).

If you can use Joda, this is super simple:
Days d = Days.daysBetween(startDate, endDate);
int days = d.getDays();
Of course you could combine these.
int days = Days.daysBetween(startDate, endDate).getDays();
Joda objects can go back and forth between the JDK's date class pretty easily.
For the first part, make a DateFormatter then parse the string based on it, like this:
DateTimeFormatter fmt = DateTimeFormat.forPattern("yyyyMMdd");
DateTime dt = fmt.parseDateTime(strInputDateTime);
(After turning the int into a string of course.)

Should dates in the future include the current day? Meaning if today is May 24th 2011, should 110529 result in 4 or 5?
public static long numberOfDays(final long date) throws ParseException {
final Calendar compare = Calendar.getInstance();
compare.setTime(new SimpleDateFormat("yyMMdd").parse(String.valueOf(date)));
final int dstOffset = compare.get(Calendar.DST_OFFSET);
final long currentTimeMillis = System.currentTimeMillis();
final long compareTimeInMillis = compare.getTimeInMillis();
long difference = 0;
if (currentTimeMillis >= compareTimeInMillis) {
difference = currentTimeMillis - compareTimeInMillis - dstOffset;
} else {
difference = compareTimeInMillis - currentTimeMillis + dstOffset;
}
return difference / (24 * 60 * 60 * 1000);
}

Since this seems like a homework question I will help you out. You will want to use Calendar.getTimeInMillis. Then you will want to create a constant that is NUMBER_OF_MILLIS_IN_DAY . From there you subtract the initialDate from the currentDate (both time in millis) and divide by the constant.

Java library for dealing with win32 FILETIME?

Are there any Java libraries around for dealing with the win32 FILETIME/ time intervals ?
It's basically a 64 bit timestamp in 100ns intervals since January 1, 1601.
(For my particular needs, converting to/from java.util.Date or an appropriate joda time equivalent would do, although I'll need access to at least microsecond resolution - which neither seems to provide.)

If you are fine with millisecond resolution, this would work:
/** Difference between Filetime epoch and Unix epoch (in ms). */
private static final long FILETIME_EPOCH_DIFF = 11644473600000L;
/** One millisecond expressed in units of 100s of nanoseconds. */
private static final long FILETIME_ONE_MILLISECOND = 10 * 1000;
public static long filetimeToMillis(final long filetime) {
return (filetime / FILETIME_ONE_MILLISECOND) - FILETIME_EPOCH_DIFF;
}
public static long millisToFiletime(final long millis) {
return (millis + FILETIME_EPOCH_DIFF) * FILETIME_ONE_MILLISECOND;
}
At this point, converting from ms to a Date object is quite straightforward.

Here's a Java 8+ java.time based solution that keeps 100-nanosecond precision:
public static final Instant ZERO = Instant.parse("1601-01-01T00:00:00Z");
public static long fromInstant(Instant instant) {
Duration duration = Duration.between(ZERO, instant);
return duration.getSeconds() * 10_000_000 + duration.getNano() / 100;
}
public static Instant toInstant(long fileTime) {
Duration duration = Duration.of(fileTime / 10, ChronoUnit.MICROS).plus(fileTime % 10 * 100, ChronoUnit.NANOS);
return ZERO.plus(duration);
}

Last time I solved this using JNI... (Although not on Windows, this was unix)
That is, a piece of C++ code that called the native OS functions, and then called that code using Java Native Interface.
A bit clunky, but it was the only way I could find (Also needed the i-node).
EDIT: Assuming the values are already obtained from some other source, Date4J can handle seconds with 9 decimals, but it is not as feature rich as Joda.

How to do calendar operations in Java GWT? How to add days to a Date?

Since GWT does not provide the GregorianCalendar class, how to do calendar operations on the client?
I have a Date a and I want the Date, which is n days after a.
Examples:
a (2000-01-01) + n (1) -> 2000-01-02
a (2000-01-01) + n (31) -> 2000-02-01

Updated answer for GWT 2.1
final Date dueDate = new Date();
CalendarUtil.addDaysToDate(dueDate, 21);
Edit: the fully qualified name of this class is com.google.gwt.user.datepicker.client.CalendarUtil.

The answer that Google seems to use (currently), is:
#SuppressWarnings("deprecation") // GWT requires Date
public static void addDaysToDate(Date date, int days) {
date.setDate(date.getDate() + days);
}
This is from the class com.google.gwt.user.datepicker.client.CalendarUtil, which is used by com.google.gwt.user.datepicker.client.DatePicker. I imagine, that there will be problems involved, when doing calculations in different timezones.
Lots of people have already voted for some kind of Joda time for GWT: http://code.google.com/p/google-web-toolkit/issues/detail?id=603 . The currently last comment states, that there's a new fork of goda time, maybe we should really check it out.

private static final long MILLISECONDS_IN_SECOND = 1000l;
private static final long SECONDS_IN_MINUTE = 60l;
private static final long MINUTES_IN_HOUR = 60l;
private static final long HOURS_IN_DAY = 24l;
private static final long MILLISECONDS_IN_DAY = MILLISECONDS_IN_SECOND *
SECONDS_IN_MINUTE *
MINUTES_IN_HOUR *
HOURS_IN_DAY;
public Date addDays (Date date, days)
{
return new Date (date.getTime () + (days * MILLISECONDS_IN_DAY));
}
this will work with leap years but will eventually stray by milliseconds on milleniums when we add or drop leap seconds.

I've created a rough implementation that emulates TimeZone, Calendar, and Locale. Feel free to try it out here:
http://code.google.com/p/gwt-calendar-class/downloads/list