We Keep Coding

Why GregorianCalendar.getTimeInMillis() changes the value of the instance?

I found a very strange behavior of GregorianCalendar.getTimeInMillis(), it seems that it changes the value of the instance content. In the following code you can see that two blocks of code differ in only one commented line, where getTimeInMillis() is called. Why is the result different when I uncomment the line?
With commented call the output is
2014-10-25T22:00:00Z -> 2014-10-26T22:00:00.000+01:00
2014-10-25T22:00:00Z -> 2014-10-27T00:00:00.000+01:00
but when I uncomment the getTimeInMillis() line, both results are the same:
2014-10-25T22:00:00Z -> 2014-10-27T00:00:00.000+01:00
2014-10-25T22:00:00Z -> 2014-10-27T00:00:00.000+01:00
Code:
package com.test;
import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.TimeZone;
import javax.xml.datatype.DatatypeFactory;
import javax.xml.datatype.XMLGregorianCalendar;
public class Main {
public static void main(String[] args) {
try {
XMLGregorianCalendar date1 = DatatypeFactory.newInstance()
.newXMLGregorianCalendar("2014-10-25T22:00:00Z");
XMLGregorianCalendar date2 = DatatypeFactory.newInstance()
.newXMLGregorianCalendar("2014-10-25T22:00:00Z");
int days = 1;
GregorianCalendar gregorianCalendar1 = date1.toGregorianCalendar();
// gregorianCalendar1.getTimeInMillis(); //UNCOMMENT THIS LINE TO GET A DIFFERENT RESULT
gregorianCalendar1.setTimeZone(TimeZone.getDefault());
gregorianCalendar1.add(Calendar.DAY_OF_MONTH, days);
XMLGregorianCalendar newXMLGregorianCalendar1 = DatatypeFactory
.newInstance().newXMLGregorianCalendar(gregorianCalendar1);
System.out.printf("%s -> %s\n", date1, newXMLGregorianCalendar1);
GregorianCalendar gregorianCalendar2 = date2.toGregorianCalendar();
gregorianCalendar2.getTimeInMillis();
gregorianCalendar2.setTimeZone(TimeZone.getDefault());
gregorianCalendar2.add(Calendar.DAY_OF_MONTH, days);
XMLGregorianCalendar newXMLGregorianCalendar2 = DatatypeFactory
.newInstance().newXMLGregorianCalendar(gregorianCalendar2);
System.out.printf("%s -> %s\n", date2, newXMLGregorianCalendar2);
} catch (Exception e) {
e.printStackTrace();
}
}
}

It's a time zone change. Not on December 31st in Shanghai, but manually, in your code.
Particularly, you are changing the time zone after having forced the calendar to compute its fields (based on the "old" time zone). This messes up the internal state of the calendar. Of course, this should not be the case, but is only one of the many strange behaviors exposed by the Calendar classes - and, most likely, mainly caused by their mutability.
Some of the potential difficulties are also stated in a comment in the implementation of Calendar#setTimeZone:
* Consider the sequence of calls:
* cal.setTimeZone(EST); cal.set(HOUR, 1); cal.setTimeZone(PST).
* Is cal set to 1 o'clock EST or 1 o'clock PST? Answer: PST.
You could possibly work around this by studying the source code of GregorianCalendar and trying to avoid the critical sequences of calls. But as others already have pointed out: The whole old Date/Time API is horribly broken. If you have the chance, you should consider using the new Date/Time API of Java 8 (or the Joda Time API, which is similar enough to Java 8 to make it easy to later change existing Joda-based code to Java 8 code).
Here is an example that demonstrates the difference between setting the time zone before the call to getTimeMillis and after the call to getTimeMillis:
import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.TimeZone;
import javax.xml.datatype.DatatypeFactory;
import javax.xml.datatype.XMLGregorianCalendar;
public class GregorianCalendarTest {
public static void main(String[] args) {
String fromSettingTimeZoneBeforeCall = createString(true);
String fromSettingTimeZoneAfterCall = createString(false);
System.out.println("Before: "+fromSettingTimeZoneBeforeCall);
System.out.println("After : "+fromSettingTimeZoneAfterCall);
}
private static String createString(boolean setTimeZoneBeforeCall)
{
try {
XMLGregorianCalendar date = DatatypeFactory.newInstance()
.newXMLGregorianCalendar("2014-10-25T22:00:00Z");
int days = 1;
GregorianCalendar gregorianCalendar = date.toGregorianCalendar();
System.out.println("After creating: "+gregorianCalendar);
if (!setTimeZoneBeforeCall)
{
gregorianCalendar.getTimeInMillis();
System.out.println("After millis : "+gregorianCalendar);
}
gregorianCalendar.setTimeZone(TimeZone.getDefault());
System.out.println("After timezone: "+gregorianCalendar);
if (setTimeZoneBeforeCall)
{
gregorianCalendar.getTimeInMillis();
System.out.println("After millis : "+gregorianCalendar);
}
gregorianCalendar.add(Calendar.DAY_OF_MONTH, days);
System.out.println("After adding : "+gregorianCalendar);
XMLGregorianCalendar newXMLGregorianCalendar = DatatypeFactory
.newInstance().newXMLGregorianCalendar(gregorianCalendar);
System.out.println("After all : "+gregorianCalendar);
return newXMLGregorianCalendar.toString();
} catch (Exception e) {
e.printStackTrace();
return null;
}
}
}
EDIT: This behavior is also described in this bug report: http://bugs.java.com/bugdatabase/view_bug.do?bug_id=5026826

Pre-Java 8 Calendar implementations have been under a lot of criticism for "weird" behavior. I think that this is due to the following documentation:
Getting and Setting Calendar Field Values
The calendar field values can be set by calling the set methods. Any field values set in a Calendar will not be interpreted until it needs to calculate its time value (milliseconds from the Epoch) or values of the calendar fields. Calling the get, getTimeInMillis, getTime, add and roll involves such calculation.
Note that the toString() method is marked as debug-only:
Return a string representation of this calendar. This method is intended to be used only for debugging purposes, and the format of the returned string may vary between implementations. The returned string may be empty but may not be null.
Though this will not probably end-up in a bug (as long as you don't use toString() in actual logic), it is better to use Joda-Time or new Java-8 Date and Time

How to check if string matches date pattern using time API?

My program is parsing an input string to a LocalDate object. For most of the time the string looks like 30.03.2014, but occasionally it looks like 3/30/2014. Depending on which, I need to use a different pattern to call DateTimeFormatter.ofPattern(String pattern) with. Basically, I need to check if the string matches the pattern dd.MM.yyyy or M/dd/yyyy before doing the parsing.
The regex approach would be something like:
LocalDate date;
if (dateString.matches("^\\d?\\d/\\d{2}/\\d{4}$")) {
date = LocalDate.parse(dateString, DateTimeFormatter.ofPattern("M/dd/yyyy"));
} else {
date = LocalDate.parse(dateString, DateTimeFormatter.ofPattern("dd.MM.yyyy"));
}
This works, but it would be nice to use the date pattern string when matching the string also.
Are there any standard ways to do this with the new Java 8 time API, without resorting to regex matching? I have looked in the docs for DateTimeFormatter but I couldn't find anything.

Okay I'm going ahead and posting it as an answer. One way is to create the class that will holds the patterns.
public class Test {
public static void main(String[] args){
MyFormatter format = new MyFormatter("dd.MM.yyyy", "M/dd/yyyy");
LocalDate date = format.parse("3/30/2014"); //2014-03-30
LocalDate date2 = format.parse("30.03.2014"); //2014-03-30
}
}
class MyFormatter {
private final String[] patterns;
public MyFormatter(String... patterns){
this.patterns = patterns;
}
public LocalDate parse(String text){
for(int i = 0; i < patterns.length; i++){
try{
return LocalDate.parse(text, DateTimeFormatter.ofPattern(patterns[i]));
}catch(DateTimeParseException excep){}
}
throw new IllegalArgumentException("Not able to parse the date for all patterns given");
}
}
You could improve this as #MenoHochschild did by directly creating an array of DateTimeFormatter from the array of String you pass in the constructor.
Another way would be to use a DateTimeFormatterBuilder, appending the formats you want. There may exists some other ways to do it, I didn't go deeply through the documentation :-)
DateTimeFormatter dfs = new DateTimeFormatterBuilder()
.appendOptional(DateTimeFormatter.ofPattern("yyyy-MM-dd"))
.appendOptional(DateTimeFormatter.ofPattern("dd.MM.yyyy"))
.toFormatter();
LocalDate d = LocalDate.parse("2014-05-14", dfs); //2014-05-14
LocalDate d2 = LocalDate.parse("14.05.2014", dfs); //2014-05-14

With DateTimeFormatter, optional patterns can be specified using square brackets.
Demo:
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.util.Locale;
import java.util.stream.Stream;
public class Main {
public static void main(String[] args) {
DateTimeFormatter dtf = DateTimeFormatter.ofPattern("[d.M.u][M/d/u][u-M-d]", Locale.ENGLISH);
Stream.of(
"3/30/2014",
"30.03.2014",
"2014-05-14",
"14.05.2014"
).forEach(s -> System.out.println(LocalDate.parse(s, dtf)));
}
}
Output:
2014-03-30
2014-03-30
2014-05-14
2014-05-14
Learn more about the the modern date-time API* from Trail: Date Time.
* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.

The approach of #ZouZou is a possible solution.
In order to avoid using exceptions for program logic as much as possible (also not so nice performancewise) following alternative might be considered:
static final String[] PATTERNS = {"dd.MM.yyyy", "M/dd/yyyy"};
static final DateTimeFormatter[] FORMATTERS = new DateTimeFormatter[PATTERNS.length];
static {
for (int i = 0; i < PATTERNS.length; i++) {
FORMATTERS[i] = DateTimeFormatter.ofPattern(PATTERNS[i]);
}
}
public static LocalDate parse(String input) {
ParsePosition pos = new ParsePosition();
for (int i = 0; i < patterns.length; i++) {
try {
TemporalAccessor tacc = FORMATTERS[i].parseUnresolved(input, pos);
if (pos.getErrorIndex < 0) {
return LocalDate.from(tacc); // possibly throwing DateTimeException => validation failure
}
} catch (DateTimeException ex) { // catches also possible DateTimeParseException
// go to next pattern
}
pos.setIndex(0);
pos.setErrorIndex(-1);
}
throw new IllegalArgumentException("Input does not match any pattern: " + input);
}
More explanation about the method parseUnresolved():
This method does only the first phase of parsing, so there is no second phase containing preliminary validation or combining effort of parsed fields. However, LocalDate.from() does validate every input, so I think this is still sufficient. And the advantage is that parseUnresolved() uses the error index of ParsePosition. This is in agreement with traditional java.text.Format-behaviour.
Unfortunately the alternative and more intuitive method DateTimeFormater.parse() first creates a DateTimeParseException and then store the error index in this exception. So I decided not to use this method in order to avoid the creation of an unnecessary exception. For me, this API-detail is a questionable design decision.

Java system time

I have this code copied from one of questions from SO:
public static String getCurrentTimeStamp() {
SimpleDateFormat sdfDate = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Date now = new Date();
String strDate = sdfDate.format(now);
return strDate;
}
I want to get only the system time and NOT the date. Then I must change second line of code to:
SimpleDateFormat sdfDate = new SimpleDateFormat(" HH:mm:ss") ;
Then, DATE() must get the current time. Clear upto this point but I can't understand the format() function used.
I mean cant we simply output variable now instead of strdate?
Is it just because that the return type of function getCurrentTimeStamp() is String?
Please clarify and if there is any other simpler and one line code for getting system time alone, do share.

I mean cant we simply output variable now instead of strdate.
Well you could return now.toString() - but that will use the format that Date.toString() happens to choose, whereas you want a specific format. The point of the SimpleDateFormat object in this case is to convert a Date (which is a point in time, without reference to any particular calendar or time zone) into a String, applying an appropriate time zone, calendar system, and text format (in your case HH:mm:ss).
You can still simplify your method somewhat though, by removing the local variables (which are each only used once):
public static String getCurrentTimeStamp() {
return new SimpleDateFormat("HH:mm:ss").format(new Date());
}
Or maybe you'd find it more readable to keep the variable for the date format, but not the date and the return value:
public static String getCurrentTimeStamp() {
DateFormat format = new SimpleDateFormat("HH:mm:ss");
return format.format(new Date());
}
Personally I'd recommend using Joda Time instead, mind you - it's a much nicer date/time API, and its formatted are thread-safe so you could easily keep a reference to a single formatting object.

public static String getCurrentTimeStampwithTimeOnly() {
return new SimpleDateFormat("HH:mm:ss").format(new Date());
}
Helps you to do this.

you can call this line any time
Date now = new Date();
The now variable will contain the current timestamp
The format function just generates a String from this timestamp
also take a look at the Calendar class ( Calendar.getInstance())

jodatime DateTime objects and Locale

Given that this field locale have been set for norwegian bokmål and norway:
Locale locale = new Locale("nb","no");
What's missing from this code fragment inside a method to return the proper string for the language bokmål?
Assert.assertNotNull(locale);//Is asserted
MutableDateTime start = new MutableDateTime(2012,1, 10,10,0,0,0 );
start.setDayOfWeek(DateTimeConstants.SATURDAY);
System.out.println(start.dayOfWeek().getAsText(locale));
System.out.println(locale.getISO3Language().toString());
The output is "Saturday, nob"
Do I need to implement locale specific weekday name strings myself? If so is there some base object or interface to override in jodatime? I can't seem to find one.

Your code works correctly for me:
[system:/tmp]$ cat Loc.java
import org.joda.time.*;
import java.util.Locale;
class Loc {
public static void main(String[] args) {
Locale locale = new Locale("nb","no");
MutableDateTime start = new MutableDateTime(2012,1, 10,10,0,0,0 );
start.setDayOfWeek(DateTimeConstants.SATURDAY);
System.out.println(start.dayOfWeek().getAsText(locale));
System.out.println(locale.getISO3Language().toString());
}
}
[system:/tmp]$ java Loc
lørdag
nob
A few things to try:
Try using new Locale("nb", "NO") (country/region is supposed to be case-insentive, but it's worth a shot)
Try using new Locale("no", "NO") or new Locale("nn", "NO") - Nyorsk may not be what you're looking for, but does it work? From some Googling, it seems like some platforms might treat nb_NO as an alias for no_NO? It might be useful just to know if that does or doesn't work.
Make sure the nb_NO locale is available. It probably is since getISO3Language() seems to work.

Determine if a String is a valid date before parsing

I have this situation where I am reading about 130K records containing dates stored as String fields. Some records contain blanks (nulls), some contain strings like this: 'dd-MMM-yy' and some contain this 'dd/MM/yyyy'.
I have written a method like this:
public Date parsedate(String date){
if(date !== null){
try{
1. create a SimpleDateFormat object using 'dd-MMM-yy' as the pattern
2. parse the date
3. return the parsed date
}catch(ParseException e){
try{
1. create a SimpleDateFormat object using 'dd/MM/yyy' as the pattern
2. parse the date
3. return parsed date
}catch(ParseException e){
return null
}
}
}else{
return null
}
}
So you may have already spotted the problem. I am using the try .. catch as part of my logic. It would be better is I can determine before hand that the String actually contains a parseable date in some format then attempt to parse it.
So, is there some API or library that can help with this? I do not mind writing several different Parse classes to handle the different formats and then creating a factory to select the correct6 one, but, how do I determine which one?
Thanks.

See Lazy Error Handling in Java for an overview of how to eliminate try/catch blocks using an Option type.
Functional Java is your friend.
In essence, what you want to do is to wrap the date parsing in a function that doesn't throw anything, but indicates in its return type whether parsing was successful or not. For example:
import fj.F; import fj.F2;
import fj.data.Option;
import java.text.SimpleDateFormat;
import java.text.ParseException;
import static fj.Function.curry;
import static fj.Option.some;
import static fj.Option.none;
...
F<String, F<String, Option<Date>>> parseDate =
curry(new F2<String, String, Option<Date>>() {
public Option<Date> f(String pattern, String s) {
try {
return some(new SimpleDateFormat(pattern).parse(s));
}
catch (ParseException e) {
return none();
}
}
});
OK, now you've a reusable date parser that doesn't throw anything, but indicates failure by returning a value of type Option.None. Here's how you use it:
import fj.data.List;
import static fj.data.Stream.stream;
import static fj.data.Option.isSome_;
....
public Option<Date> parseWithPatterns(String s, Stream<String> patterns) {
return stream(s).apply(patterns.map(parseDate)).find(isSome_());
}
That will give you the date parsed with the first pattern that matches, or a value of type Option.None, which is type-safe whereas null isn't.
If you're wondering what Stream is... it's a lazy list. This ensures that you ignore patterns after the first successful one. No need to do too much work.
Call your function like this:
for (Date d: parseWithPatterns(someString, stream("dd/MM/yyyy", "dd-MM-yyyy")) {
// Do something with the date here.
}
Or...
Option<Date> d = parseWithPatterns(someString,
stream("dd/MM/yyyy", "dd-MM-yyyy"));
if (d.isNone()) {
// Handle the case where neither pattern matches.
}
else {
// Do something with d.some()
}

Don't be too hard on yourself about using try-catch in logic: this is one of those situations where Java forces you to so there's not a lot you can do about it.
But in this case you could instead use DateFormat.parse(String, ParsePosition).

You can take advantage of regular expressions to determine which format the string is in, and whether it matches any valid format. Something like this (not tested):
(Oops, I wrote this in C# before checking to see what language you were using.)
Regex test = new Regex(#"^(?:(?<formatA>\d{2}-[a-zA-Z]{3}-\d{2})|(?<formatB>\d{2}/\d{2}/\d{3}))$", RegexOption.Compiled);
Match match = test.Match(yourString);
if (match.Success)
{
if (!string.IsNullOrEmpty(match.Groups["formatA"]))
{
// Use format A.
}
else if (!string.IsNullOrEmpty(match.Groups["formatB"]))
{
// Use format B.
}
...
}

If you formats are exact (June 7th 1999 would be either 07-Jun-99 or 07/06/1999: you are sure that you have leading zeros), then you could just check for the length of the string before trying to parse.
Be careful with the short month name in the first version, because Jun may not be June in another language.
But if your data is coming from one database, then I would just convert all dates to the common format (it is one-off, but then you control the data and its format).

In this limited situation, the best (and fastest method) is certinally to parse out the day, then based on the next char either '/' or '-' try to parse out the rest. and if at any point there is unexpected data, return NULL then.

Assuming the patterns you gave are the only likely choices, I would look at the String passed in to see which format to apply.
public Date parseDate(final String date) {
if (date == null) {
return null;
}
SimpleDateFormat format = (date.charAt(2) == '/') ? new SimpleDateFormat("dd/MMM/yyyy")
: new SimpleDateFormat("dd-MMM-yy");
try {
return format.parse(date);
} catch (ParseException e) {
// Log a complaint and include date in the complaint
}
return null;
}
As others have mentioned, if you can guarantee that you will never access the DateFormats in a multi-threaded manner, you can make class-level or static instances.

Looks like three options if you only have two, known formats:
check for the presence of - or / first and start with that parsing for that format.
check the length since "dd-MMM-yy" and "dd/MM/yyyy" are different
use precompiled regular expressions
The latter seems unnecessary.

Use regular expressions to parse your string. Make sure that you keep both regex's pre-compiled (not create new on every method call, but store them as constants), and compare if it actually is faster then the try-catch you use.
I still find it strange that your method returns null if both versions fail rather then throwing an exception.

you could use split to determine which format to use
String[] parts = date.split("-");
df = (parts.length==3 ? format1 : format2);
That assumes they are all in one or the other format, you could improve the checking if need be

An alternative to creating a SimpleDateFormat (or two) per iteration would be to lazily populate a ThreadLocal container for these formats. This will solve both Thread safety concerns and concerns around object creation performance.

A simple utility class I have written for my project. Hope this helps someone.
Usage examples:
DateUtils.multiParse("1-12-12");
DateUtils.multiParse("2-24-2012");
DateUtils.multiParse("3/5/2012");
DateUtils.multiParse("2/16/12");
public class DateUtils {
private static List<SimpleDateFormat> dateFormats = new ArrayList<SimpleDateFormat>();
private Utils() {
dateFormats.add(new SimpleDateFormat("MM/dd/yy")); // must precede yyyy
dateFormats.add(new SimpleDateFormat("MM/dd/yyyy"));
dateFormats.add(new SimpleDateFormat("MM-dd-yy"));
dateFormats.add(new SimpleDateFormat("MM-dd-yyyy"));
}
private static Date tryToParse(String input, SimpleDateFormat format) {
Date date = null;
try {
date = format.parse(input);
} catch (ParseException e) {
}
return date;
}
public static Date multiParse(String input) {
Date date = null;
for (SimpleDateFormat format : dateFormats) {
date = tryToParse(input, format);
if (date != null) break;
}
return date;
}
}

On one hand I see nothing wrong with your use of try/catch for the purpose, it’s the option I would use. On the other hand there are alternatives:
Take a taste from the string before deciding how to parse it.
Use optional parts of the format pattern string.
For my demonstrations I am using java.time, the modern Java date and time API, because the Date class used in the question was always poorly designed and is now long outdated. For a date without time of day we need a java.time.LocalDate.
try-catch
Using try-catch with java.time looks like this:
DateTimeFormatter ddmmmuuFormatter = DateTimeFormatter.ofPattern("dd-MMM-uu", Locale.ENGLISH);
DateTimeFormatter ddmmuuuuFormatter = DateTimeFormatter.ofPattern("dd/MM/uuuu");
String dateString = "07-Jun-09";
LocalDate result;
try {
result = LocalDate.parse(dateString, ddmmmuuFormatter);
} catch (DateTimeParseException dtpe) {
result = LocalDate.parse(dateString, ddmmuuuuFormatter);
}
System.out.println("Date: " + result);
Output is:
Date: 2009-06-07
Suppose instead we defined the string as:
String dateString = "07/06/2009";
Then output is still the same.
Take a taste
If you prefer to avoid the try-catch construct, it’s easy to make a simple check to decide which of the formats your string conforms to. For example:
if (dateString.contains("-")) {
result = LocalDate.parse(dateString, ddmmmuuFormatter);
} else {
result = LocalDate.parse(dateString, ddmmuuuuFormatter);
}
The result is the same as before.
Use optional parts in the format pattern string
This is the option I like the least, but it’s short and presented for some measure of completeness.
DateTimeFormatter dateFormatter
= DateTimeFormatter.ofPattern("[dd-MMM-uu][dd/MM/uuuu]", Locale.ENGLISH);
LocalDate result = LocalDate.parse(dateString, dateFormatter);
The square brackets denote optional parts of the format. So Java first tries to parse using dd-MMM-uu. No matter if successful or not it then tries to parse the remainder of the string using dd/MM/uuuu. Given your two formats one of the attempts will succeed, and you have parsed the date. The result is still the same as above.
Link
Oracle tutorial: Date Time explaining how to use java.time.