Develop Reference

Add n number of days using simpledateformat in java

We have a java code snippet here
import java.text.SimpleDateFormat;
import java.util.Date;
public class SimpleDateFormatExample {
public static void main(String[] args) {
Date date = new Date();
int days = 5;
SimpleDateFormat formatter = new SimpleDateFormat("dd/MM/yyyy");
String strDate= formatter.format(date.getTime() + (days*86400000));
System.out.println(strDate);
}
}
to add n no. of days to today's date. The result will be correct upto n=24 but gives previous month' after n=24. Why it is so?

The problem is the the int is overflowing
consider
int days = 25;
int d = days*86400000;
System.out.println(d);
try
int days = 25;
long d = days*86400000L;
System.out.println(d);

tl;dr
LocalDate // Represent a date-only, without a time-of-day and without a time zone.
.now() // Capture the current date, as seen through your JVM’s current default time zone. Better to pass a `ZoneId` as the optional argument.
.plusDays( 5 ) // Add five days, returning a new `LocalDate` object. Per the Immutable Objects pattern, a new object is produced rather than changing (“mutating”) the original.
.format( // Generate text representing the date value of our `LocalDate` object.
DateTimeFormatter.ofPattern( "dd/MM/uuuu" ) // Define a formatting pattern to suit your taste. Or call the `.ofLocalized…` methods to localize automatically.
) // Returns a `String`.
java.time
Date class represents a moment in UTC, a date with a time-of-day, and an offset-from-UTC of zero. Wrong class to use when working with date-only values.
Avoid using the terrible old legacy date-time classes such as Calendar, Date, and SimpleDateFormat. These classes were supplanted years ago by the java.time classes.
Do not track days as a count of seconds or milliseconds. Days are not always 24 hours long, and years are not always 365 days long.
LocalDate
Instead, use LocalDate class.
LocalDate today = LocalDate.now() ;
LocalDate later = today.plusDays( 5 ) ;
Convert
Best to avoid the legacy classes altogether. But if you must interoperate with old code not yet updated to java.time classes, you can convert back-and-forth. Call new methods added to the old classes.
For Date you need to add a time-of-day. I expect you will want to go with the first moment of the day. And I'll assume you want to frame the date as UTC rather than a time zone. We must go through a OffsetDateTime object to add the time-of-day and offset. For the offset, we use the constant ZoneOffset.UTC. Then we extract the more basic Instant class object to convert to a java.util.Date.
OffsetDateTime odt = OffsetDateTime.of( later , LocalTime.MIN , ZoneOffset.UTC ) ; // Combine the date with time-of-day and with an offset-from-UTC.
Instant instant = odt.toInstant() ; // Convert to the more basic `Instant` class, a moment in UTC, always UTC by definition.
java.util.Date d = java.util.Date.from( instant ) ; // Convert from modern class to legacy class.
Going the other direction:
Instant instant = d.toInstant() ; // Convert from legacy class to modern class.
About java.time
The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.
Where to obtain the java.time classes?
Java SE 8, Java SE 9, Java SE 10, Java SE 11, and later - Part of the standard Java API with a bundled implementation.
Java 9 adds some minor features and fixes.
Java SE 6 and Java SE 7
Most of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
Later versions of Android bundle implementations of the java.time classes.
For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Use days*86400000L to make this a long calculation otherwise the int value overflows.

Try this one in your code:
Calendar cal = Calendar.getInstance();
cal.setTime(new Date());
cal.add(Calendar.DATE, 5);
strDate = formatter.format(cal.getTime());

Converting Date to 08:00:00.000+0000

I am calling a rest web service that accepts Date. On client side, i have calling this service using JDK 8 OffsetDateTime Class.
Value that is going from my client side : 2018-07-01T05:30+05:30
Value that is accepted by Service : 2018-07-01T08:00:00.000+0000
Below is the code:
Calendar cal = Calendar.getInstance(TimeZone.getTimeZone(ZoneId.of("UTC")));
cal.set(2018, 05, 31);
cal.set(Calendar.HOUR, 0);
cal.set(Calendar.MINUTE, 0);
cal.set(Calendar.SECOND, 0);
cal.set(Calendar.MILLISECOND, 0);
cal.set(Calendar.HOUR_OF_DAY, 0);
OffsetDateTime offsetDateTime = OffsetDateTime.ofInstant(cal.getTime().toInstant(), ZoneId.systemDefault());
Value of offsetDateTime that is coming with above code is 2018-07-01T05:30+05:30.
I am in IST time zone.
Can someone help as to what needs to be done to change date to 2018-07-01T08:00:00.000+0000.

tl;dr
If you want 8 AM on first day of July at UTC…
OffsetDateTime.of(
2018 , 7 , 1 , // Date (year, month 1-12 is Jan-Dec, day-of-month)
8 , 0 , 0 , 0 , // Time (hour, minute, second, nano)
ZoneOffset.UTC // Offset-from-UTC (0 = UTC)
) // Returns a `OffsetDateTime` object.
.format( // Generates a `String` object with text representing the value of the `OffsetDateTime` object.
DateTimeFormatter.ofPattern( "uuuu-MM-dd'T'HH:mm:ss.SSSZ" , Locale.US )
) // Returns a `String` object.
2018-07-01T08:00:00.000+0000
Avoid legacy date-time classes
Never use Calendar or Date classes. They were completely supplanted by the modern java.time classes such as OffsetDateTime. You are mixing the legacy classes with the modern, and that makes no sense.
java.time
Your Question is not clear about what are your inputs and what are your outputs versus your expectations.
If you goal is 8 AM on July 1 in UTC:
LocalDate ld = LocalDate.of( 2018 , Month.JULY , 1 ) ;
LocalTime lt = LocalTime.of( 8 , 0 ) ;
OffsetDateTime odt = OffsetDateTime.of( ld , lt , ZoneOffset.UTC ) ;
odt.toString(): 2018-07-01T08:00Z
That string format complies with ISO 8061 standard. If your destination refuses that input and accepts only 2018-07-01T08:00:00.000+0000, then we must defining a formatting pattern.
DateTimeFormatter f = DateTimeFormatter.ofPattern( "uuuu-MM-dd'T'HH:mm:ss.SSSZ" , Locale.US );
String output = odt.format( f );
2018-07-01T08:00:00.000+0000
About java.time
The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.
Where to obtain the java.time classes?
Java SE 8, Java SE 9, Java SE 10, Java SE 11, and later - Part of the standard Java API with a bundled implementation.
Java 9 adds some minor features and fixes.
Java SE 6 and Java SE 7
Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
Later versions of Android bundle implementations of the java.time classes.
For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

i think the below code will work
public static Date ConvertToGMT() {
Date date = new Date();
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
dateFormat.setTimeZone(TimeZone.getTimeZone("UTC"));
Date utc = new Date(dateFormat.format(date));
return utc;
}

You can do it like so,
offsetDateTime.atZoneSameInstant(ZoneId.of("Asia/Kolkata"))
Update
If you need an instance of OffsetDateTime here it is.
offsetDateTime.atZoneSameInstant(ZoneId.of("Asia/Kolkata")).toOffsetDateTime();

It’s not the answer you asked for, but it may be the answer you prefer in the end: Check once more whether the service you are calling accepts the format that you are already giving it. Both formats conform with ISO 8601, so it seems that the service accepts this standard format. If so, it should accept yours too.
In any case, use OffsetDateTime and the other classes from java.time exclusively and avoid the old and outdated Calendar and TimeZone classes. Basil Bourque’s answer shows the good solution.
Link: Wikipedia article: ISO 8601

Date operations in Java already passed or near in the future

In Java (I'm a junior) - how do you perform various date operations?
Assess "storedDate is one week away from today" and "storedDate has passed"
I am not sure if its the best approach or how to do the condition check
java.sql.Date dueDate = (Date) loggedUser.get("dueDate");
I've seen various calculations like this - but not sure an if condition could be met like this?
Calendar c= Calendar.getInstance();
//c.set(Calendar.DAY_OF_WEEK,Calendar.SUNDAY);
c.set(Calendar.HOUR_OF_DAY,0);
c.set(Calendar.MINUTE,0);
c.set(Calendar.SECOND,0);
DateFormat df=new SimpleDateFormat("EEE yyyy/MM/dd HH:mm:ss");
System.out.println(df.format(c.getTime()));// This past Sunday [ May include today ]
c.add(Calendar.DATE,7);
System.out.println(df.format(c.getTime()));// Next Sunday

Step 1: Learn to read API pages.
Here is the Calendar API Page, read it.
For your class project, you don't care about timezone (since you didn't mention it in your post).
Get a date parameter: public void methodName(Date dateParameter)
Create a Calendar containing now: Calendar myCalendar = Calendar.getInstance();
Set a calendar to a date value: myCalendar.setTime(dateParameter)
"Calculate" one week from today: myCalendar.roll(Calendar.DAY_OF_YEAR, 7)
The Calendar object stores Millisecond, Second, Minute, and Hour.
Clear these using the set method.
Compare Calendar objects using the after method.

tl;dr
LocalDate.now( ZoneId.of( "Pacific/Auckland" ) )
.with(
TemporalAdjusters.previousOrSame​( DayOfWeek.SUNDAY )
)
java.time
The modern approach uses java.time classes.
Get current moment in UTC.
Instant instant = Instant.now() ;
If you care about day-of-week, you care about date. Determining a date requires a time zone. For any given moment the date varies around the globe by zone.
ZoneId z = ZoneId.of( "Africa/Tunis" ) ;
ZonedDateTime zdt = instant.atZone( z ) ; // Same point on the timeline, different wall-clock time.
To focus on date-only without time-of-day, extract a LocalDate.
LocalDate ld = zdt.toLocalDate() ;
To adjust into other moments, use a TemporalAdjuster. Find implementations in TemporalAdjusters.
LocalDate previousOrSameSunday = ld.with(
TemporalAdjusters.previousOrSame​( DayOfWeek.SUNDAY )
) ;
…and…
LocalDate nextOrSameSunday = ld.with(
TemporalAdjusters.nextOrSame​( DayOfWeek.SUNDAY )
) ;
To compare, look for isBefore, isAfter, isEqual, and equals methods on the various java.time classes.
thisLocalDate.isBefore( thatLocalDate)
About java.time
The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
Where to obtain the java.time classes?
Java SE 8, Java SE 9, and later
Built-in.
Part of the standard Java API with a bundled implementation.
Java 9 adds some minor features and fixes.
Java SE 6 and Java SE 7
Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
See How to use ThreeTenABP….
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Get Last Day (Date) of Week for a given Date

How to get the last week Day (saturday) Date for a particular Date. Means if I give Input as 06-04-2012
(MM-dd-YYYY)
The output should be 06-09-2012 as seen in this calendar.

Calendar cal = Calendar.getInstance();
int currentDay = cal.get(Calendar.DAY_OF_WEEK);
int leftDays= Calendar.SATURDAY - currentDay;
cal.add(Calendar.DATE, leftDays);
See
example

tl;dr
LocalDate.parse(
"06-04-2012" ,
DateTimeFormatter.ofPattern( "MM-dd-uuuu" )
).with( TemporalAdjusters.nextOrSame( DayOfWeek.SATURDAY ) )
.format( DateTimeFormatter.ofPattern( "MM-dd-uuuu" ) )
Using java.time
The modern way is with java.time classes.
First parse your input string as a LocalDate.
DateTimeFormatter f = DateTimeFormatter.ofPattern( "MM-dd-uuuu" );
LocalDate ld = LocalDate.parse( "06-04-2012" , f );
ld.toString(): 2012-06-04
Then get the next Saturday, or use the date itself if it is a Saturday. To specify a Saturday, use the enum DayOfWeek.SATURDAY. To find that next or same date that is a Saturday, use an implementation of TemporaAdjuster found in the TemporalAdjusters (note plural name) class.
LocalDate nextOrSameSaturday =
ld.with( TemporalAdjusters.nextOrSame( DayOfWeek.SATURDAY ) ) ;
To generate a String, I suggest calling toString to use standard ISO 8601 format.
String output = ld.toString();
2012-06-09
But if you insist, you may use the same formatter as used in parsing.
String output = ld.format( f );
06-09-2012
About java.time
The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
Where to obtain the java.time classes?
Java SE 8 and SE 9 and later
Built-in.
Part of the standard Java API with a bundled implementation.
Java 9 adds some minor features and fixes.
Java SE 6 and SE 7
Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
See How to use ThreeTenABP….
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Look at either JODA time or (if you cannot add new libraries) the Calendar class.

public Calendar lastDayOfWeek(Calendar calendar){
Calendar cal = (Calendar) calendar.clone();
int day = cal.get(Calendar.DAY_OF_YEAR);
while(cal.get(Calendar.DAY_OF_WEEK) != Calendar.SATURDAY){
cal.set(Calendar.DAY_OF_YEAR, ++day);
}
return cal;
}

how to convert timestamp string to java.util.Date

I need to convert a timestamp string to java.util.Date. E.g.:
MMDDYYHHMMSS to MM-DD-YY HH-MM-SS
Where MM is month, DD is date, YY is year, HH is hours, MM is minutes and SS is seconds.

You can do it like this:
DateFormat format = new SimpleDateFormat("MMddyyHHmmss");
Date date = format.parse("022310141505");
but I would strongly recommend that you use Joda Time instead. It's a better date/time library by a long, long way. In particular, the formatters/parsers in Joda Time are thread-safe, so you can reuse them freely and statically; java.text.SimpleDateFormat isn't thread-safe, so you either need to create one per thread or serialize access to it with a synchronized block.

tl;dr
java.time.LocalDateTime.parse(
"012318123456" ,
DateTimeFormatter.ofPattern( "MMdduuHHmmss" )
).format(
DateTimeFormatter.ofPattern( "MM-dd-uu HH-mm-ss" )
)
01-23-18 12-34-56
java.time
The modern approach uses the java.time classes.
Define a formatting pattern to match your input string.
DateTimeFormatter f = DateTimeFormatter.ofPattern( "MMdduuHHmmss" ) ;
Your two-digit year will be interpreted as being 21st century ( 20xx ).
Parse as a LocalDateTime because your input string lacks any indicator of time zone or offset-from-UTC.
LocalDateTime ldt = LocalDateTime.parse( "012318123456" , f ) ;
ldt.toString(): 2018-01-23T12:34:56
Generate a string in your desired format.
DateTimeFormatter fOut = DateTimeFormatter.ofPattern( "MM-dd-uu HH-mm-ss" ) ;
String output = ldt.format( fOut );
01-23-18 12-34-56
ISO 8601
Both of your formats are terrible, for multiple reasons.
When serializing date-time values, use the standard ISO 8601 formats whenever possible. They are designed to be practical, easy to parse by machine, easy to read by humans across cultures.
For a date-time time such as yours, the T in the middle separates the date portion from the time-of-day portion.
2018-01-23T12:34:56
About java.time
The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
Where to obtain the java.time classes?
Java SE 8, Java SE 9, and later
Built-in.
Part of the standard Java API with a bundled implementation.
Java 9 adds some minor features and fixes.
Java SE 6 and Java SE 7
Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
Later versions of Android bundle implementations of the java.time classes.
For earlier Android, the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

use a SimpleDateFormat with an appropriate format string (be careful to use the correct format letters, uppercase and lowercase have different meanings!).