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Not able to convert date to epoch time [duplicate]

This question already has answers here:
Convert String to java.util.Date
(4 answers)
Display current time in 12 hour format with AM/PM
(15 answers)
Java - parse date with AM/PM next to seconds (no space)
(5 answers)
Closed 3 years ago.
This is my sample code
String dt = "Oct 24 2019 12:00:00.000 AM UTC";
String dt1="Oct 24 2019 11:59:59.000 PM UTC";
SimpleDateFormat df = new SimpleDateFormat("MMM dd yyyy HH:mm:ss.SSS
aa zzz");
Date date = df.parse(dt);
Date date1 = df.parse(dt1);
long epoch = date.getTime();
long epoch1 = date1.getTime();
System.out.println(epoch);
System.out.println(epoch1);
Here specifying the AM and PM but its not taking the value for that and throwing the exception as
{"error_code":"INVALID_PARAMETER_VALUE","message":"Time range must have a start time earlier than the end time"}
How to specify AM/PM in the java code.
How can I take yesterday's date and time for today in java code as an input to convert to epoch.

tl;dr
ZonedDateTime
.parse(
"Oct 24 2019 12:00:00.000 AM UTC" ,
DateTimeFormatter.ofPattern( "MMM d uuuu hh:mm:ss.SSS a z" ).withLocale( Locale.US )
)
.toInstant()
.toEpochMilli()
1571875200000
java.time
Two problems:
As commented by Heuberger, you are using incorrect formatting codes.
You are using terrible date-time classes supplanted years ago by the modern java.time classes.
Your inputs, renamed for clarity.
String inputStart = "Oct 24 2019 12:00:00.000 AM UTC";
String inputStop = "Oct 24 2019 11:59:59.000 PM UTC";
Define a formatting pattern to match your inputs.
DateTimeFormatter f = DateTimeFormatter.ofPattern( "MMM d uuuu hh:mm:ss.SSS a z" ).withLocale( Locale.US );
Parse inputs.
ZonedDateTime zdtStart = ZonedDateTime.parse( inputStart , f );
ZonedDateTime zdtStop = ZonedDateTime.parse( inputStop , f );
Calculate elapsed time. We should get one second less than 24 hours.
Duration d = Duration.between( zdtStart , zdtStop );
Apparently you want the count of milliseconds since the epoch reference of first moment of 1970 in UTC, 1970-01-01T00:00Z. First extract the building-block class, Instant, from each ZonedDateTime. An Instant represents a moment in UTC. This class lets you interrogate for the count since epoch. Note that java.time classes resolve to nanoseconds. So asking for milliseconds can result in data loss, ignoring any microseconds or nanoseconds.
Instant start = zdtStart.toInstant() ;
Instant stop = zdtStop.toInstant() ;
long millisStart = start.toEpochMilli() ;
long milliStop = stop.toEpochMilli() ;
Dump to console.
System.out.println( "zdtStart = " + zdtStart );
System.out.println( "zdtStop = " + zdtStop );
System.out.println( "d = " + d );
System.out.println( "start = " + start );
System.out.println( "stop = " + stop );
System.out.println( "millisStart = " + millisStart );
System.out.println( "milliStop = " + milliStop );
zdtStart = 2019-10-24T00:00Z[UTC]
zdtStop = 2019-10-24T23:59:59Z[UTC]
d = PT23H59M59S
start = 2019-10-24T00:00:00Z
stop = 2019-10-24T23:59:59Z
millisStart = 1571875200000
milliStop = 1571961599000
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.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
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.

You need to use the following date format:
SimpleDateFormat df = new SimpleDateFormat("MMM dd yyyy hh:mm:ss.SSS a zzz",Locale.US);
For AM/PM Marker, 'a' is used. For reference
e.g.
import java.util.*;
import java.text.*;
public class Main
{
public static void main(String[] args) {
try{
String dt = "Oct 24 2019 12:00:00.000 AM UTC";
String dt1="Oct 24 2019 11:59:59.000 PM UTC";
SimpleDateFormat df = new SimpleDateFormat("MMM dd yyyy hh:mm:ss.SSS a zzz",Locale.US);
Date date = df.parse(dt);
Date date1 = df.parse(dt1);
long epoch = date.getTime();
long epoch1 = date1.getTime();
System.out.println(epoch);
System.out.println(epoch1);
}catch(Exception e){
e.printStackTrace();
}
}
}
output:
1571918400000
1571918399000

Get day, month and year separately using SimpleDateFormat

I have a SimleDateFormat like this
SimpleDateFormat format = new SimpleDateFormat("MMM dd,yyyy hh:mm");
String date = format.format(Date.parse(payback.creationDate.date));
I'm giving date with the format like "Jan,23,2014".
Now, I want to get day, month and year separately. How can I implement this?

If you need to get the values separately, then use more than one SimpleDateFormat.
SimpleDateFormat dayFormat = new SimpleDateFormat("dd");
String day = dayFormat.format(Date.parse(payback.creationDate.date));
SimpleDateFormat monthFormat = new SimpleDateFormat("MM");
String month = monthFormat .format(Date.parse(payback.creationDate.date));
etc.

SimpleDateFormat format = new SimpleDateFormat("MMM dd,yyyy hh:mm", Locale.ENGLISH);
Date theDate = format.parse("JAN 13,2014 09:15");
Calendar myCal = new GregorianCalendar();
myCal.setTime(theDate);
System.out.println("Day: " + myCal.get(Calendar.DAY_OF_MONTH));
System.out.println("Month: " + myCal.get(Calendar.MONTH) + 1);
System.out.println("Year: " + myCal.get(Calendar.YEAR));

Wow, SimpleDateFormat for getting string parts? It can be solved much easier if your input string is like "Jan,23,2014":
String input = "Jan,23,2014";
String[] out = input.split(",");
System.out.println("Year = " + out[2]);
System.out.println("Month = " + out[0]);
System.out.println("Day = " + out[1]);
Output:
Year = 2014
Month = Jan
Day = 23
But if you really want to use SimpleDateFormat because of some reason, the solution will be the following:
String input = "Jan,23,2014";
SimpleDateFormat format = new SimpleDateFormat("MMM,dd,yyyy");
Date date = format.parse(input);
Calendar calendar = Calendar.getInstance(TimeZone.getDefault());
calendar.setTime(date);
System.out.println(calendar.get(Calendar.YEAR));
System.out.println(calendar.get(Calendar.DAY_OF_MONTH));
System.out.println(new SimpleDateFormat("MMM").format(calendar.getTime()));
Output:
2014
23
Jan

The accepted answer here suggests to use more than one SimpleDateFormat, but it's possible to do this using one SimpleDateFormat instance and calling applyPattern.
Note: I believe this post would also be helpful for those who were searching for setPattern() just like me.
Date date=new Date();
SimpleDateFormat simpleDateFormat = new SimpleDateFormat();
simpleDateFormat.applyPattern("dd");
System.out.println("Day : " + simpleDateFormat.format(date));
simpleDateFormat.applyPattern("MMM");
System.out.println("Month : " + simpleDateFormat.format(date));
simpleDateFormat.applyPattern("yyyy");
System.out.println("Year : " + simpleDateFormat.format(date));

tl;dr
Use LocalDate class.
LocalDate
.parse(
"Jan,23,2014" ,
DateTimeFormatter.ofPattern( "MMM,dd,uuuu" , Locale.US )
)
.getYear()
… or .getMonthValue() or .getDayOfMonth.
java.time
The other Answers use outmoded classes. The java.time classes supplant those troublesome old legacy classes.
LocalDate
The LocalDate class represents a date-only value without time-of-day and without time zone.
String input = "Jan,23,2014";
DateTimeFormatter f = DateTimeFormatter.ofPattern( "MMM,d,uuuu" );
LocalDate ld = LocalDate.parse( input , f );
Interrogate for the parts you want.
int year = ld.getYear();
int month = ld.getMonthValue();
int dayOfMonth = ld.getDayOfMonth();
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 this to parse "Jan,23,2014"
SimpleDateFormat fmt = new SimpleDateFormat("MMM','dd','yyyy");
Date dt = fmt.parse("Jan,23,2014");
then you can get whatever part of the date.

Are you accepting this ?
int day = 25 ; //25
int month =12; //12
int year = 1988; // 1988
Calendar c = Calendar.getInstance();
c.set(year, month-1, day, 0, 0);
SimpleDateFormat format = new SimpleDateFormat("MMM dd,yyyy hh:mm");
System.out.println(format.format(c.getTime()));
Display as Dec 25,1988 12:00
UPDATE : based on Comment
DateFormat format = new SimpleDateFormat("MMM");
System.out.println(format.format(format.parse("Jan,23,2014")));
NOTE: Date.parse() is #deprecated and as per API it is recommend to use DateFormat.parse

public static String getDate(long milliSeconds, String dateFormat) {
// Create a DateFormatter object for displaying date in specified
// format.
SimpleDateFormat formatter = new SimpleDateFormat(dateFormat,
Locale.getDefault());
// Create a calendar object that will convert the date and time value in
// milliseconds to date.
Calendar calendar = Calendar.getInstance();
calendar.setTimeInMillis(milliSeconds);
return formatter.format(calendar.getTime());
}

How to convert date to string and to date again?

Hi i want to convert the current date to this format YYYY-MM-DD. However, it will convert the date into String format, but i want to convert it back into Date format. So can anyone advise on this?
This is my code so far
DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
Date date = new Date();
String datestring = dateFormat.format(date);

try this:
String DATE_FORMAT_NOW = "yyyy-MM-dd";
Date date = new Date();
SimpleDateFormat sdf = new SimpleDateFormat(DATE_FORMAT_NOW);
String stringDate = sdf.format(date );
try {
Date date2 = sdf.parse(stringDate);
} catch(ParseException e){
//Exception handling
} catch(Exception e){
//handle exception
}

Use DateFormat#parse(String):
Date date = dateFormat.parse("2013-10-22");

tl;dr
How to convert date to string and to date again?
LocalDate.now().toString()
2017-01-23
…and…
LocalDate.parse( "2017-01-23" )
java.time
The Question uses troublesome old date-time classes bundled with the earliest versions of Java. Those classes are now legacy, supplanted by the java.time classes built into Java 8, Java 9, and later.
Determining today’s date requires a time zone. For any given moment the date varies around the globe by zone.
If not supplied by you, your JVM’s current default time zone is applied. That default can change at any moment during runtime, and so is unreliable. I suggest you always specify your desired/expected time zone.
ZoneId z = ZoneId.of( "America/Montreal" ) ;
LocalDate ld = LocalDate.now( z ) ;
ISO 8601
Your desired format of YYYY-MM-DD happens to comply with the ISO 8601 standard.
That standard happens to be used by default by the java.time classes when parsing/generating strings. So you can simply call LocalDate::parse and LocalDate::toString without specifying a formatting pattern.
String s = ld.toString() ;
To parse:
LocalDate ld = LocalDate.parse( s ) ;
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.

try this function
public static Date StringToDate(String strDate) throws ModuleException {
Date dtReturn = null;
SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-mm-dd");
try {
dtReturn = simpleDateFormat.parse(strDate);
} catch (ParseException e) {
e.printStackTrace();
}
return dtReturn;
}

Convert Date to String using this function
public String convertDateToString(Date date, String format) {
String dateStr = null;
DateFormat df = new SimpleDateFormat(format);
try {
dateStr = df.format(date);
} catch (Exception ex) {
System.out.println(ex);
}
return dateStr;
}
From Convert Date to String in Java
. And convert string to date again
public Date convertStringToDate(String dateStr, String format) {
Date date = null;
DateFormat df = new SimpleDateFormat(format);
try {
date = df.parse(dateStr);
} catch (Exception ex) {
System.out.println(ex);
}
return date;
}
From Convert String to date in Java

Android: how to get the current day of the week (Monday, etc...) in the user's language?

I want to know what the current day of the week is (Monday, Tuesday...) in the user's local language. For example, "Lundi" "Mardi" etc... if the user is French.
I have read this post, it but it only returns an int, not a string with the day in the user's language: What is the easiest way to get the current day of the week in Android?
More generally, how do you get all the days of the week and all the months of the year written in the user's language ?
I think that this is possible, as for example the Google agenda gives the days and months written in the user's local language.

Use SimpleDateFormat to format dates and times into a human-readable string, with respect to the users locale.
Small example to get the current day of the week (e.g. "Monday"):
SimpleDateFormat sdf = new SimpleDateFormat("EEEE");
Date d = new Date();
String dayOfTheWeek = sdf.format(d);

Try this:
int dayOfWeek = date.get(Calendar.DAY_OF_WEEK);
String weekday = new DateFormatSymbols().getShortWeekdays()[dayOfWeek];

I know already answered but who looking for 'Fri' like this
for Fri -
SimpleDateFormat sdf = new SimpleDateFormat("EEE");
Date d = new Date();
String dayOfTheWeek = sdf.format(d);
and who wants full date string they can use 4E for Friday
For Friday-
SimpleDateFormat sdf = new SimpleDateFormat("EEEE");
Date d = new Date();
String dayOfTheWeek = sdf.format(d);
Enjoy...

To make things shorter You can use this:
android.text.format.DateFormat.format("EEEE", date);
which will return day of the week as a String.

Hers's what I used to get the day names (0-6 means monday - sunday):
public static String getFullDayName(int day) {
Calendar c = Calendar.getInstance();
// date doesn't matter - it has to be a Monday
// I new that first August 2011 is one ;-)
c.set(2011, 7, 1, 0, 0, 0);
c.add(Calendar.DAY_OF_MONTH, day);
return String.format("%tA", c);
}
public static String getShortDayName(int day) {
Calendar c = Calendar.getInstance();
c.set(2011, 7, 1, 0, 0, 0);
c.add(Calendar.DAY_OF_MONTH, day);
return String.format("%ta", c);
}

Try this...
//global declaration
private TextView timeUpdate;
Calendar calendar;
.......
timeUpdate = (TextView) findViewById(R.id.timeUpdate); //initialize in onCreate()
.......
//in onStart()
calendar = Calendar.getInstance();
//date format is: "Date-Month-Year Hour:Minutes am/pm"
SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy HH:mm a"); //Date and time
String currentDate = sdf.format(calendar.getTime());
//Day of Name in full form like,"Saturday", or if you need the first three characters you have to put "EEE" in the date format and your result will be "Sat".
SimpleDateFormat sdf_ = new SimpleDateFormat("EEEE");
Date date = new Date();
String dayName = sdf_.format(date);
timeUpdate.setText("" + dayName + " " + currentDate + "");
The result is...

tl;dr
String output =
LocalDate.now( ZoneId.of( "America/Montreal" ) )
.getDayOfWeek()
.getDisplayName( TextStyle.FULL , Locale.CANADA_FRENCH ) ;
java.time
The java.time classes built into Java 8 and later and back-ported to Java 6 & 7 and to Android include the handy DayOfWeek enum.
The days are numbered according to the standard ISO 8601 definition, 1-7 for Monday-Sunday.
DayOfWeek dow = DayOfWeek.of( 1 );
This enum includes the getDisplayName method to generate a String of the localized translated name of the day.
The Locale object specifies a human language to be used in translation, and specifies cultural norms to decide issues such as capitalization and punctuation.
String output = DayOfWeek.MONDAY.getDisplayName( TextStyle.FULL , Locale.CANADA_FRENCH ) ;
To get today’s date, use the LocalDate class. Note that a time zone is crucial as for any given moment the date varies around the globe.
ZoneId z = ZoneId.of( "America/Montreal" );
LocalDate today = LocalDate.now( z );
DayOfWeek dow = today.getDayOfWeek();
String output = dow.getDisplayName( TextStyle.FULL , Locale.CANADA_FRENCH ) ;
Keep in mind that the locale has nothing to do with the time zone.two separate distinct orthogonal issues. You might want a French presentation of a date-time zoned in India (Asia/Kolkata).
Joda-Time
UPDATE: The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
The Joda-Time library provides Locale-driven localization of date-time values.
DateTimeZone zone = DateTimeZone.forID( "America/Montreal" );
DateTime now = DateTime.now( zone );
Locale locale = Locale.CANADA_FRENCH;
DateTimeFormatter formatterUnJourQuébécois = DateTimeFormat.forPattern( "EEEE" ).withLocale( locale );
String output = formatterUnJourQuébécois.print( now );
System.out.println("output: " + output );
output: samedi
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….

Sorry for late reply.But this would work properly.
daytext=(textview)findviewById(R.id.day);
Calender c=Calender.getInstance();
SimpleDateFormat sd=new SimpleDateFormat("EEEE");
String dayofweek=sd.format(c.getTime());
daytext.setText(dayofweek);

I just use this solution in Kotlin:
var date : String = DateFormat.format("EEEE dd-MMM-yyyy HH:mm a" , Date()) as String

If you are using ThreetenABP date library bt Jake Warthon you can do:
dayOfWeek.getDisplayName(TextStyle.FULL, Locale.getDefault()
on your dayOfWeek instance. More at:
https://github.com/JakeWharton/ThreeTenABP https://www.threeten.org/threetenbp/apidocs/org/threeten/bp/format/TextStyle.html

//selected date from calender
SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy"); //Date and time
String currentDate = sdf.format(myCalendar.getTime());
//selcted_day name
SimpleDateFormat sdf_ = new SimpleDateFormat("EEEE");
String dayofweek=sdf_.format(myCalendar.getTime());
current_date.setText(currentDate);
lbl_current_date.setText(dayofweek);
Log.e("dayname", dayofweek);

How can I change the date format in Java? [duplicate]

This question already has answers here:
java.util.Date format conversion yyyy-mm-dd to mm-dd-yyyy
(8 answers)
Closed 3 years ago.
I need to change the date format using Java from
dd/MM/yyyy to yyyy/MM/dd

How to convert from one date format to another using SimpleDateFormat:
final String OLD_FORMAT = "dd/MM/yyyy";
final String NEW_FORMAT = "yyyy/MM/dd";
// August 12, 2010
String oldDateString = "12/08/2010";
String newDateString;
SimpleDateFormat sdf = new SimpleDateFormat(OLD_FORMAT);
Date d = sdf.parse(oldDateString);
sdf.applyPattern(NEW_FORMAT);
newDateString = sdf.format(d);

SimpleDateFormat sdf = new SimpleDateFormat("yyyy/MM/dd");
sdf.format(new Date());
This should do the trick

tl;dr
LocalDate.parse(
"23/01/2017" ,
DateTimeFormatter.ofPattern( "dd/MM/uuuu" , Locale.UK )
).format(
DateTimeFormatter.ofPattern( "uuuu/MM/dd" , Locale.UK )
)
2017/01/23
Avoid legacy date-time classes
The answer by Christopher Parker is correct but outdated. The troublesome old date-time classes such as java.util.Date, java.util.Calendar, and java.text.SimpleTextFormat are now legacy, supplanted by the java.time classes.
Using java.time
Parse the input string as a date-time object, then generate a new String object in the desired format.
The LocalDate class represents a date-only value without time-of-day and without time zone.
DateTimeFormatter fIn = DateTimeFormatter.ofPattern( "dd/MM/uuuu" , Locale.UK ); // As a habit, specify the desired/expected locale, though in this case the locale is irrelevant.
LocalDate ld = LocalDate.parse( "23/01/2017" , fIn );
Define another formatter for the output.
DateTimeFormatter fOut = DateTimeFormatter.ofPattern( "uuuu/MM/dd" , Locale.UK );
String output = ld.format( fOut );
2017/01/23
By the way, consider using standard ISO 8601 formats for strings representing date-time values.
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.
Joda-Time
UPDATE: The Joda-Time project is now in maintenance mode, with the team advising migration to the java.time classes. This section here is left for the sake of history.
For fun, here is his code adapted for using the Joda-Time library.
// © 2013 Basil Bourque. This source code may be used freely forever by anyone taking full responsibility for doing so.
// import org.joda.time.*;
// import org.joda.time.format.*;
final String OLD_FORMAT = "dd/MM/yyyy";
final String NEW_FORMAT = "yyyy/MM/dd";
// August 12, 2010
String oldDateString = "12/08/2010";
String newDateString;
DateTimeFormatter formatterOld = DateTimeFormat.forPattern(OLD_FORMAT);
DateTimeFormatter formatterNew = DateTimeFormat.forPattern(NEW_FORMAT);
LocalDate localDate = formatterOld.parseLocalDate( oldDateString );
newDateString = formatterNew.print( localDate );
Dump to console…
System.out.println( "localDate: " + localDate );
System.out.println( "newDateString: " + newDateString );
When run…
localDate: 2010-08-12
newDateString: 2010/08/12

Use SimpleDateFormat
String DATE_FORMAT = "yyyy/MM/dd";
SimpleDateFormat sdf = new SimpleDateFormat(DATE_FORMAT);
System.out.println("Formated Date " + sdf.format(date));
Complete Example:
import java.text.SimpleDateFormat;
import java.util.Date;
public class JavaSimpleDateFormatExample {
public static void main(String args[]) {
// Create Date object.
Date date = new Date();
// Specify the desired date format
String DATE_FORMAT = "yyyy/MM/dd";
// Create object of SimpleDateFormat and pass the desired date format.
SimpleDateFormat sdf = new SimpleDateFormat(DATE_FORMAT);
/*
* Use format method of SimpleDateFormat class to format the date.
*/
System.out.println("Today is " + sdf.format(date));
}
}

SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
Date myDate = sdf.parse("28/12/2013");
sdf.applyPattern("yyyy/MM/dd")
String myDateString = sdf.format(myDate);
Now myDateString = 2013/12/28

This is just Christopher Parker's answer adapted to use the new1 classes from Java 8:
final DateTimeFormatter OLD_FORMATTER = DateTimeFormatter.ofPattern("dd/MM/yyyy");
final DateTimeFormatter NEW_FORMATTER = DateTimeFormatter.ofPattern("yyyy/MM/dd");
String oldString = "26/07/2017";
LocalDate date = LocalDate.parse(oldString, OLD_FORMATTER);
String newString = date.format(NEW_FORMATTER);
1 well, not that new anymore, Java 9 should be released soon.

Or you could go the regex route:
String date = "10/07/2010";
String newDate = date.replaceAll("(\\d+)/(\\d+)/(\\d+)", "$3/$2/$1");
System.out.println(newDate);
It works both ways too. Of course this won't actually validate your date and will also work for strings like "21432/32423/52352". You can use "(\\d{2})/(\\d{2})/(\\d{4}" to be more exact in the number of digits in each group, but it will only work from dd/MM/yyyy to yyyy/MM/dd and not the other way around anymore (and still accepts invalid numbers in there like 45). And if you give it something invalid like "blabla" it will just return the same thing back.

many ways to change date format
private final String dateTimeFormatPattern = "yyyy/MM/dd";
private final Date now = new Date();
final DateFormat format = new SimpleDateFormat(dateTimeFormatPattern);
final String nowString = format.format(now);
final Instant instant = now.toInstant();
final DateTimeFormatter formatter =
DateTimeFormatter.ofPattern(
dateTimeFormatPattern).withZone(ZoneId.systemDefault());
final String formattedInstance = formatter.format(instant);
/* Java 8 needed*/
LocalDate date = LocalDate.now();
String text = date.format(formatter);
LocalDate parsedDate = LocalDate.parse(text, formatter);

To Change the format of Date you have Require both format look below.
String stringdate1 = "28/04/2010";
try {
SimpleDateFormat format1 = new SimpleDateFormat("dd/MM/yyyy");
Date date1 = format1.parse()
SimpleDateFormat format2 = new SimpleDateFormat("yyyy/MM/dd");
String stringdate2 = format2.format(date1);
} catch (ParseException e) {
e.printStackTrace();
}
here stringdate2 have date format of yyyy/MM/dd. and it contain 2010/04/28.

SimpleDateFormat format1 = new SimpleDateFormat("yyyy/MM/dd");
System.out.println(format1.format(date));