Develop Reference

Getting the first date of next month

Hi i am trying to get the current year in the below code however it is returning a 1970 year instead of 2020 last month this was working correctly but since we in January 2020, it is now returning a date from 1970, please assist
public String firstDateOfNextMonth(){
DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
Calendar today = Calendar.getInstance();
Calendar next = Calendar.getInstance();
today.clear();
Date date;
next.clear();
next.set(Calendar.YEAR, today.get(Calendar.YEAR));
next.set(Calendar.MONTH, today.get(Calendar.MONTH)+ 1);
next.set(Calendar.DAY_OF_MONTH, 1);
date = next.getTime();
Log.d(TAG, "The Date: " + dateFormat.format(date));
return dateFormat.format(date);
}

If you have Java 8 or above, then you have java.time and you won't have to rely on outdated datetime implementations and you can do it this way:
public static String getFirstOfNextMonth() {
// get a reference to today
LocalDate today = LocalDate.now();
// having today,
LocalDate firstOfNextMonth = today
// add one to the month
.withMonth(today.getMonthValue() + 1)
// and take the first day of that month
.withDayOfMonth(1);
// then return it as formatted String
return firstOfNextMonth.format(DateTimeFormatter.ISO_LOCAL_DATE);
}
which prints the following when called today (2020-01-03) like System.out.println(getFirstOfNextMonth());:
2020-02-01
You might have to involve an external library, the ThreeTenAbp if you want it to work in Android below API level 26. Its use is explained in this question.

not sure why the today date gets cleared, remove today.clear() at line 4

today.clear(); initalize all elements of a date with the value 0
removing this line will give you the right answer

tl;dr
LocalDate // Represent a date-only value without a time-of-day and without a time zone.
.now( // Determine the current date as seen through the wall-clock time used by people in certain region (a time zone).
ZoneId.of( "America/Montreal" ) // Real time zone names have names in the format of `Continent/Region`. Never use 2-4 letter pseudo-zones such as `IST`, `PST`, or `CST`, which are neither standardized nor unique.
) // Return a `LocalDate`.
.with( // Move from one date another by passing a `TemporalAdjuster` implementation.
TemporalAdjusters // Class providing several implementations of `TemporalAdjuster`.
.firstDayOfNextMonth() // This adjuster finds the date of the first of next month, as its name suggests.
) // Returns another `LocalDate` object. The original `LocalDate` object is unaltered.
.toString() // Generate text in standard ISO 8601 format of YYYY-MM-DD.
See this code run live at IdeOne.com.
2020-02-01
Details
You are using terrible date-time classes that were made obsolete years ago by the unanimous adoption of JSR 310 defining the java.time classes.
The Answer by deHaar is correct. Here is an even shorter solution.
TemporalAdjuster
To move from one date to another, the java.time classes include the TemporalAdjuster interface. Pass one of these objects to the with method found on many of the other java.time classes.
TemporalAdjusters.firstDayOfNextMonth()
Several implementations of that interface are found in the class TemporalAdjusters (note the s plural). One of those is firstDayOfNextMonth(), just what you need.
Get today's date. A time zone is required, as for any given moment the date varies around the globe by time zone. If omitted, your JVM's current default time zone is implicitly applied. Better to be explicit.
ZoneId z = ZoneId.of( "Asia/Tokyo" ) ;
LocalDate today = LocalDate.now( z ) ;
Get your TemporalAdjuster object.
TemporalAdjuster ta = TemporalAdjusters.firstDayOfNextMonth() ;
Apply that adjuster to get another LocalDate object. Note that java.time classes are immutable by design. So we get a new object rather than altering the original.
LocalDate firstOfNextMonth = today.with( ta ) ;
We can shorten this code to a one-liner, if desired.
LocalDate firstOfNextMonth =
LocalDate
.now(
ZoneId.of( "Africa/Tunis" )
)
.with(
TemporalAdjusters.firstDayOfNextMonth()
)
;
Text
Your desired output format of YYYY-MM-DD complies with the ISO 8601 standard used by default in the java.time classes when parsing/generating text. So no formatting pattern need be specified.
String output = firstOfNextMonth.toString() ;
2020-02-01
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 are using Calendar.clear() which clears all the fields of your calendar, and essentially reverts it to 1/1/1970 (epoch time 0).
remove today.clear() and you'll get the correct answer
see more here

Remove next.clear();. As Calendar next= Calendar.getInstance(); initiates next with the current date, in your cases Fri Jan 03 2020 15:07:53. And when you do next.clear(), it sets to the inital epoch.
Epoch, also known as Unix timestamps, is the number of seconds (not
milliseconds!) that have elapsed since January 1, 1970 at 00:00:00 GMT
(1970-01-01 00:00:00 GMT).

Java Calendar WEEK_OF_YEAR not ISO-8601compliant?

The ISO-8601 standard states that
"The first week of a year is the week that contains the first Thursday
of the year (and, hence, always contains 4 January)."
Meaning the first week of the year is not that which contains January the 1st but the first one that contains at leat four days into the new year.
Acording to that Mon, January 11 2016 is on week #2. Here is a list of week numbers for 2016.
Ubuntu reflects that in its time widget:
And the cal command does also:
Oracle supports it with the "iw" parameter of TO_CHAR:
> select to_char(to_date('11/01/2016','dd/mm/yyyy'),'iw') weekno from dual;
> WEEKNO
02
But Java says Mon, January 11 2016 is week #3
Calendar c = Calendar.getInstance();
System.out.println(c.getTime());
System.out.println(c.get(Calendar.WEEK_OF_YEAR));
Output:
Mon Jan 11 09:02:35 VET 2016
3
Java thinks the first week of the year is the one that contains January the 1st.
- Is there a way for Java to use the ISO-8601-copliant week numbering?

As I noted in my comment, the default behavior is locale specific. Some locales will give 3, some will give 2.
Luckily, you can specify the number of days that has to be present in the first week of the year, for a given Calendar. As you write above, for ISO 8601, this number is 4, thus the following code should work:
Calendar c = Calendar.getInstance();
c.setMinimalDaysInFirstWeek(4); // For ISO 8601
System.out.println(c.getTime());
System.out.println(c.get(Calendar.WEEK_OF_YEAR));
This should make the output correct regardless of locale.
Test output:
Mon Jan 11 14:54:22 CET 2016
2

tl;dr
myZonedDateTime.get( IsoFields.WEEK_OF_WEEK_BASED_YEAR )
…and…
myZonedDateTime.get( IsoFields.WEEK_BASED_YEAR )
Avoid legacy date-time classes
As the correct Answer by haraldK explains, the Calendar class’s definition of week varies by locale. While well-intentioned, this is confusing.
You should be avoiding Calendar and related classes such as Date. They are now supplanted by the java.time classes.
ISO 8601 week
As for ISO 8601 week, be clear that means:
The first day is Monday, running through Sunday.
Week number one of a week-based year contains the first Thursday of the calendar year.
A week-based year has either 52 or 53 weeks.
The first/last few days of a calendar year may appear in the previous/next week-based year.
java.time
The java.time classes include limited support for ISO 8601 standard weeks. Call the get method on various classes such as LocalDate and ZonedDateTime. Pass the TemporalField implementations found as constants in the IsoFields class.
int week = myZonedDateTime.get( IsoFields.WEEK_OF_WEEK_BASED_YEAR ) ;
int weekBasedYear = myZonedDateTime.get( IsoFields.WEEK_BASED_YEAR ) ;
ThreeTen-Extra
Even better, add the ThreeTen-Extra library to your project to use YearWeek class.
org.threeten.extra.YearWeek yw = YearWeek.from( myZonedDateTime ) ;
Beware of calendaring software settings
Never assume the definition of a week number. Be sure the source of such a number has the same definition of week as you, such as ISO 8601 definition.
For example, the Calendar app supplied by Apple with macOS defaults to a "Gregorian" calendar definition of week. As for what that means, I do not know as I could not find any documentation about their intent/definition. For ISO 8601 weeks, you must change a setting away from default.
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.
Using a JDBC driver compliant with JDBC 4.2 or later, you may exchange java.time objects directly with your database. No need for strings nor java.sql.* classes.
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.

Understanding java.util.Calendar WEEK_OF_YEAR [duplicate]

This question already has answers here:
Why dec 31 2010 returns 1 as week of year?
(6 answers)
Closed 5 years ago.
I'm trying to understand how java.util.Calendar.get(java.util.Calendar.WEEK_OF_YEAR) works, but it seems that I'm missing some points.
String time = "1998-12-31"; // year month day
java.util.Calendar date = java.util.Calendar.getInstance();
date.setTime((new java.text.SimpleDateFormat("yyyy-MM-dd")).parse(time));
System.err.println("Week of year = " + date.get(java.util.Calendar.WEEK_OF_YEAR));
// Week of year = 1 Why ???
Why date.get(java.util.Calendar.WEEK_OF_YEAR) returns 1 for the last week of the year?
Moreover, WEEK_OF_YEAR for "1998-01-01" is 1 and for "1998-12-23" it is 52.
Does anybody have an explanation for this behavior?

From java.util.Calendar javadoc:
First Week
Calendar defines a locale-specific seven day week using two
parameters: the first day of the week and the minimal days in first
week (from 1 to 7). These numbers are taken from the locale resource
data when a Calendar is constructed. They may also be specified
explicitly through the methods for setting their values.
When setting or getting the WEEK_OF_MONTH or WEEK_OF_YEAR fields,
Calendar must determine the first week of the month or year as a
reference point. The first week of a month or year is defined as the
earliest seven day period beginning on getFirstDayOfWeek() and
containing at least getMinimalDaysInFirstWeek() days of that month or
year. Weeks numbered ..., -1, 0 precede the first week; weeks numbered
2, 3,... follow it. Note that the normalized numbering returned by
get() may be different. For example, a specific Calendar subclass may
designate the week before week 1 of a year as week n of the previous
year.
So it's locale-specific. In your case, if the week contains days from new year, it is counted as week 1 from the new year.
You can change this behavior by using Calendar#setMinimalDaysInFirstWeek(int).

tl;dr
java.time.LocalDate.parse( "1998-12-31" )
.get( IsoFields.WEEK_OF_WEEK_BASED_YEAR )
53
Or, add a library, and then…
org.threeten.extra.YearWeek.from( // Convert from a `LocalDate` object to a `YearWeek` object representing the entire week of that date’s week-based year.
LocalDate.parse( "1998-12-31" ) // Parse string into a `LocalDate` objects.
).getWeek() // Extract an integer number of that week of week-based-year, either 1-52 or 1-53 depending on the year.
53
Details
I'm trying to understand how java.util.Calendar.get(java.util.Calendar.WEEK_OF_YEAR) works
Don’t! That class is a bloody mess, and best left forgotten.
The answer by npe is correct. In Calendar, the definition of a week varies by locale. A well-intentioned feature, but confusing.
Standard week definition
There are many ways to define “a week” and “first week of the year”.
However, there is one major standard definition: the ISO 8601 standard. That standard defines weeks of the year, including the first week of the year.
the week with the year's first Thursday
A standard week begins with Monday and ends with Sunday.
Week # 1 of a standard week-based-year has the first Thursday of the calendar-year.
java.time
The java.time classes supplanted the troublesome legacy date-time classes. These modern classes support the ISO 8601 week through the IsoFields class, holding three constants that implement TemporalField:
WEEK_OF_WEEK_BASED_YEAR
WEEK_BASED_YEAR
WEEK_BASED_YEARS
Call LocalDate::get to access the TemporalField.
LocalDate ld = LocalDate.parse( "1998-12-31" ) ;
int weekOfWeekBasedYear = ld.get( IsoFields.WEEK_OF_WEEK_BASED_YEAR ) ;
int yearOfWeekBasedYear = ld.get( IsoFields.WEEK_BASED_YEAR ) ;
ld.toString(): 1998-12-31
weekOfWeekBasedYear: 53
yearOfWeekBasedYear: 1998
Notice the day after, the first day of the new calendar year 1999, also is in the same week, week # 53 of week-based 1998.
LocalDate firstOf1999 = ld.plusDays( 1 );
int weekOfWeekBasedYear_FirstOf1999 = firstOf1999.get( IsoFields.WEEK_OF_WEEK_BASED_YEAR ) ;
int yearOfWeekBasedYear_FirstOf1999 = firstOf1999.get( IsoFields.WEEK_BASED_YEAR ) ;
firstOf1999.toString(): 1999-01-01
weekOfWeekBasedYear_FirstOf1999: 53
yearOfWeekBasedYear_FirstOf1999: 1998
ISO 8601 string format
The ISO 8601 standard defines a textual format as well as a meaning for week-based-year values: yyyy-Www. For a specific date, add day-of-week numbered 1-7 for Monday-Sunday: yyyy-Www-d.
Construct such a string.
String outputWeek = ld.format( DateTimeFormatter.ISO_WEEK_DATE ) ; // yyyy-Www
1998-W53
String outputDate = outputWeek + "-" + ld.getDayOfWeek().getValue() ; // yyyy-Www-d
1998-W53-4
YearWeek
This work is much easier if you add the ThreeTen-Extra library to your project. Then use the YearWeek class.
YearWeek yw = YearWeek.from( ld ) ; // Determine ISO 8601 week of a `LocalDate`.
Generate the standard string.
String output = yw.toString() ;
1998-W53
And parse.
YearWeek yearWeek = YearWeek.parse( "1998-W53" ) ;
yearWeek.toString(): 1998-W53
Determine a date. Pass a java.time.DayOfWeek enum object for day-of-week Monday-Sunday.
LocalDate localDate = yw.atDay( DayOfWeek.MONDAY ) ;
localDate.toString(): 1998-12-28
I strongly recommending adding this library to your project. Then you can pass around smart objects rather than dumb ints. Doing so makes your code more self-documenting, provides type-safety, and ensures valid 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.
Using a JDBC driver compliant with JDBC 4.2 or later, you may exchange java.time objects directly with your database. No need for strings nor java.sql.* classes.
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.
Joda-Time
UPDATE: The Joda-Time project is now in maintenance mode, with the team advising migration to the java.time classes. This section left intact as history.
The excellent Joda-Time framework uses ISO 8601 for its defaults. Its classes include this week-of-year information. Joda-Time is a popular replacement for the notoriously troublesome java.util.Date & java.util.Calendar classes bundled with Java.
Example Code
Here is some example code to get first moment of the first day of the first week of the year of the current date-time.
Note the call to withTimeAtStartOfDay to get the first moment of the day.
DateTimeZone timeZone = DateTimeZone.forID( "Europe/Paris" );
DateTime now = new DateTime( timeZone );
DateTime firstWeekStart = now.withWeekOfWeekyear(1).withDayOfWeek(1).withTimeAtStartOfDay();
DateTime firstWeekStop = firstWeekStart.plusWeeks( 1 );
Interval firstWeek = new Interval( firstWeekStart, firstWeekStop );
Dump to console…
System.out.println( "now: " + now );
System.out.println( "firstWeekStart: " + firstWeekStart );
System.out.println( "firstWeekStop: " + firstWeekStop );
System.out.println( "firstWeek: " + firstWeek );
When run…
now: 2014-02-07T12:49:33.623+01:00
firstWeekStart: 2013-12-30T00:00:00.000+01:00
firstWeekStop: 2014-01-06T00:00:00.000+01:00
firstWeek: 2013-12-30T00:00:00.000+01:00/2014-01-06T00:00:00.000+01:00

Calendar returns wrong month [duplicate]

This question already has answers here:
Why is January month 0 in Java Calendar?
(18 answers)
Closed 2 years ago.
Calendar rightNow = Calendar.getInstance();
String month = String.valueOf(rightNow.get(Calendar.MONTH));
After the execution of the above snippet, month gets a value of 10 instead of 11. How come?

Months are indexed from 0 not 1 so 10 is November and 11 will be December.

They start from 0 - check the docs

As is clear by the many answers: the month starts with 0.
Here's a tip: you should be using SimpleDateFormat to get the String-representation of the month:
Calendar rightNow = Calendar.getInstance();
java.text.SimpleDateFormat df1 = new java.text.SimpleDateFormat("MM");
java.text.SimpleDateFormat df2 = new java.text.SimpleDateFormat("MMM");
java.text.SimpleDateFormat df3 = new java.text.SimpleDateFormat("MMMM");
System.out.println(df1.format(rightNow.getTime()));
System.out.println(df2.format(rightNow.getTime()));
System.out.println(df3.format(rightNow.getTime()));
Output:
11
Nov
November
Note: the output may vary, it is Locale-specific.

As several people have pointed out, months returned by the Calendar and Date classes in Java are indexed from 0 instead of 1. So 0 is January, and the current month, November, is 10.
You might wonder why this is the case. The origins lie with the POSIX standard functions ctime, gmtime and localtime, which accept or return a time_t structure with the following fields (from man 3 ctime):
int tm_mday; /* day of month (1 - 31) */
int tm_mon; /* month of year (0 - 11) */
int tm_year; /* year - 1900 */
This API was copied pretty much exactly into the Java Date class in Java 1.0, and from there mostly intact into the Calendar class in Java 1.1. Sun fixed the most glaring problem when they introduced Calendar – the fact that the year 2001 in the Gregorian calendar was represented by the value 101 in their Date class. But I'm not sure why they didn't change the day and month values to at least both be consistent in their indexing, either from zero or one. This inconsistency and related confusion still exists in Java (and C) to this day.

Months start from zero, like indexes for lists.
Therefore Jan = 0, Feb = 1, etc.

From the API:
The first month of the year is JANUARY
which is 0; the last depends on the
number of months in a year.
http://java.sun.com/j2se/1.5.0/docs/api/java/util/Calendar.html

tl;dr
LocalDate.now() // Returns a date-only `LocalDate` object for the current month of the JVM’s current default time zone.
.getMonthValue() // Returns 1-12 for January-December.
Details
Other answers are correct but outdated.
The troublesome old date-time classes had many poor design choices and flaws. One was the zero-based counting of month numbers 0-11 rather than the obvious 1-12.
java.time
The java.time framework is built into Java 8 and later. These classes supplant the old troublesome date-time classes such as java.util.Date, .Calendar, & java.text.SimpleDateFormat.
Now in maintenance mode, the Joda-Time project also advises migration to java.time.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations.
Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport and further adapted to Android in ThreeTenABP.
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time.
Months 1-12
In java.time the month number is indeed the expected 1-12 for January-December.
The LocalDate class represents a date-only value without time-of-day and without time zone.
Time zone
A time zone is crucial in determining a date. For any given moment, the date varies around the globe by zone. For example, a few minutes after midnight in Paris France is a new day while still “yesterday” in Montréal Québec.
Specify a proper time zone name in the format of continent/region, such as America/Montreal, Africa/Casablanca, or Pacific/Auckland. Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!).
LocalDate today = LocalDate.now( ZoneId.of( "America/Montreal" ) );
int month = today.getMonthValue(); // Returns 1-12 as values.
If you want a date-time for a time zone, use ZonedDateTime object in the same way.
ZonedDateTime now = ZonedDateTime.now( ZoneId.of( "America/Montreal" ) );
int month = now.getMonthValue(); // Returns 1-12 as values.
Convert legacy classes
If you have a GregorianCalendar object in hand, convert to ZonedDateTime using new toZonedDateTime method added to the old class. For more conversion info, see Convert java.util.Date to what “java.time” type?
ZonedDateTime zdt = myGregorianCalendar.toZonedDateTime();
int month = zdt.getMonthValue(); // Returns 1-12 as values.
Month enum
The java.time classes include the handy Month enum, by the way. Use instances of this class in your code rather than mere integers to make your code more self-documenting, provide type-safety, and ensure valid values.
Month month = today.getMonth(); // Returns an instant of `Month` rather than integer.
The Month enum offers useful methods such as generating a String with the localized name of the month.
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.

cal.get(Calendar.MONTH) + 1;
The above statement gives the exact number of the month. As get(Calendar.Month) returns month starting from 0, adding 1 to the result would give the correct output. And keep in mind to subtract 1 when setting the month.
cal.set(Calendar.MONTH, (8 - 1));
Or use the constant variables provided.
cal.set(Calendar.MONTH, Calendar.AUGUST);

It would be better to use
Calendar.JANUARY
which is zero ...

Get name of the month in Java

I wanna programmatically convert an integer in the range 1-12 to corresponding month name. (e.g. 1 -> January, 2 -> February) etc using Java Calendar class in one statement.
Note : I want to do it using Java Calendar class only. Don't suggest any switch-case or string array solution.
Thanks.

The Calendar class is not the best class to use when it comes obtaining the localized month name in one statement.
The following is an example of obtaining the month name of a desired month specified by a int value (where January is 1), using only the Calendar class:
// Month as a number.
int month = 1;
// Sets the Calendar instance to the desired month.
// The "-1" takes into account that Calendar counts months
// beginning from 0.
Calendar c = Calendar.getInstance();
c.set(Calendar.MONTH, month - 1);
// This is to avoid the problem of having a day that is greater than the maximum of the
// month you set. c.getInstance() copies the whole current dateTime from system
// including day, if you execute this on the 30th of any month and set the Month to 1
// (February) getDisplayName will get you March as it automatically jumps to the next
// Month
c.set(Calendar.DAY_OF_MONTH, 1);
// Returns a String of the month name in the current locale.
c.getDisplayName(Calendar.MONTH, Calendar.LONG, Locale.getDefault());
The above code will return the month name in the system locale.
If another locale is required, one can specify another Locale by replacing the Locale.getDefault() with a specific locale such as Locale.US.

Use DateFormatSymbols
Proudly copied and pasted from bluebones.net:
import java.text.*;
String getMonthForInt(int m) {
String month = "invalid";
DateFormatSymbols dfs = new DateFormatSymbols();
String[] months = dfs.getMonths();
if (m >= 0 && m <= 11 ) {
month = months[m];
}
return month;
}

Did you read the API? The method getDisplayName(...) looks like a good place to start. Doing it in one statement is a terrible requirement.

tl;dr
Month.of( 12 ).getDisplayName( TextStyle.FULL , Locale.US )
…or…
Month.DECEMBER.getDisplayName( TextStyle.FULL , Locale.US )
December
Using java.time
The modern way to get the localized name of a month is with the java.time.Month enum. This class is part of the java.time package than now supplants the troublesome old legacy date-time classes such as Date and Calendar.
To localize, specify:
TextStyle to determine how long or abbreviated should the string be.
Locale to determine (a) the human language for translation of name of day, name of month, and such, and (b) the cultural norms deciding issues of abbreviation, capitalization, punctuation, separators, and such.
Example code.
Month month = Month.of( 7 );
String outputConstantName = month.toString();
String outputMonthNameEnglish = month.getDisplayName( TextStyle.FULL , Locale.US );
String outputMonthQuébec = month.getDisplayName( TextStyle.FULL , Locale.CANADA_FRENCH );
month.toString(): JULY
outputMonthNameEnglish: July
outputMonthQuébec: juillet
Using the Month enum objects by name rather than month number can be handy, easier to read, and less error-prone.
String output = Month.JULY.getDisplayName( TextStyle.FULL , Locale.US ) ;
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.