Develop Reference

Get correct week Start date and end date when monday , tuesday falls in last year last week

I am getting week number and year from DB(using function DATE_PART('week',alarm_date - (interval '1 days') * 0) AS week, DATE_PART('year',alarm_date) AS yearNo , ISO compliant) and on the basis of week number and year i want to calculate weekStartDate and weekEndDate between user provided startdate and end date with week start date for example {"startDate":"2021-12-28","endDate":"2022-01-06","weekStartDay":"Sunday"}
private String getWeek(LocalDate startTime, LocalDate endTime, int yearNo, int weekNumber, int weekStartDay){
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Calendar cal = Calendar.getInstance();
// make it ISO compliant since postgres is using ISO time to calculate week number
cal.setMinimalDaysInFirstWeek(4);
cal.set(Calendar.YEAR, yearNo);
cal.set(Calendar.WEEK_OF_YEAR, weekNumber);
//ISO week starts on Monday and ends on Sunday
cal.set(Calendar.DAY_OF_WEEK, weekStartDay);// weekStartDay configurabale, as per
//user input Calendar.SUNDAY or Calendar.MONDAY or Calendar.TUEDAY; etc
String weekStartDate = sdf.format(cal.getTime());
cal.add(Calendar.DATE, 6);
String weekEndDate = sdf.format(cal.getTime());
LocalDate weekStart = LocalDate.of(Integer.valueOf(weekStartDate.substring(0, 4)),
Integer.valueOf(weekStartDate.substring(5, 7)), Integer.valueOf(weekStartDate.substring(8)));
if(weekStart.isBefore(startTime)) {
weekStart = startTime;
}
LocalDate weekEnd = LocalDate.of(Integer.valueOf(weekEndDate.substring(0, 4)),
Integer.valueOf(weekEndDate.substring(5, 7)), Integer.valueOf(weekEndDate.substring(8)));
if(weekEnd.isAfter(endTime)) {
weekEnd = endTime;
}
String weekStr = weekStart.toString()+"_"+weekEnd.toString();
return weekStr;
}
But when weekStartDay falls in lastweek of previous year then it gives the wrong vale of weekStartDate and weekStartEnd so please suggest me how to set yearnumber in java

Avinash, not the full answer but in-line with what Tom mentioned. A lot of convinient date time API's can be used since Java-8. Please feel free to customize this approach for the exact logic you may need
import java.time.DayOfWeek;
import java.time.temporal.ChronoField;
import java.time.temporal.IsoFields;
import static java.time.temporal.IsoFields.WEEK_OF_WEEK_BASED_YEAR;
import java.time.LocalDate;
import java.time.YearMonth;
import java.time.temporal.TemporalAdjusters;
public class SO75013931 {
public static void main(String[] args) {
System.out.println(getWeekStart(LocalDate.parse("2021-12-28"),DayOfWeek.SUNDAY));
System.out.println(getWeekEnd(LocalDate.parse("2021-12-28"),DayOfWeek.SUNDAY));
}
//28-12-2021 TO 01-01-2022 && 02-01-2022 to 06-01-2022
private static String getWeekStart(LocalDate localDate, DayOfWeek weekStartDay) {
LocalDate weekStart = localDate.with(TemporalAdjusters.previous(weekStartDay));
return weekStart.isBefore(localDate) ? localDate.toString(): weekStart.toString();
}
private static String getWeekEnd(LocalDate localDate, DayOfWeek weekStartDay) {
LocalDate weekEnd = localDate.with(TemporalAdjusters.next(weekStartDay.minus(1)));
return weekEnd.isBefore(localDate) ? localDate.toString(): weekEnd.toString();
}
}
This would give
2021-12-28
2022-01-01
Again, the idea was to recommend usage of these API's as opposed to older util API's

How to obtain elapsed years from string

I have the string date "3.9.1991". And I want to obtain how many years have passed since the date. For example 23. How can I achieve it using Calendar or Date?
EDIT:
I have been trying this:
private String parseVkBirthday(String birthdayString) {
// 3.9.1991
SimpleDateFormat formatter = new SimpleDateFormat("d.M.yyyy");
String formattedDate = null;
Calendar date = Calendar.getInstance();
int year = 0;
try {
Date currentDate = new Date();
Date birthdayDate = formatter.parse(birthdayString);
Log.d(LOG_TAG, "year1 = " + currentDate.getTime() + " year2 = " + birthdayDate.getTime());
long diff = currentDate.getTime() - birthdayDate.getTime();
birthdayDate.setTime(diff);
date.setTime(birthdayDate);
year = date.get(Calendar.YEAR);
} catch (ParseException e) {
e.printStackTrace();
}
return "" + year;
}
But it returns me 1993.
Answer:
There are while 3 ways for solving this problem:
1) As codeaholicguy answered, we can use Joda-Time library(what I prefer for Android).
2) As Basil Bourque answered, we can use ThreeTen-Backport library for using java.time classes from java 8.
3) And we can use java 8 and classes from java.time.
Thanks to everyone.

Use SimpleDateFormat and Period of Joda-Time library, example below:
String pattern = "dd.MM.yyyy";
SimpleDateFormat format = new SimpleDateFormat(pattern);
Date date = format.parse("3.9.1991");
System.out.println(date);
Period period = new Period(date.getTime(), (new Date()).getTime());
System.out.println(period.getYears());

String fullDate="3.9.1991";
String[] splitDate=fullDate.split(".");
int year=Integer.parseInt(splitDate[2]);
int currentYear = Calendar.getInstance().get(Calendar.YEAR);
int passedYears=currentYear-year;

Calendar.YEAR can be used to add or subtract year from current date in the same fashion we added days and month into date.
http://javarevisited.blogspot.in/2012/12/how-to-add-subtract-days-months-years-to-date-time-java.html
sample program:
import java.util.Calendar;
public class Years {
public static void main(String[] args) {
//create Calendar instance
Calendar now = Calendar.getInstance();
System.out.println("Current date : " + (now.get(Calendar.MONTH) + 1)
+ "-"
+ now.get(Calendar.DATE)
+ "-"
+ now.get(Calendar.YEAR));
//add year to current date using Calendar.add method
now.add(Calendar.YEAR,1);
System.out.println("date after one year : " + (now.get(Calendar.MONTH) + 1)
+ "-"
+ now.get(Calendar.DATE)
+ "-"
+ now.get(Calendar.YEAR));
//substract year from current date
now =Calendar.getInstance();
now.add(Calendar.YEAR,-100);
System.out.println("date before 100 years : " + (now.get(Calendar.MONTH) + 1)
+ "-"
+ now.get(Calendar.DATE)
+ "-"
+ now.get(Calendar.YEAR));
}
}
http://forgetcode.com/Java/1568-Adding-or-Subtracting-Years-to-Current-Date#

Based on example code by xrcwrn with the Joda-Time 2.8 library:
// get the current year with #xrcwm's code
Calendar mydate = new GregorianCalendar();
String mystring = "3.9.1991";
Date thedate = new SimpleDateFormat("d.m.yyyy", Locale.ENGLISH).parse(mystring);
DateTime myDateTime = new DateTime(thedate.getTime()); // joda DateTime object
// get he current date
DateTime currentDateTime = new DateTime();
// get the years value
long years = Years.between(currentDateTime, myDateTime).getYears()
The code above should give you the correct value. Mind you, this code may have some syntax errors.
As a side note, Java 8 has a time package which seems to provide more of the same functionality.

java.time
The new java.time package in Java 8 and later supplants the old java.util.Date/.Calendar & SimpleTextFormat classes.
First parse the string using new DateTimeFormatter class. Do not use SimpleTextFormat. And read the doc as there may be subtle differences in the symbol codes between the old and new classes.
Get today's date, to calculate elapsed years. Note that we need a time zone. Time zone is crucial in determining a date. A new day dawns earlier in Paris, for example, than it does in Montréal.
The Period class considers a span of time as a number of years, months and days not tied to any points on the timeline.
The between method uses the "Half-Open" approach common to date-time handling. The beginning is inclusive while the ending is exclusive.
The default formatting of java.time follows the ISO 8601 standard. Apply formatter if you wish a different string representation of your date-time values.
String input = "3.9.1991" ;
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("d.M.yyyy") ;
LocalDate then = LocalDate.parse( input, formatter ) ;
ZoneId zone = ZoneId.of( "America/Montreal" ) ;
LocalDate today = LocalDate.now( zone ) ; // Specify time zone to get your locality’s current date.
Period period = Period.between( then , today ) ;
int years = period.getYears() ;
System.out.println( "Between " + then + " and " + today + " is " + years + " years.");
Between 1991-09-03 and 2015-07-09 is 23 years.
Joda-Time
Android currently lacks Java 8 features. So you cannot use java.time. Unless perhaps the ThreeTen-Backport project (a) supports the classes used in the above example and (b) works on Android (I do not know about either).
Alternatively, you can use Joda-Time, the third-party library that inspired java.time. The Joda-Time code version of the above code example would be very similar. In this case, java.time and Joda-Time parallel one another with similar classes.

Calendar mydate = new GregorianCalendar();
String mystring = "3.9.1991";
Date thedate = new SimpleDateFormat("d.m.yyyy", Locale.ENGLISH).parse(mystring);
mydate.setTime(thedate);
//breakdown
System.out.println("year -> "+mydate.get(Calendar.YEAR));
Reference

Android get week day names between two dates

I want to get day names between two dates with simple Java, without using any third party library.
I want to get names like Saturday, Sunday, Monday between two days inclusive both.

/**
*
* #param startDate
* #param endDate
* #return Start Date and End Date are <b>Inclusive</b>, days returned between these two dates
*/
protected List<String> getWeekDayNames(Date startDate, Date endDate) {
List<String> days = new ArrayList<String>();
Calendar startCal = Calendar.getInstance();
startCal.setTime(startDate);
Calendar endCal = Calendar.getInstance();
endCal.setTime(endDate);
if (startCal.getTimeInMillis() == endCal.getTimeInMillis()) {
days.add(this.formatDayOfWeek(startCal.getTime()));
return Collections.unmodifiableList(days);
}
// swap values
if (startCal.getTimeInMillis() > endCal.getTimeInMillis()) {
startCal.setTime(endDate);
endCal.setTime(startDate);
}
do {
days.add(this.formatDayOfWeek(startCal.getTime()));
startCal.add(Calendar.DAY_OF_MONTH, 1);
} while (startCal.getTimeInMillis() <= endCal.getTimeInMillis());
return Collections.unmodifiableList(days);
}
Usage:
Calendar cal = Calendar.getInstance();
cal.add(Calendar.DAY_OF_MONTH, 15);
List<String> list = new Test().getWeekDayNames(new Date(), cal.getTime());
System.out.println(list);
Output:
[SATURDAY, SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY]

Joda-Time
Usually I would suggest the Joda-Time library, a popular replacement for the notoriously troublesome java.util.Date & java.util.Calendar classes bundled with Java. But the Question requires no third-party libraries.
java.time.*
So, instead of Joda-Time, my code example below uses the new java.time.* package bundled with Java 8. These classes are inspired by Joda-Time, but are entirely re-architected. They are defined by JSR 310. For more information, see the new Tutorial from Oracle.
The solution is quite simple. Boils down to this one-line fragment…
DayOfWeek.from( zonedDateTime ).getDisplayName( TextStyle.FULL, Locale.US );
For fun, I tossed in an extra line to show how easy it is to localize. In this case I show the French as well as US English word for day-of-week.
Here is the entire snippet, ready to run if you import java.time.* and java.time.format.*.
ZoneId timeZone = ZoneId.of( "America/New_York" );
ZonedDateTime start = ZonedDateTime.now( timeZone );
ZonedDateTime stop = start.plusDays( 2 );
// Usually spans of time are handled in a "half-open" manner, meaning start is inclusive and stop is exclusive.
// But the Question required both start and stop to be inclusive. So add "1".
long days = java.time.temporal.ChronoUnit.DAYS.between( start, stop ) + 1L;
System.out.println( days + " days from " + start + " to " + stop + " inclusive…");
for ( int i = 0; i < days; i++ ) {
ZonedDateTime zonedDateTime = start.plusDays( i );
String dayOfWeek = DayOfWeek.from( zonedDateTime ).getDisplayName( TextStyle.FULL, java.util.Locale.US );
String dayOfWeek_Français = DayOfWeek.from( zonedDateTime ).getDisplayName( TextStyle.FULL, java.util.Locale.FRENCH );
System.out.println( "zonedDateTime: " + zonedDateTime + " dayOfWeek: " + dayOfWeek + " dayOfWeek_Français: " + dayOfWeek_Français );
}
When run…
3 days from 2014-02-08T06:06:33.335-05:00[America/New_York] to 2014-02-10T06:06:33.335-05:00[America/New_York] inclusive…
zonedDateTime: 2014-02-08T06:06:33.335-05:00[America/New_York] dayOfWeek: Saturday dayOfWeek_Français: samedi
zonedDateTime: 2014-02-09T06:06:33.335-05:00[America/New_York] dayOfWeek: Sunday dayOfWeek_Français: dimanche
zonedDateTime: 2014-02-10T06:06:33.335-05:00[America/New_York] dayOfWeek: Monday dayOfWeek_Français: lundi

How to check if time stamp (epoch time) is of today's or yesterday's [android]

I want to convert the time stamp (epoch time) to human readable string.
For that i am using calendar.setTimeInMillis(timeSinceEpoch) function to create the calender object and to get the date time string in human readable format.
I am confused on, How can I find out that the time stamp (epoch time) is of today or yesterday or it is in the same week as per the system's current date and time?
Is there any API's to achieve this in android?
Thanks.

You can use methods:
public static long diff(long time, int field) {
long fieldTime = getFieldInMillis(field);
Calendar cal = Calendar.getInstance();
long now = cal.getTimeInMillis();
return (time/fieldTime - now / fieldTime);
}
private static final long getFieldInMillis(int field) {
// TODO cache values
final Calendar cal = Calendar.getInstance();
long now = cal.getTimeInMillis();
cal.add(field, 1);
long after = cal.getTimeInMillis();
return after - now;
}
and use them this way:
diff(time, Calendar.DAY_OF_YEAR); // 0 - today, 1 - tomorrow, -1 - yesterday
diff(time, Calendar.WEEK_OF_YEAR); // 0 - this week, -1 - last week etc.

Its really simple:
Calendar c=Calendar.getInstance();
c.getTimeInMillis();
String cur_day=String.format("%te %B %tY",c,c,c); // This will give date like 22 February 2012
c.setTimeInMillis(time);//set your saved timestamp
String that_day=String.format("%te %B %tY",c,c,c); //this will convert timestamp into format like 22 February 2012
//you can compare days,months,year,hours,minutes,seconds and milliseconds using above method.you can find various formats in below link
For more formats,please refer http://download.oracle.com/javase/1.5.0/docs/api/java/util/Formatter.html

You can try to use this for detect today date:
public static boolean isDateToday(long milliSeconds) {
Calendar calendar = Calendar.getInstance();
calendar.setTimeInMillis(milliSeconds);
Date getDate = calendar.getTime();
calendar.setTimeInMillis(System.currentTimeMillis());
calendar.set(Calendar.HOUR_OF_DAY, 0);
calendar.set(Calendar.MINUTE, 0);
calendar.set(Calendar.SECOND, 0);
Date startDate = calendar.getTime();
return getDate.compareTo(startDate) > 0;
}

i think this is the most efficient way of figuring out if two timestamps are on the same day. plus it's language-independent:
int secondsInADay = 60*60*24;
int daysSinceEpoch1 = timestamp1/secondsInADay;
int daysSinceEpoch2 = timestamp2/secondsInADay;
if( daysSinceEpoch1 == daysSinceEpoch2 )
;//sameday
else if( daysSinceEpoch1 - daysSinceEpoch2 == 1 )
;//timestamp2 is a day before timetamp1
else if( ......
set timestamp1 to the current time if you want to compare to today

java.time
The legacy date-time API (java.util date-time types and their formatting type, SimpleDateFormat etc.) is outdated and error-prone. It is recommended to stop using it completely and switch to java.time, the modern date-time API*.
Solution using java.time, the modern API:
The modern date-time API is rich with intuitive concepts e.g. it provides us with the class, Instant which represents an instantaneous point on the timeline. There is a class called ZonedDateTime which represents a date-time with a time-zone in the ISO-8601 calendar system. In order to switch to a different time unit (e.g. day, hour, minute, week, month etc.), the API provides methods named as prepositions and other spoken English constructs. Learn more about the modern date-time API from Trail: Date Time.
The concepts like today, yesterday, same week etc. are bounds to a timezone e.g a moment today in London can be tomorrow in Singapore. Also, the start of the week is Locale-sensitive e.g. for Locale.France, it starts on Monday whereas for Locale.US, it starts on Sunday.
Demo:
import java.time.DayOfWeek;
import java.time.Instant;
import java.time.LocalDate;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.temporal.TemporalAdjusters;
import java.util.stream.Stream;
public class Main {
public static void main(String[] args) {
// Test
Stream.of(
1621533017083L,
1621446617083L,
1621619417083L,
1621189684296L,
1621209600000L,
1621814400000L
).forEach(millis -> {
System.out.println(millis);
System.out.println(Instant.ofEpochMilli(millis));
System.out.println("Today: " + isToday(millis, "Europe/London"));
System.out.println("Yesterday: " + isYesterday(millis, "Europe/London"));
System.out.println("In the current week: " + isInTheSameWeek(millis, "Europe/London"));
System.out.println();
});
}
static boolean isToday(long epochMillis, String timezone) {
ZoneId zoneId = ZoneId.of(timezone);
// The start of the day today at this timezone
ZonedDateTime zdtStartOfDayToday = LocalDate.now().atStartOfDay(zoneId);
long millisStartOfDayToday = zdtStartOfDayToday.toInstant().toEpochMilli();
// The start of the next day at this timezone
ZonedDateTime zdtStartOfDayNextDay = LocalDate.now().plusDays(1).atStartOfDay(zoneId);
long millisStartOfDayNextDay = zdtStartOfDayNextDay.toInstant().toEpochMilli();
return (epochMillis >= millisStartOfDayToday && epochMillis < millisStartOfDayNextDay);
}
static boolean isYesterday(long epochMillis, String timezone) {
ZoneId zoneId = ZoneId.of(timezone);
// The start of the day today at this timezone
ZonedDateTime zdtStartOfDayToday = LocalDate.now().atStartOfDay(zoneId);
long millisStartOfDayToday = zdtStartOfDayToday.toInstant().toEpochMilli();
// The start of the day yesterday at this timezone
ZonedDateTime zdtStartOfDayYesterday = LocalDate.now().minusDays(1).atStartOfDay(zoneId);
long millisStartOfDayYesterday = zdtStartOfDayYesterday.toInstant().toEpochMilli();
return (epochMillis >= millisStartOfDayYesterday && epochMillis < millisStartOfDayToday);
}
static boolean isInTheSameWeek(long epochMillis, String timezone) {
ZoneId zoneId = ZoneId.of(timezone);
// The start of the day today at this timezone
ZonedDateTime zdtStartOfDayToday = LocalDate.now().atStartOfDay(zoneId);
// The start of the week at this timezone
ZonedDateTime zdtStartOfTheWeek = zdtStartOfDayToday.with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));
long millisStartOfTheWeek = zdtStartOfTheWeek.toInstant().toEpochMilli();
// The start of the next week at this timezone
ZonedDateTime zdtStartOfTheNextWeek = zdtStartOfDayToday.with(TemporalAdjusters.next(DayOfWeek.MONDAY));
long millisStartOfTheNextWeek = zdtStartOfTheNextWeek.toInstant().toEpochMilli();
return (epochMillis >= millisStartOfTheWeek && epochMillis < millisStartOfTheNextWeek);
}
}
Output:
1621533017083
2021-05-20T17:50:17.083Z
Today: true
Yesterday: false
In the current week: true
1621446617083
2021-05-19T17:50:17.083Z
Today: false
Yesterday: true
In the current week: true
1621619417083
2021-05-21T17:50:17.083Z
Today: false
Yesterday: false
In the current week: true
1621189684296
2021-05-16T18:28:04.296Z
Today: false
Yesterday: false
In the current week: false
1621209600000
2021-05-17T00:00:00Z
Today: false
Yesterday: false
In the current week: true
1621814400000
2021-05-24T00:00:00Z
Today: false
Yesterday: false
In the current week: false
ONLINE DEMO
* 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.

time should be in milli seconds
DateUtils.isToday(time)

How to get name of the first day of a month?

How would I go about getting the first day of the month? So for January, of this year, it would return Sunday. And then for February it would return Wednesday.

To get the first date of the current month, use java.util.Calendar. First get an instance of it and set the field Calendar.DAY_OF_MONTH to the first date of the month. Since the first day of any month is 1, inplace of cal.getActualMinimum(Calendar.DAY_OF_MONTH), 1 can be used here.
private Date getFirstDateOfCurrentMonth() {
Calendar cal = Calendar.getInstance();
cal.set(Calendar.DAY_OF_MONTH, cal.getActualMinimum(Calendar.DAY_OF_MONTH));
return cal.getTime();
}

You can create a Calendar with whatever date you want and then do set(Calendar.DAY_OF_MONTH, 1) to get the first day of a month.
Calendar cal = Calendar.getInstance();
cal.set(Calendar.DATE, 25);
cal.set(Calendar.MONTH, Calendar.JANUARY);
cal.set(Calendar.YEAR, 2012);
cal.set(Calendar.DAY_OF_MONTH, 1);
Date firstDayOfMonth = cal.getTime();
DateFormat sdf = new SimpleDateFormat("EEEEEEEE");
System.out.println("First Day of Month: " + sdf.format(firstDayOfMonth));

public int getFirstDay(){
Calendar c=new GregorianCalendar();
c.set(Calendar.DAY_OF_MONTH, 1);
return c.get(Calendar.DAY_OF_WEEK);
}
From there you can see if the int is equal to Calendar.SUNDAY, Calendar.MONDAY, etc.

In the Java 8 you can use the TemporalAdjusters:
This is an example:
import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.temporal.TemporalAdjusters;
/**
* Dates in Java8
*
*/
public class App {
public static void main(String[] args) {
LocalDate localDate = LocalDate.now();
System.out.println("Day of Month: " + localDate.getDayOfMonth());
System.out.println("Month: " + localDate.getMonth());
System.out.println("Year: " + localDate.getYear());
System.out.printf("first day of Month: %s%n",
localDate.with(TemporalAdjusters.firstDayOfMonth()));
System.out.printf("first Monday of Month: %s%n", localDate
.with(TemporalAdjusters.firstInMonth(DayOfWeek.MONDAY)));
System.out.printf("last day of Month: %s%n",
localDate.with(TemporalAdjusters.lastDayOfMonth()));
System.out.printf("first day of next Month: %s%n",
localDate.with(TemporalAdjusters.firstDayOfNextMonth()));
System.out.printf("first day of next Year: %s%n",
localDate.with(TemporalAdjusters.firstDayOfNextYear()));
System.out.printf("first day of Year: %s%n",
localDate.with(TemporalAdjusters.firstDayOfYear()));
LocalDate tomorrow = localDate.plusDays(1);
System.out.println("Day of Month: " + tomorrow.getDayOfMonth());
System.out.println("Month: " + tomorrow.getMonth());
System.out.println("Year: " + tomorrow.getYear());
}
}
The results would be:
Day of Month: 16
Month: MAY
Year: 2014
first day of Month: 2014-05-01
first Monday of Month: 2014-05-05
last day of Month: 2014-05-31
first day of next Month: 2014-06-01
first day of next Year: 2015-01-01
first day of Year: 2014-01-01
Last in Month Tuesday: 2014-05-27
Day of Month: 17
Month: MAY
Year: 2014

TemporalAdjuster
In java 8 you can use the new LocalDate and LocalTime API.
To achieve the coveted result, give a try to the following. It prints the name of the given day.
import java.time.*;
import java.time.temporal.*;
public class Main {
public static void main(String[] args) {
LocalDate l = LocalDate.now().with(TemporalAdjusters.firstDayOfMonth());
System.out.println(l.getDayOfWeek());
}
}
Explanation:
now gives you the current date.
by calling with you can pass a TemporalAdjuster which are a key tool for modifying temporal objects.
getDayOfWeek Gets the day-of-week field, which is an enum DayOfWeek.
This includes textual names of the values.
TemporalAdjuster has a brother class, called TemporalAdjusters which contains static methods regarding the adjustments, like the one you are looking for, the firstDayOfMonth

Create java.util.Date or java.util.Calendar object, set date value and use java.text.SimpleDateFormat class method to format it.
Calendar cal=Calendar.getInstance();
cal.set(Calendar.DATE,1);
cal.set(Calendar.MONTH,0);
cal.set(Calendar.YEAR,2012);
SimpleDateFormat sdf=new SimpleDateFormat("EEEE");
System.out.println(sdf.format(cal.getTime()));

java.time
Since Java 8 we can also use YearMonth class which allows us to create LocalDate objects with specified days (first, last). Then We can simply convert these dates to DayOfWeek Enum (Tutorial) and read its name property.
YearMonth ym = YearMonth.of(2012, 1);
String firstDay = ym.atDay(1).getDayOfWeek().name();
String lastDay = ym.atEndOfMonth().getDayOfWeek().name();
System.out.println(firstDay);
System.out.println(lastDay);
result:
SUNDAY
TUESDAY

java.time, soft-coded
The Answer by Pshemo was good and clever, but is hard-coded to English. Let's take a stab at it allowing for localization.
ZoneId zoneId = ZoneId.of( "America/Montreal" ); // Time zone is crucial to determining a date.
YearMonth yearMonth = YearMonth.now( zoneId );
LocalDate firstOfMonth = yearMonth.atDay( 1 );
Formatter
Generate a textual representation of that LocalDate. We specify a desired Locale, in this case CANADA_FRENCH. If omitted, your JVM’s current default Locale is implicitly applied; better to specify explicit Locale.
DateTimeFormatter formatter = DateTimeFormatter.ofPattern( "EEEE" ).withZone( zoneId ).withLocale( Locale.CANADA_FRENCH ); // Exactly four 'E' letters means full form. http://docs.oracle.com/javase/8/docs/api/java/time/format/TextStyle.html#FULL
String output = formatter.format( firstOfMonth );
Dump to console.
System.out.println( "output: " + output );
When run.
samedi
DayOfWeek Enum
Another route is to use the DayOfWeek enum. This enum includes a getDisplayName method where you specify both the text style (full name versus abbreviation) and a Locale.
DayOfWeek dayOfWeek = firstOfMonth.getDayOfWeek() ;
String output = dayOfWeek.getDisplayName( TextStyle.FULL_STANDALONE , Locale.CANADA_FRENCH ) ;

Simple and tricky answer
val calendar = Date()
val sdf = SimpleDateFormat("yyyy-MM-01 00:00:00", Locale.getDefault())
var result = sdf.format(calendar)
yyyy-MM-01 -> dd change to 01