Get first next Monday after certain date?

Question

I know there is the same question here, but I have tried the answer provided and it returned an output that I don't understand. I am confused by the answer and I don't think the output is correct.

I need help, thank you :)

GregorianCalendar date1 = new GregorianCalendar( 2014, 05, 12 ); //05 is june as month start from 0 -11

while( date1.get( Calendar.DAY_OF_WEEK ) != Calendar.MONDAY )
    date1.add( Calendar.DATE, 1 );  

System.out.println(date1);

Here is the output:

java.util.GregorianCalendar[time=1405267200000,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="Asia/Singapore",offset=28800000,dstSavings=0,useDaylight=false,transitions=9,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2014,MONTH=6,WEEK_OF_YEAR=29,WEEK_OF_MONTH=3,DAY_OF_MONTH=14,DAY_OF_YEAR=195,DAY_OF_WEEK=2,DAY_OF_WEEK_IN_MONTH=2,AM_PM=0,HOUR=0,HOUR_OF_DAY=0,MINUTE=0,SECOND=0,MILLISECOND=0,ZONE_OFFSET=28800000,DST_OFFSET=0]

Where on the output should I extract to retrieve Monday's date?

Answer 1

Java 8+

LocalDate ld = LocalDate.of(2014, Month.JUNE, 12);
System.out.println(ld);
ld = ld.with(TemporalAdjusters.next(DayOfWeek.MONDAY));
System.out.println(ld);

Which prints...

2014-06-12
2014-06-16

Because it's possible that the date my actually be a Monday, you could also use...

ld = ld.with(TemporalAdjusters.nextOrSame(DayOfWeek.MONDAY));

Java <= 7

You should be using the ThreeTen Backport, which gives you the support of the Java 8 Date/Time API

Original Answer

Instead of System.out.println(date1); use System.out.println(date1.getTime());

getTime returns an instance of Date which represents the current state of the Calendar

Which will output Mon Jul 14 00:00:00 EST 2014

System.out.println(date1) is the equivlent of using System.out.println(date1.toString()), which, in this case, is dumping a bunch of useful info about the state of the Calendar object, but not really human readable data.

System.out.println(date1.getTime()) will use the Date's to toString method to display a date value, formatted based on the current local settings, which will provide more useful information.

Updated

Instead of using GregorianCalendar, you should use the system Calendar, for example...

Calendar date1 = Calendar.getInstance();
date1.set(2014, 06, 12);

Also, months are 0 indexed, meaning that Janurary is actually 0 not 1, so in your example, you've specified the month as July, not June.

So, instead, using...

Calendar date1 = Calendar.getInstance();
date1.set(2014, 05, 12);

while (date1.get(Calendar.DAY_OF_WEEK) != Calendar.MONDAY) {
    date1.add(Calendar.DATE, 1);
}

System.out.println(date1.getTime());

Which outputted...

Mon Jun 16 16:22:26 EST 2014

Which is next Monday from today...more or less ;)

Answer 2

tl;dr

LocalDate.of( 2014 , Month.JUNE , 12 )                    // Represent a date-only value, without time-of-day and without time zone.
         .with(                                           // Move from one date to another by passing an implementation of the `TemporalAdjuster` interface. 
             TemporalAdjusters.next( DayOfWeek.MONDAY )   // Use an existing implementation found on utility class `TemporalAdjusters` (plural versus singular).
         )                                                // Returns another fresh `LocalDate` object rather than altering the original, per immutable objects design.

Using java.time

You are using troublesome old date-time classes that are now legacy, supplanted by the java.time classes.

The LocalDate class represents a date-only value without time-of-day and without time zone.

LocalDate ld = LocalDate.of( 2014 , Month.JUNE , 12 ) ;

To get the following Monday, use a TemporalAdjuster implementation found in TemporalAdjusters class.

LocalDate nextMonday = ld.with( TemporalAdjusters.next( DayOfWeek.MONDAY ) ) ;

If you want to go with the original date if it is itself a Monday, then use nextOrSame adjuster.

LocalDate nextOrSameMonday = ld.with( TemporalAdjusters.nextOrSame( DayOfWeek.MONDAY ) ) ;

---

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.

Answer 3

A minor change to your code:

    GregorianCalendar date1 = new GregorianCalendar(2014, 6, 12);

    while (date1.get(Calendar.DAY_OF_WEEK) != Calendar.MONDAY) {
        date1.add(Calendar.DATE, 1);
    }

    SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy");
    System.out.println(sdf.format(date1.getTime()));

Yields:

 14-07-2014

EDIT: As per the JavaDoc documentation, the constructor you are using, expects a 0 based month index, so for the month of June, you will need to pass in a value of 5, not 6.

Answer 4

Just a suggestion - Instead of looping, you could just use the switch statement along with the Calendar.add() like this:

int weekdayNum = c.get(Calendar.DAY_OF_WEEK);

    switch (weekdayNum) {
    case 1: {
        c.add(Calendar.DAY_OF_MONTH, 1);
        break;
    }
    case 3: {
        c.add(Calendar.DAY_OF_MONTH, 6);
        break;
    }
    case 4: {
        c.add(Calendar.DAY_OF_MONTH, 5);
        break;
    }
    case 5: {
        c.add(Calendar.DAY_OF_MONTH, 4);
        break;
    }
    case 6: {
        c.add(Calendar.DAY_OF_MONTH, 3);
        break;
    }
    case 7: {
        c.add(Calendar.DAY_OF_MONTH, 2);
        break;
    }
    default:
        break;
    }

Answer 5

Or Just do some simple math. Since Calender.Monday equals 2, it is easy to calculate the number of days that need to be added to the current date.

Calendar date1 = Calendar.getInstance();
date1.set(2014, 05, 12);
int dayOfWeek = date1.get(Calendar.DAY_OF_WEEK);
date1.add(Calendar.DATE, (9 - dayOfWeek) % 7);