使用Calendar Java获取本月的最后一天XY

Question

I need to get the last date of a given month, in my case I need to get the last Date of June. My code is following:

cal.set(Calendar.DAY_OF_MONTH,
            Calendar.getInstance().getActualMinimum(Calendar.DAY_OF_MONTH));
    int month = cal.get(Calendar.MONTH) + 1;

    if (month <= 6) {
        cal.set(Calendar.DAY_OF_YEAR, Calendar.getInstance()
                .getActualMaximum(Calendar.JUNE));
        return (Calendar) cal;
    } else {
        cal.set(Calendar.DAY_OF_YEAR, Calendar.getInstance()
                .getActualMaximum(Calendar.DAY_OF_YEAR));
        return (Calendar) cal;
    }

At first I get the actual month and wether it's the first half of the year or the second in need another date, always the last date of that half year. With the code above the return is

2015-01-31

and not 2015-06-31 as I thought it should be. How could I possibly fix this?

Answer 1

Your code is all over the place at the moment, unfortunately - you're creating new calendars multiple times for no obvious reason, and you're calling Calendar.getActualMaximum passing in the wrong kind of constant (a value rather than a field).

You want something like:

int month = cal.get(Calendar.MONTH) <= Calendar.JUNE
    ? Calendar.JUNE : Calendar.DECEMBER;
cal.set(Calendar.MONTH, month);
cal.set(Calendar.DAY_OF_MONTH, cal.getActualMaximum(Calenday.DAY_OF_MONTH));
return cal;

However, I would strongly recommend using java.time if you're on Java 8, and Joda Time if you're not - both are much, much better APIs than java.util.Calendar .

Answer 2

java.time

Much easier now with the modern java.time classes. Specifically, the YearMonth , Month , and LocalDate classes.

LocalDate

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

If no time zone is specified, the JVM implicitly applies its current default time zone. That default may change at any moment, so your results may vary. Better to specify your desired/expected time zone explicitly as an argument.

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(!).

ZoneId z = ZoneId.of( "America/Montreal" ) ;  
LocalDate today = LocalDate.now( z ) ;

If you want to use the JVM's current default time zone, ask for it and pass as an argument. If omitted, the JVM's current default is applied implicitly. Better to be explicit, as the default may be changed at any moment during runtime by any code in any thread of any app within the JVM.

ZoneId z = ZoneId.systemDefault() ;  // Get JVM’s current default time zone.

Or specify a date. You may set the month by a number, with sane numbering 1-12 for January-December.

LocalDate ld = LocalDate.of( 1986 , 2 , 23 ) ;  // Years use sane direct numbering (1986 means year 1986). Months use sane numbering, 1-12 for January-December.

Or, better, use the Month enum objects pre-defined, one for each month of the year. Tip: Use these Month objects throughout your codebase rather than a mere integer number to make your code more self-documenting, ensure valid values, and provide type-safety .

LocalDate ld = LocalDate.of( 1986 , Month.FEBRUARY , 23 ) ;

YearMonth

With a LocalDate in hand, get the year-month of that date.

YearMonth ym = YearMonth.from( ld ) ;

See which half year it is in.

Set < Month > firstHalfOfYear = EnumSet.range( Month.JANUARY , Month.JUNE ); // Populate the set with first six months of the year.
boolean isFirstHalf = firstHalfOfYear.contains( ym.getMonth() );

Knowing which half of the year, get the end of June or the end of December in the same year.

LocalDate result = null;
if ( isFirstHalf ) {
    result = ym.withMonth( Month.JUNE.getValue() ).atEndOfMonth();
} else {  // Else in last half of year.
    result = ym.withMonth( Month.DECEMBER.getValue() ).atEndOfMonth();
}

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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 , 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 (<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 .