gregoriancalendar.java

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/* java.util.GregorianCalendar
   Copyright (C) 1998, 1999, 2001, 2002, 2003, 2004
   Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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GNU Classpath is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
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02110-1301 USA.

Linking this library statically or dynamically with other modules is
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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
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this exception to your version of the library, but you are not
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exception statement from your version. */


package java.util;


/**
 * <p>
 * This class represents the Gregorian calendar, that is used in most
 * countries all over the world.  It does also handle the Julian calendar
 * for dates smaller than the date of the change to the Gregorian calendar.
 * The Gregorian calendar differs from the Julian calendar by a different
 * leap year rule (no leap year every 100 years, except if year is divisible
 * by 400).
 * </p>
 * <p>
 * This change date is different from country to country, and can be changed with
 * <code>setGregorianChange</code>.  The first countries to adopt the Gregorian
 * calendar did so on the 15th of October, 1582.  This date followed October
 * the 4th, 1582 in the Julian calendar system.  The non-existant days that were
 * omitted when the change took place are interpreted as Gregorian dates.
 * </p>
 * <p>
 * Prior to the changeover date, New Year's Day occurred on the 25th of March.
 * However, this class always takes New Year's Day as being the 1st of January.
 * Client code should manually adapt the year value, if required, for dates
 * between January the 1st and March the 24th in years prior to the changeover.
 * </p>
 * <p>
 * Any date infinitely forwards or backwards in time can be represented by
 * this class.  A <em>proleptic</em> calendar system is used, which allows
 * future dates to be created via the existing rules.  This allows meaningful
 * and consistent dates to be produced for all years.  However, dates are only
 * historically accurate following March the 1st, 4AD when the Julian calendar
 * system was adopted.  Prior to this, leap year rules were applied erraticly.
 * </p>
 * <p>
 * There are two eras available for the Gregorian calendar, namely BC and AD.
 * </p>
 * <p>
 * Weeks are defined as a period of seven days, beginning on the first day
 * of the week, as returned by <code>getFirstDayOfWeek()</code>, and ending
 * on the day prior to this.
 * </p>
 * <p>
 * The weeks of the year are numbered from 1 to a possible 53.  The first week
 * of the year is defined as the first week that contains at least the minimum
 * number of days of the first week in the new year (retrieved via
 * <code>getMinimalDaysInFirstWeek()</code>).  All weeks after this are numbered
 * from 2 onwards.
 * </p>
 * <p>
 * For example, take the year 2004.  It began on a Thursday.  The first week
 * of 2004 depends both on where a week begins and how long it must minimally
 * last.  Let's say that the week begins on a Monday and must have a minimum
 * of 5 days.  In this case, the first week begins on Monday, the 5th of January.
 * The first 4 days (Thursday to Sunday) are not eligible, as they are too few
 * to make up the minimum number of days of the first week which must be in
 * the new year.  If the minimum was lowered to 4 days, then the first week
 * would instead begin on Monday, the 29th of December, 2003.  This first week
 * has 4 of its days in the new year, and is now eligible.
 * </p>
 * <p>
 * The weeks of the month are numbered from 0 to a possible 6.  The first week
 * of the month (numbered 1) is a set of days, prior to the first day of the week,
 * which number at least the minimum number of days in a week.  Unlike the first
 * week of the year, the first week of the month only uses days from that particular
 * month.  As a consequence, it may have a variable number of days (from the minimum
 * number required up to a full week of 7) and it need not start on the first day of
 * the week.  It must, however, be following by the first day of the week, as this
 * marks the beginning of week 2.  Any days of the month which occur prior to the
 * first week (because the first day of the week occurs before the minimum number
 * of days is met) are seen as week 0.
 * </p>
 * <p>
 * Again, we will take the example of the year 2004 to demonstrate this.  September
 * 2004 begins on a Wednesday.  Taking our first day of the week as Monday, and the
 * minimum length of the first week as 6, we find that week 1 runs from Monday,
 * the 6th of September to Sunday the 12th.  Prior to the 6th, there are only
 * 5 days (Wednesday through to Sunday).  This is too small a number to meet the
 * minimum, so these are classed as being days in week 0.  Week 2 begins on the
 * 13th, and so on.  This changes if we reduce the minimum to 5.  In this case,
 * week 1 is a truncated week from Wednesday the 1st to Sunday the 5th, and week
 * 0 doesn't exist.  The first seven day week is week 2, starting on the 6th.
 * </p>
 * <p>
 * On using the <code>clear()</code> method, the Gregorian calendar returns
 * to its default value of the 1st of January, 1970 AD 00:00:00 (the epoch).
 * The day of the week is set to the correct day for that particular time.
 * The day is also the first of the month, and the date is in week 0.
 * </p>
 *
 * @see Calendar
 * @see TimeZone
 * @see Calendar#getFirstDayOfWeek()
 * @see Calendar#getMinimalDaysInFirstWeek()
 */
public class GregorianCalendar extends Calendar
{
  /**
   * Constant representing the era BC (Before Christ).
   */
  public static final int BC = 0;

  /**
   * Constant representing the era AD (Anno Domini).
   */
  public static final int AD = 1;

  /**
   * The point at which the Gregorian calendar rules were used.
   * This is locale dependent; the default for most catholic
   * countries is midnight (UTC) on October 5, 1582 (Julian),
   * or October 15, 1582 (Gregorian).
   *
   * @serial the changeover point from the Julian calendar
   *         system to the Gregorian.
   */
  private long gregorianCutover;

  /**
   * For compatability with Sun's JDK.
   */
  static final long serialVersionUID = -8125100834729963327L;

  /**
   * The name of the resource bundle. Used only by getBundle()
   */
  private static final String bundleName = "gnu.java.locale.Calendar";

  /**
   * Days in the epoch. Relative Jan 1, year '0' which is not a leap year.
   * (although there is no year zero, this does not matter.)
   * This is consistent with the formula:
   * = (year-1)*365L + ((year-1) >> 2)
   *
   * Plus the gregorian correction:
   *  Math.floor((year-1) / 400.) - Math.floor((year-1) / 100.);
   * For a correct julian date, the correction is -2 instead.
   *
   * The gregorian cutover in 1582 was 10 days, so by calculating the
   * correction from year zero, we have 15 non-leap days (even centuries)
   * minus 3 leap days (year 400,800,1200) = 12. Subtracting two corrects
   * this to the correct number 10.
   */
  private static final int EPOCH_DAYS = 719162;

  /**
   * Constructs a new GregorianCalender representing the current
   * time, using the default time zone and the default locale.
   */
  public GregorianCalendar()
  {
    this(TimeZone.getDefault(), Locale.getDefault());
  }

  /**
   * Constructs a new GregorianCalender representing the current
   * time, using the specified time zone and the default locale.
   *
   * @param zone a time zone.
   */
  public GregorianCalendar(TimeZone zone)
  {
    this(zone, Locale.getDefault());
  }

  /**
   * Constructs a new GregorianCalender representing the current
   * time, using the default time zone and the specified locale.
   *
   * @param locale a locale.
   */
  public GregorianCalendar(Locale locale)
  {
    this(TimeZone.getDefault(), locale);
  }

  /**
   * Constructs a new GregorianCalender representing the current
   * time with the given time zone and the given locale.
   *
   * @param zone a time zone.
   * @param locale a locale.
   */
  public GregorianCalendar(TimeZone zone, Locale locale)
  {
    this(zone, locale, false);
    setTimeInMillis(System.currentTimeMillis());
    complete();
  }

  /**
   * Common constructor that all constructors should call.
   * @param zone a time zone.
   * @param locale a locale.
   * @param unused unused parameter to make the signature differ from
   * the public constructor (TimeZone, Locale).
   */
  private GregorianCalendar(TimeZone zone, Locale locale, boolean unused)
  {
    super(zone, locale);
    ResourceBundle rb = ResourceBundle.getBundle(bundleName, locale,
                                                 ClassLoader
                                                 .getSystemClassLoader());
    gregorianCutover = ((Date) rb.getObject("gregorianCutOver")).getTime();
  }

  /**
   * Constructs a new GregorianCalendar representing midnight on the
   * given date with the default time zone and locale.
   * @param year corresponds to the YEAR time field.
   * @param month corresponds to the MONTH time field.
   * @param day corresponds to the DAY time field.
   */
  public GregorianCalendar(int year, int month, int day)
  {
    this(TimeZone.getDefault(), Locale.getDefault(), false);
    set(year, month, day);
  }

  /**
   * Constructs a new GregorianCalendar representing midnight on the
   * given date with the default time zone and locale.
   *
   * @param year corresponds to the YEAR time field.
   * @param month corresponds to the MONTH time field.
   * @param day corresponds to the DAY time field.
   * @param hour corresponds to the HOUR_OF_DAY time field.
   * @param minute corresponds to the MINUTE time field.
   */
  public GregorianCalendar(int year, int month, int day, int hour, int minute)
  {
    this(TimeZone.getDefault(), Locale.getDefault(), false);
    set(year, month, day, hour, minute);
  }

  /**
   * Constructs a new GregorianCalendar representing midnight on the
   * given date with the default time zone and locale.
   *
   * @param year corresponds to the YEAR time field.
   * @param month corresponds to the MONTH time field.
   * @param day corresponds to the DAY time field.
   * @param hour corresponds to the HOUR_OF_DAY time field.
   * @param minute corresponds to the MINUTE time field.
   * @param second corresponds to the SECOND time field.
   */
  public GregorianCalendar(int year, int month, int day, int hour, int minute,
                           int second)
  {
    this(TimeZone.getDefault(), Locale.getDefault(), false);
    set(year, month, day, hour, minute, second);
  }

  /**
   * Sets the date of the switch from Julian dates to Gregorian dates.
   * You can use <code>new Date(Long.MAX_VALUE)</code> to use a pure
   * Julian calendar, or <code>Long.MIN_VALUE</code> for a pure Gregorian
   * calendar.
   *
   * @param date the date of the change.
   */
  public void setGregorianChange(Date date)
  {
    gregorianCutover = date.getTime();
  }

  /**
   * Gets the date of the switch from Julian dates to Gregorian dates.
   *
   * @return the date of the change.
   */
  public final Date getGregorianChange()
  {
    return new Date(gregorianCutover);
  }

  /**
   * <p>
   * Determines if the given year is a leap year.  The result is
   * undefined if the Gregorian change took place in 1800, so that
   * the end of February is skipped, and that year is specified.
   * (well...).
   * </p>
   * <p>
   * To specify a year in the BC era, use a negative value calculated
   * as 1 - y, where y is the required year in BC.  So, 1 BC is 0,
   * 2 BC is -1, 3 BC is -2, etc.
   * </p>
   *
   * @param year a year (use a negative value for BC).
   * @return true, if the given year is a leap year, false otherwise.
   */
  public boolean isLeapYear(int year)
  {
    // Only years divisible by 4 can be leap years
    if ((year & 3) != 0)
      return false;

    // Is the leap-day a Julian date? Then it's a leap year
    if (! isGregorian(year, 31 + 29 - 1))
      return true;

    // Apply gregorian rules otherwise
    return ((year % 100) != 0 || (year % 400) == 0);
  }

  /**
   * Retrieves the day of the week corresponding to the specified
   * day of the specified year.
   *
   * @param year the year in which the dayOfYear occurs.
   * @param dayOfYear the day of the year (an integer between 0 and
   *        and 366)
   */
  private int getWeekDay(int year, int dayOfYear)
  {
    boolean greg = isGregorian(year, dayOfYear);
    int day = (int) getLinearDay(year, dayOfYear, greg);

    // The epoch was a thursday.
    int weekday = (day + THURSDAY) % 7;
    if (weekday <= 0)
      weekday += 7;
    return weekday;
  }

  /**
   * Returns the day of the week for the first day of a given month (0..11)
   */
  private int getFirstDayOfMonth(int year, int month)
  {
    int[] dayCount = { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };

    if (month > 11)
      {
	year += (month / 12);
	month = month % 12;
      }

    if (month < 0)
      {
	year += (int) month / 12;
	month = month % 12;
	if (month < 0)
	  {
	    month += 12;
	    year--;
	  }
      }

    int dayOfYear = dayCount[month] + 1;
    if (month > 1)
      if (isLeapYear(year))
	dayOfYear++;

    boolean greg = isGregorian(year, dayOfYear);
    int day = (int) getLinearDay(year, dayOfYear, greg);

    // The epoch was a thursday.
    int weekday = (day + THURSDAY) % 7;
    if (weekday <= 0)
      weekday += 7;
    return weekday;
  }

  /**
   * Takes a year, and a (zero based) day of year and determines
   * if it is gregorian or not.
   */
  private boolean isGregorian(int year, int dayOfYear)
  {
    int relativeDay = (year - 1) * 365 + ((year - 1) >> 2) + dayOfYear
                      - EPOCH_DAYS; // gregorian days from 1 to epoch.
    int gregFactor = (int) Math.floor((double) (year - 1) / 400.)
                     - (int) Math.floor((double) (year - 1) / 100.);

    return ((relativeDay + gregFactor) * 60L * 60L * 24L * 1000L >= gregorianCutover);
  }

  /**
   * Check set fields for validity, without leniency.
   *
   * @throws IllegalArgumentException if a field is invalid
   */
  private void nonLeniencyCheck() throws IllegalArgumentException
  {
    int[] month_days = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
    int year = fields[YEAR];
    int month = fields[MONTH];
    int leap = isLeapYear(year) ? 1 : 0;

    if (isSet[ERA] && fields[ERA] != AD && fields[ERA] != BC)
      throw new IllegalArgumentException("Illegal ERA.");
    if (isSet[YEAR] && fields[YEAR] < 1)
      throw new IllegalArgumentException("Illegal YEAR.");
    if (isSet[MONTH] && (month < 0 || month > 11))
      throw new IllegalArgumentException("Illegal MONTH.");
    if (isSet[WEEK_OF_YEAR])
      {
	int daysInYear = 365 + leap;
	daysInYear += (getFirstDayOfMonth(year, 0) - 1); // pad first week
	int last = getFirstDayOfMonth(year, 11) + 4;
	if (last > 7)
	  last -= 7;
	daysInYear += 7 - last;
	int weeks = daysInYear / 7;
	if (fields[WEEK_OF_YEAR] < 1 || fields[WEEK_OF_YEAR] > weeks)
	  throw new IllegalArgumentException("Illegal WEEK_OF_YEAR.");
      }

    if (isSet[WEEK_OF_MONTH])
      {
	int weeks = (month == 1 && leap == 0) ? 4 : 5;
	if (fields[WEEK_OF_MONTH] < 1 || fields[WEEK_OF_MONTH] > weeks)
	  throw new IllegalArgumentException("Illegal WEEK_OF_MONTH.");