date.java

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/*
 * @(#)Date.java	1.74 03/01/23
 *
 * Copyright 2003 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util;

import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.TimeZone;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
import java.lang.ref.SoftReference;

/**
 * The class <code>Date</code> represents a specific instant
 * in time, with millisecond precision.
 * <p>
 * Prior to JDK&nbsp;1.1, the class <code>Date</code> had two additional
 * functions.  It allowed the interpretation of dates as year, month, day, hour,
 * minute, and second values.  It also allowed the formatting and parsing
 * of date strings.  Unfortunately, the API for these functions was not
 * amenable to internationalization.  As of JDK&nbsp;1.1, the
 * <code>Calendar</code> class should be used to convert between dates and time
 * fields and the <code>DateFormat</code> class should be used to format and
 * parse date strings.
 * The corresponding methods in <code>Date</code> are deprecated.
 * <p>
 * Although the <code>Date</code> class is intended to reflect 
 * coordinated universal time (UTC), it may not do so exactly, 
 * depending on the host environment of the Java Virtual Machine. 
 * Nearly all modern operating systems assume that 1&nbsp;day&nbsp;=
 * 24&nbsp;&times;&nbsp;60&nbsp;&times;&nbsp;60&nbsp;= 86400 seconds 
 * in all cases. In UTC, however, about once every year or two there 
 * is an extra second, called a "leap second." The leap 
 * second is always added as the last second of the day, and always 
 * on December 31 or June 30. For example, the last minute of the 
 * year 1995 was 61 seconds long, thanks to an added leap second. 
 * Most computer clocks are not accurate enough to be able to reflect 
 * the leap-second distinction. 
 * <p>
 * Some computer standards are defined in terms of Greenwich mean 
 * time (GMT), which is equivalent to universal time (UT).  GMT is 
 * the "civil" name for the standard; UT is the 
 * "scientific" name for the same standard. The 
 * distinction between UTC and UT is that UTC is based on an atomic 
 * clock and UT is based on astronomical observations, which for all 
 * practical purposes is an invisibly fine hair to split. Because the 
 * earth's rotation is not uniform (it slows down and speeds up 
 * in complicated ways), UT does not always flow uniformly. Leap 
 * seconds are introduced as needed into UTC so as to keep UTC within 
 * 0.9 seconds of UT1, which is a version of UT with certain 
 * corrections applied. There are other time and date systems as 
 * well; for example, the time scale used by the satellite-based 
 * global positioning system (GPS) is synchronized to UTC but is 
 * <i>not</i> adjusted for leap seconds. An interesting source of 
 * further information is the U.S. Naval Observatory, particularly 
 * the Directorate of Time at:
 * <blockquote><pre>
 *     <a href=http://tycho.usno.navy.mil>http://tycho.usno.navy.mil</a>
 * </pre></blockquote>
 * <p>
 * and their definitions of "Systems of Time" at:
 * <blockquote><pre>
 *     <a href=http://tycho.usno.navy.mil/systime.html>http://tycho.usno.navy.mil/systime.html</a>
 * </pre></blockquote>
 * <p>
 * In all methods of class <code>Date</code> that accept or return 
 * year, month, date, hours, minutes, and seconds values, the 
 * following representations are used: 
 * <ul>
 * <li>A year <i>y</i> is represented by the integer 
 *     <i>y</i>&nbsp;<code>-&nbsp;1900</code>. 
 * <li>A month is represented by an integer from 0 to 11; 0 is January, 
 *     1 is February, and so forth; thus 11 is December. 
 * <li>A date (day of month) is represented by an integer from 1 to 31 
 *     in the usual manner. 
 * <li>An hour is represented by an integer from 0 to 23. Thus, the hour 
 *     from midnight to 1 a.m. is hour 0, and the hour from noon to 1 
 *     p.m. is hour 12. 
 * <li>A minute is represented by an integer from 0 to 59 in the usual manner.
 * <li>A second is represented by an integer from 0 to 61; the values 60 and 
 *     61 occur only for leap seconds and even then only in Java 
 *     implementations that actually track leap seconds correctly. Because 
 *     of the manner in which leap seconds are currently introduced, it is 
 *     extremely unlikely that two leap seconds will occur in the same 
 *     minute, but this specification follows the date and time conventions 
 *     for ISO C.
 * </ul>
 * <p>
 * In all cases, arguments given to methods for these purposes need 
 * not fall within the indicated ranges; for example, a date may be 
 * specified as January 32 and is interpreted as meaning February 1.
 *
 * @author  James Gosling
 * @author  Arthur van Hoff
 * @author  Alan Liu
 * @version 1.74, 01/23/03
 * @see     java.text.DateFormat
 * @see     java.util.Calendar
 * @see     java.util.TimeZone
 * @since   JDK1.0
 */
public class Date implements java.io.Serializable, Cloneable, Comparable {
    /* DEFAULT ZONE SYNCHRONIZATION: Part of the usage model of Date
     * is that a Date object behaves like a Calendar object whose zone
     * is the current default TimeZone.  As a result, we must be
     * careful about keeping this phantom calendar in sync with the
     * default TimeZone.  There are three class and instance variables
     * to watch out for to achieve this.  (1)staticCal. Whenever this
     * object is used, it must be reset to the default zone. This is a
     * cheap operation which can be done directly (just a reference
     * assignment), so we just do it every time. (2)simpleFormatter.
     * Likewise, the DateFormat object we use to implement toString()
     * must be reset to the current default zone before use.  Again,
     * this is a cheap reference assignment. (3)cal. This is a little
     * more tricky.  Unlike the other cached static objects, cal has
     * state, and we don't want to monkey with it willy-nilly.  The
     * correct procedure is to change the zone in a way that doesn't
     * alter the time of this object.  This means getting the millis
     * (forcing a fields->time conversion), setting the zone, and then
     * restoring the millis.  The zone must be set before restoring
     * the millis.  Since this is an expensive operation, we only do
     * this when we have to. - liu 1.2b4 */

    /* If cal is null, then fastTime indicates the time in millis.
     * Otherwise, fastTime is ignored, and cal indicates the time.
     * The cal object is only created if a setXxx call is made to
     * set a field.  For other operations, staticCal is used instead.
     */
    private transient Calendar cal;
    private transient long fastTime;

    private static Calendar staticCal = null;
    private static Calendar utcCal = null;
    private static int defaultCenturyStart = 0;

    /* use serialVersionUID from modified java.util.Date for
     * interoperability with JDK1.1. The Date was modified to write
     * and read only the UTC time.
     */
    private static final long serialVersionUID = 7523967970034938905L;

    /**
     * Caches for the DateFormatters used by various toString methods.
     */
    private static SoftReference simpleFormatter = null;
    private static SoftReference gmtFormatter = null;
    
    /**
     * Allocates a <code>Date</code> object and initializes it so that 
     * it represents the time at which it was allocated, measured to the 
     * nearest millisecond. 
     *
     * @see     java.lang.System#currentTimeMillis()
     */
    public Date() {
        this(System.currentTimeMillis());
    }

    /**
     * Allocates a <code>Date</code> object and initializes it to 
     * represent the specified number of milliseconds since the 
     * standard base time known as "the epoch", namely January 1, 
     * 1970, 00:00:00 GMT. 
     *
     * @param   date   the milliseconds since January 1, 1970, 00:00:00 GMT.
     * @see     java.lang.System#currentTimeMillis()
     */
    public Date(long date) {
        cal = null;
        fastTime = date;
    }

    /**
     * Allocates a <code>Date</code> object and initializes it so that 
     * it represents midnight, local time, at the beginning of the day 
     * specified by the <code>year</code>, <code>month</code>, and 
     * <code>date</code> arguments. 
     *
     * @param   year    the year minus 1900.
     * @param   month   the month between 0-11.
     * @param   date    the day of the month between 1-31.
     * @see     java.util.Calendar
     * @deprecated As of JDK version 1.1,
     * replaced by <code>Calendar.set(year + 1900, month, date)</code>
     * or <code>GregorianCalendar(year + 1900, month, date)</code>.
     */
    public Date(int year, int month, int date) {
        this(year, month, date, 0, 0, 0);
    }

    /**
     * Allocates a <code>Date</code> object and initializes it so that 
     * it represents the instant at the start of the minute specified by 
     * the <code>year</code>, <code>month</code>, <code>date</code>, 
     * <code>hrs</code>, and <code>min</code> arguments, in the local 
     * time zone. 
     *
     * @param   year    the year minus 1900.
     * @param   month   the month between 0-11.
     * @param   date    the day of the month between 1-31.
     * @param   hrs     the hours between 0-23.
     * @param   min     the minutes between 0-59.
     * @see     java.util.Calendar
     * @deprecated As of JDK version 1.1,
     * replaced by <code>Calendar.set(year + 1900, month, date,
     * hrs, min)</code> or <code>GregorianCalendar(year + 1900,
     * month, date, hrs, min)</code>.
     */
    public Date(int year, int month, int date, int hrs, int min) {
        this(year, month, date, hrs, min, 0);
    }

    /**
     * Allocates a <code>Date</code> object and initializes it so that 
     * it represents the instant at the start of the second specified 
     * by the <code>year</code>, <code>month</code>, <code>date</code>, 
     * <code>hrs</code>, <code>min</code>, and <code>sec</code> arguments, 
     * in the local time zone. 
     *
     * @param   year    the year minus 1900.
     * @param   month   the month between 0-11.
     * @param   date    the day of the month between 1-31.
     * @param   hrs     the hours between 0-23.
     * @param   min     the minutes between 0-59.
     * @param   sec     the seconds between 0-59.
     * @see     java.util.Calendar
     * @deprecated As of JDK version 1.1,
     * replaced by <code>Calendar.set(year + 1900, month, date,
     * hrs, min, sec)</code> or <code>GregorianCalendar(year + 1900,
     * month, date, hrs, min, sec)</code>.
     */
    public Date(int year, int month, int date, int hrs, int min, int sec) {
        cal = null;
        if (staticCal == null)
            makeStaticCalendars();
        synchronized (staticCal) {
            staticCal.setTimeZone(TimeZone.getDefault());
            staticCal.clear();
            staticCal.set(year + 1900, month, date, hrs, min, sec);
            fastTime = staticCal.getTimeInMillis();
        }
    }

    /**
     * Allocates a <code>Date</code> object and initializes it so that 
     * it represents the date and time indicated by the string 
     * <code>s</code>, which is interpreted as if by the 
     * {@link Date#parse} method. 
     *
     * @param   s   a string representation of the date.
     * @see     java.text.DateFormat
     * @see     java.util.Date#parse(java.lang.String)
     * @deprecated As of JDK version 1.1,
     * replaced by <code>DateFormat.parse(String s)</code>.
     */
    public Date(String s) {
        this(parse(s));
    }

    /**
     * Return a copy of this object.
     */
    public Object clone() {
        Date d = null;
        try {
            d = (Date)super.clone();
            if (d.cal != null) d.cal = (Calendar)d.cal.clone();
        } catch (CloneNotSupportedException e) {} // Won't happen
        return d;
    }
    
    /**
     * Determines the date and time based on the arguments. The 
     * arguments are interpreted as a year, month, day of the month, 
     * hour of the day, minute within the hour, and second within the 
     * minute, exactly as for the <tt>Date</tt> constructor with six 
     * arguments, except that the arguments are interpreted relative 
     * to UTC rather than to the local time zone. The time indicated is 
     * returned represented as the distance, measured in milliseconds, 
     * of that time from the epoch (00:00:00 GMT on January 1, 1970).
     *
     * @param   year    the year minus 1900.
     * @param   month   the month between 0-11.
     * @param   date    the day of the month between 1-31.
     * @param   hrs     the hours between 0-23.
     * @param   min     the minutes between 0-59.
     * @param   sec     the seconds between 0-59.
     * @return  the number of milliseconds since January 1, 1970, 00:00:00 GMT for
     *          the date and time specified by the arguments. 
     * @see     java.util.Calendar
     * @deprecated As of JDK version 1.1,