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package horstmann.ch03_day3;
public class Day
{
/**
Constructs a day with a given year, month, and day
of the Julian/Gregorian calendar. The Julian calendar
is used for all days before October 15, 1582
@param aYear a year != 0
@param aMonth a month between 1 and 12
@param aDate a date between 1 and 31
*/
public Day(int aYear, int aMonth, int aDate)
{
year = aYear;
month = aMonth;
date = aDate;
ymdValid = true;
julianValid = false;
}
/**
Returns the year of this day
@return the year
*/
public int getYear()
{
ensureYmd();
return year;
}
/**
Returns the month of this day
@return the month
*/
public int getMonth()
{
ensureYmd();
return month;
}
/**
Returns the day of the month of this day
@return the day of the month
*/
public int getDate()
{
ensureYmd();
return date;
}
/**
Returns a day that is a certain number of days away from
this day
@param n the number of days, can be negative
@return a day that is n days away from this one
*/
public Day addDays(int n)
{
ensureJulian();
return new Day(julian + n);
}
/**
Returns the number of days between this day and another
day
@param other the other day
@return the number of days that this day is away from
the other (>0 if this day comes later)
*/
public int daysFrom(Day other)
{
ensureJulian();
other.ensureJulian();
return julian - other.julian;
}
private Day(int aJulian)
{
julian = aJulian;
ymdValid = false;
julianValid = true;
}
/**
Computes the Julian day number of this day if
necessary
*/
private void ensureJulian()
{
if (julianValid) return;
julian = toJulian(year, month, date);
julianValid = true;
}
/**
Converts this Julian day mumber to a calendar date if necessary.
*/
private void ensureYmd()
{
if (ymdValid) return;
int[] ymd = fromJulian(julian);
year = ymd[0];
month = ymd[1];
date = ymd[2];
ymdValid = true;
}
/**
Computes the Julian day number of the given day day.
@param year a year
@param month a month
@param date a day of the month
@return The Julian day number that begins at noon of
the given day
Positive year signifies CE, negative year BCE.
Remember that the year after 1 BCE is 1 CE.
A convenient reference point is that May 23, 1968 noon
is Julian day number 2440000.
Julian day number 0 is a Monday.
This algorithm is from Press et al., Numerical Recipes
in C, 2nd ed., Cambridge University Press 1992
*/
private static int toJulian(int year, int month, int date)
{
int jy = year;
if (year < 0) jy++;
int jm = month;
if (month > 2) jm++;
else
{
jy--;
jm += 13;
}
int jul = (int) (java.lang.Math.floor(365.25 * jy)
+ java.lang.Math.floor(30.6001 * jm) + date + 1720995.0);
int IGREG = 15 + 31 * (10 + 12 * 1582);
// Gregorian Calendar adopted Oct. 15, 1582
if (date + 31 * (month + 12 * year) >= IGREG)
// Change over to Gregorian calendar
{
int ja = (int) (0.01 * jy);
jul += 2 - ja + (int) (0.25 * ja);
}
return jul;
}
/**
Converts a Julian day number to a calendar date.
This algorithm is from Press et al., Numerical Recipes
in C, 2nd ed., Cambridge University Press 1992
@param j the Julian day number
@return an array whose 0 entry is the year, 1 the month,
and 2 the day of the month.
*/
@SuppressWarnings("cast")
private static int[] fromJulian(int j)
{
int ja = j;
int JGREG = 2299161;
// The Julian day number of the adoption of the Gregorian calendar
if (j >= JGREG)
// Cross-over to Gregorian Calendar produces this correction
{
int jalpha = (int) (((float) (j - 1867216) - 0.25)
/ 36524.25);
ja += 1 + jalpha - (int) (0.25 * jalpha);
}
int jb = ja + 1524;
int jc = (int) (6680.0 + ((float) (jb - 2439870) - 122.1)
/ 365.25);
int jd = (int) (365 * jc + (0.25 * jc));
int je = (int) ((jb - jd) / 30.6001);
int date = jb - jd - (int) (30.6001 * je);
int month = je - 1;
if (month > 12) month -= 12;
int year = jc - 4715;
if (month > 2) --year;
if (year <= 0) --year;
return new int[] { year, month, date };
}
private int year;
private int month;
private int date;
private int julian;
private boolean ymdValid;
private boolean julianValid;
}
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