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package horstmann.ch03_day1;
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;
}
/**
Returns the year of this day
@return the year
*/
public int getYear()
{
return year;
}
/**
Returns the month of this day
@return the month
*/
public int getMonth()
{
return month;
}
/**
Returns the day of the month of this day
@return the day of the month
*/
public int getDate()
{
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)
{
Day result = this;
while (n > 0)
{
result = result.nextDay();
n--;
}
while (n < 0)
{
result = result.previousDay();
n++;
}
return result;
}
/**
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)
{
int n = 0;
Day d = this;
while (d.compareTo(other) > 0)
{
d = d.previousDay();
n++;
}
while (d.compareTo(other) < 0)
{
d = d.nextDay();
n--;
}
return n;
}
/**
Compares this day with another day.
@param other the other day
@return a positive number if this day comes after the
other day, a negative number if this day comes before
the other day, and zero if the days are the same
*/
private int compareTo(Day other)
{
if (year > other.year) return 1;
if (year < other.year) return -1;
if (month > other.month) return 1;
if (month < other.month) return -1;
return date - other.date;
}
/**
Computes the next day.
@return the day following this day
*/
private Day nextDay()
{
int y = year;
int m = month;
int d = date;
if (y == GREGORIAN_START_YEAR
&& m == GREGORIAN_START_MONTH
&& d == JULIAN_END_DAY)
d = GREGORIAN_START_DAY;
else if (d < daysPerMonth(y, m))
d++;
else
{
d = 1;
m++;
if (m > DECEMBER)
{
m = JANUARY;
y++;
if (y == 0) y++;
}
}
return new Day(y, m, d);
}
/**
Computes the previous day.
@return the day preceding this day
*/
private Day previousDay()
{
int y = year;
int m = month;
int d = date;
if (y == GREGORIAN_START_YEAR
&& m == GREGORIAN_START_MONTH
&& d == GREGORIAN_START_DAY)
d = JULIAN_END_DAY;
else if (d > 1)
d--;
else
{
m--;
if (m < JANUARY)
{
m = DECEMBER;
y--;
if (y == 0) y--;
}
d = daysPerMonth(y, m);
}
return new Day(y, m, d);
}
/**
Gets the days in a given month
@param y the year
@param m the month
@return the last day in the given month
*/
private static int daysPerMonth(int y, int m)
{
int days = DAYS_PER_MONTH[m - 1];
if (m == FEBRUARY && isLeapYear(y))
days++;
return days;
}
/**
Tests if a year is a leap year
@param y the year
@return true if y is a leap year
*/
private static boolean isLeapYear(int y)
{
if (y % 4 != 0) return false;
if (y < GREGORIAN_START_YEAR) return true;
return (y % 100 != 0) || (y % 400 == 0);
}
private int year;
private int month;
private int date;
private static final int[] DAYS_PER_MONTH
= { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
private static final int GREGORIAN_START_YEAR = 1582;
private static final int GREGORIAN_START_MONTH = 10;
private static final int GREGORIAN_START_DAY = 15;
private static final int JULIAN_END_DAY = 4;
private static final int JANUARY = 1;
private static final int FEBRUARY = 2;
private static final int DECEMBER = 12;
}
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