如何在 Java 中编写一个方法，当我以 computeDayOfWeek(Month, Date) 格式输入日期时，它会告诉我星期几？
【问题讨论】：
不需要外部库。 java.util.Calendar 类处理这些东西。
Calendar c = Calendar.getInstance();
c.setTime(put a java.util.Date instance here);
int dow = c.get(Calendar.DAY_OF_WEEK); //1=sunday, 2=monday, 3=Calendar.WEDNESDAY...
【讨论】：
如果您正在寻找 Java。您可以使用 Joda-Time API 来实现： 在此处查看文档：http://joda-time.sourceforge.net/userguide.html
示例：
DateTime dt = new DateTime()
System.out.println(dt.dayOfWeek().getAsText());
【讨论】：
如果您需要自己计算而不是使用预制的库函数，请查看Zeller's congruence。
我去年实现了这个，然后纠正了大约 40 个学生的解决方案（其中相当一部分人是第一个真正的程序）试图自己做 - 这里有很多方法会出错。确保测试不同日期（一月、二月、其他月份、正常年份、闰年、2000 年（闰年）和 1900 年（非闰年））。
【讨论】：
大约 10 年前，我写了一个有趣的小课堂。我只是为了好玩而从未使用过它，但请随意使用和改进它（给我发邮件改进）。非常可怕的代码，因为我当时还是个初学者。
package se.danero.calendar.util;
import java.lang.NullPointerException;
/**
* This is a Calendar that is meant to be used as a utility. To come up with this class, I analyzed the gregorian calendar, and I searched the web for
* useful, free algorithms. Then I put them together to create this version of of the Calendar.<BR>
* <BR>
* <B>The analyze</B><BR>
* In my analyze of the gregorian calendar, I have come to the conclution that<BR>
* there are 8 different types of year endings. These endings have a recurring<BR>
* cycle of 28 years. The pattern repeats itself over 28 years, continiously.<BR>
* <BR>
* The pattern consists of 8 different types of year-endings. These are listed below <BR>
* <BR>
* <B>Type</B><BR>
* 1 = 4 times over a period of 28 year period<BR>
* 2 = 4 times over a period of 28 year period<BR>
* 3 = 3 times ... <BR>
* 4 = 4 times ... <BR>
* 5 = 4 times ... <BR>
* 6 = 4 times ... <BR>
* 7 = 4 times ... <BR>
* 8 = 1 time ... <BR>
* <BR>
*
* @author Robert Nordin, robert.nordin@bredband.net
* @version 0.5
*
*/
public class Calendar {
// ****************************************************************************
// *** MEMBERS
// ****************************************************************************
// *** Type of year break; beginning with break 1980/1981 ***
private final int types[] = { 1, 2, 3, 4, 5, 6, 1, 2, 4, 7, 5, 6, 2, 3, 4, 7, 6, 1, 2, 3, 7, 5, 6, 1, 8, 4, 7, 5 };
// *** Week of the last week of the year and the first week of the new year ***
private final int weeks[][] = { { 52, 1 }, { 52, 53 }, { 52, 1 }, { 52, 1 }, { 52, 1 }, { 52, 1 }, { 52, 1 }, { 52, 53 } };
// Date of the last day of the last week and the last date of the first
// week in the new year
// private final int dates[][] = {{28, 4}, {27, 3}, { 2, 9}, { 1, 8},
// {30, 6}, {29, 5}, {31, 7}, {26, 2}};
private boolean initiated = false;
private int year = 0;
private int preMonths = 0;
private int aftMonths = 0;
// ****************************************************************************
// *** CONSTRUCTORS
// ****************************************************************************
public Calendar() {
}
// ****************************************************************************
public Calendar(int year) throws OutOfRangeException {
if (!isYearValid(year)) {
throw new OutOfRangeException(OutOfRangeException.TYPE_YEAR, "Year[" + year + "]");
} else {
this.year = year;
initiated = true;
}
}
// ****************************************************************************
public Calendar(int year, int preMonths, int aftMonths) throws OutOfRangeException {
if (!isYearValid(year)) {
throw new OutOfRangeException(OutOfRangeException.TYPE_YEAR, "Year[" + year + "]");
} else {
this.year = year;
initiated = true;
}
this.preMonths = ((preMonths >= 0) && (preMonths <= 12)) ? preMonths : 0;
this.aftMonths = ((aftMonths >= 0) && (aftMonths <= 12)) ? aftMonths : 0;
}
// ****************************************************************************
private boolean isYearValid(int year) {
return ((year >= Date.MIN_YEAR) && (year <= Date.MAX_YEAR)) ? true : false;
}
// ****************************************************************************
private int[] getWeeks(int type) {
int retval[] = null;
int arrayindex = type - 1;
try {
retval = weeks[arrayindex];
} catch (Exception ex) {
System.err.println("getWeeks(): Type is not inside valid range");
}
return retval;
}
// ****************************************************************************
private int getType(int index) {
int retval = types[index];
if ((retval < 1) || (retval > 8)) {
System.err.println("getType(): Index is not inside valid range");
}
return retval;
}
// ****************************************************************************
private int getFirstWeek(int type) {
int retval = 1;
if (type == 3 || type == 4) {
retval = 52;
} else
if (type == 2 || type == 8) {
retval = 53;
}
return retval;
}
// ****************************************************************************
private int getLastWeek(int type) {
int retval = 52;
if (type == 1 || type == 5 || type == 6) {
retval = 1;
} else
if (type == 2 || type == 8) {
retval = 53;
}
return retval;
}
// ****************************************************************************
public String intToWeekday(int day, boolean longVersion) {
String retval = "";
/*
if (day == 1) {
retval = (longVersion) ? "Monday" : "Mon";
} else
if (day == 2) {
retval = (longVersion) ? "Tuesday" : "Tue";
} else
if (day == 3) {
retval = (longVersion) ? "Wednesday" : "Wed";
} else
if (day == 4) {
retval = (longVersion) ? "Thursday" : "Thu";
} else
if (day == 5) {
retval = (longVersion) ? "Friday" : "Fri";
} else
if (day == 6) {
retval = (longVersion) ? "Saturday" : "Sat";
} else
if (day == 7) {
retval = (longVersion) ? "Sunday" : "Sun";
} else {
System.err.println("intToWeekday(): Day is not inside valid range");
}
*/
switch (day) {
case 1:
retval = (longVersion) ? "Monday" : "Mon";
break;
case 2:
retval = (longVersion) ? "Tuesday" : "Tue";
break;
case 3:
retval = (longVersion) ? "Wednesday" : "Wed";
break;
case 4:
retval = (longVersion) ? "Thursday" : "Thu";
break;
case 5:
retval = (longVersion) ? "Friday" : "Fri";
break;
case 6:
retval = (longVersion) ? "Saturday" : "Sat";
break;
case 7:
retval = (longVersion) ? "Sunday" : "Sun";
break;
default:
System.err.println("intToWeekday(): Day is not inside valid range");
}
return retval;
}
// ****************************************************************************
private int getYearType(int year) {
int retval = -1;
if (year >= 1980) {
retval = getType(((year - 1980) % 28));
} else
if (year == 1979) {
retval = 5;
} else {
System.err.println("getYearType(): Year is not inside valid range");
}
return retval;
}
/**
* Compute the sequential number of a day in the year (1-Jan=1, 31-Dec=365/366). The algorithm used: J.D. Robertson: Remark on Algorithm 398,
* Comm. ACM 13, 10 (Oct. 1972), p. 918
*
* @param date
* a date of the day you query about
* @return returns an integer between 1 and 365/366. 366 if it is a leapyear.
* @throw NullPointerException if the date is null.
*/
public int getDayInYear(Date date) throws NullPointerException {
if (date == null)
throw new NullPointerException("Date cannot be null");
int y = date.getYear();
int m = date.getMonth();
int d = date.getDay();
int lm = (m + 10) / 13;
return (int) (((long) 3055 * (long) (m + 2)) / 100 - 2 * lm - 91
+ (1 - (y - (y / 4) * 4 + 3) / 4 + (y - (y / 100) * 100 + 99) / 100 - (y - (y / 400) * 400 + 399) / 400) * lm + d);
}
/**
* Returns an array of dates that conforms to a year and a week.
*
* @param year
* the year in the form eg. 1980. Must be within Date.MIN_YEAR and Date.MAX_YEAR.
* @param week
* the week. Must be less than 54
* @return returns a full week of valid ISODates if inparameters are within the given range. returns a full week of null pointers if not.
*/
public Date[] getWeekDates(int year, int week) {
Date retval[] = new Date[7];
if ((isYearValid(year)) && ((week > 0) && (week < 54))) {
int dayCounter = 0;
for (int i = 1; i < 13; i++) {
for (int j = 1; j <= Date.getDaysInMonth(year, i); j++) {
Date tmp = new Date(year, i, j);
if (this.getWeekNumber(tmp) == week) {
retval[dayCounter] = tmp;
if (dayCounter == 7) {
i = 13;
j = 31;
}
if (dayCounter < 6) {
dayCounter++;
}
}
}
}
}
return retval;
}
/**
* Returns the weeknumber of the given date.
*
* @param date
* a date that you want to know in what week it is.
* @return returns the weeknumber as an integer.
* @throws NullPointerException
* if the parameter is null.
*/
public int getWeekNumber(Date date) throws NullPointerException {
int retval = -1;
if (date == null)
throw new NullPointerException("ISODate cannot be null");
// *** Incoming data ***
int year = date.getYear();
int month = date.getMonth();
int day = date.getDay();
if (isYearValid(year)) {
int targetDay = getDayInYear(date);
int firstWeek = getFirstWeek(getYearType((year - 1)));
int lastWeek = getLastWeek(getYearType(year));
Date oneone = new Date(year, 1, 1);
int firstDay = getDayInWeek(oneone);
int dayCounter = 0;
int weekCounter = firstWeek;
int dayInWeek = firstDay;
while (dayCounter != targetDay) {
if (dayInWeek == 8) {
dayInWeek = 1;
if ((weekCounter > 51) && (dayCounter < 10)) {
weekCounter = 1;
} else
if ((dayCounter > 360) && (weekCounter > lastWeek)) {
weekCounter = lastWeek;
} else {
weekCounter++;
}
}
dayCounter++;
dayInWeek++;
}
retval = weekCounter;
}
return retval;
}
/**
* Returns an integer that tells you what day of the week the date specifies. The algorithm used is Zeller's congruence for the Gregorian
* calendar.
*
* @param date
* a date that points out the day you want to know the weekday of.
*
* @return returns the weekday in the form of an integer. 1=Monday, 2=Tuesday...
* @throws NullPointerException
* if the parameter is null.
*/
public int getDayInWeek(Date date) throws NullPointerException {
if (date == null)
throw new NullPointerException("ISODate cannot be null");
int year = date.getYear();
int month = date.getMonth();
int day = date.getDay();
if (month < 3) {
month += 12;
year--;
}
return ((13 * month + 3) / 5 + day + year + year / 4 - year / 100 + year / 400) % 7 + 1;
}
}
【讨论】：