**6.34（打印日历）编程练习题3.21使用Zeller一致性原理来计算某天是星期几。使用Zeller的算法简化程序清单6-12以获得每月开始的第一天是星期几。

**6.34(Print calendar) Programming Exercise 3.21 uses Zeller’s congruence to calculate the day of the week. Simplify Listing 6.12, PrintCalendar.java, using Zeller’s algorithm to get the start day of the month.

``````// https://cn.fankuiba.com
import java.util.Scanner;

public class Ans6_34_page205 {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter full year (e.g., 2012): ");
int year = input.nextInt();
System.out.print("What day is January 1, "+year+" ? ");
int week = input.nextInt();

int month = 1, day = 0;
String monthString = "";
boolean leapYear;

if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0 && year % 3200 != 0) || year % 172800 == 0)
leapYear = true;
else
leapYear = false;

for (; month <= 12; month++) {
switch (month) {
case 1:
monthString = "January";
break;
case 2:
day += 31;
monthString = "February";
break;
case 3:
monthString = "March";
if (leapYear)
day += 29;
else
day += 28;
break;
case 4:
day += 31;
monthString = "April";
break;
case 5:
day += 30;
monthString = "May";
break;
case 6:
day += 31;
monthString = "June";
break;
case 7:
day += 30;
monthString = "July";
break;
case 8:
day += 31;
monthString = "August";
break;
case 9:
day += 31;
monthString = "September";
break;
case 10:
day += 30;
monthString = "October";
break;
case 11:
day += 31;
monthString = "November";
break;
case 12:
day += 30;
monthString = "December";
}
int days = (week + day) % 7;
System.out.print("\n           " + monthString + " " + year + "\n---------------------------------");
System.out.printf("\n%-5s%-5s%-5s%-5s%-5s%-5s%-5s\n", "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat");

for (int n =1;n<=days;n++) {
System.out.printf("%-5s", "");
}

int j = 1;
if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 ||
month == 12) {
for (; j <= 31; j++) {
System.out.printf("%-5d", j);
if ((days+j) % 7 == 0)
System.out.println();
}
}
else if (month == 2 && leapYear) {
for (; j <= 29; j++) {
System.out.printf("%-5d", j);
if ((days+j) % 7 == 0)
System.out.println();
}
}
else if (month == 2) {
for (; j <= 28; j++) {
System.out.printf("%-5d", j);
if ((days+j) % 7 == 0)
System.out.println();
}
}
else {
for (; j <= 30; j++) {
System.out.printf("%-5d", j);
if ((days + j) % 7 == 0)
System.out.println();
}
}
System.out.print("\n");
switch (days) {
case 0:
System.out.print("Sun");break;
case 1:
System.out.print("Mon");break;
case 2:
System.out.print("Tue");break;
case 3:
System.out.print("Wed");break;
case 4:
System.out.print("Thu");break;
case 5:
System.out.print("Fri");break;
case 6:
System.out.print("Sat");
}
System.out.println(" starts on the first day of "+monthString);
}
}
}``````