第六章第三十四题（打印日历）(Print calendar)

**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.

下面是参考答案代码:

import java.util.*;

public class PrintCalendarQuestion34 {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);

    // Prompt the user to enter year
    System.out.print("Enter full year (e.g., 2012): ");
    int year = input.nextInt();

    // Prompt the user to enter month
    System.out.print("Enter month as a number between 1 and 12: ");
    int month = input.nextInt();

    // Print calendar for the month of the year
    printMonth(year, month);
    
    input.close();
  }

  /** Print the calendar for a month in a year */
  public static void printMonth(int year, int month) {
    // Print the headings of the calendar
    printMonthTitle(year, month);

    // Print the body of the calendar
    printMonthBody(year, month);
  }

  /** Print the month title, e.g., March 2012 */
  public static void printMonthTitle(int year, int month) {
    System.out.println(" " + getMonthName(month) + " " + year);
    System.out.println("-----------------------------");
              
    System.out.println(" Sun Mon Tue Wed Thu Fri Sat");
  }

  /** Get the English name for the month */
  public static String getMonthName(int month) {
    String monthName = "";
    switch (month) {
    case 1:
      monthName = "January";
      break;
    case 2:
      monthName = "February";
      break;
    case 3:
      monthName = "March";
      break;
    case 4:
      monthName = "April";
      break;
    case 5:
      monthName = "May";
      break;
    case 6:
      monthName = "June";
      break;
    case 7:
      monthName = "July";
      break;
    case 8:
      monthName = "August";
      break;
    case 9:
      monthName = "September";
      break;
    case 10:
      monthName = "October";
      break;
    case 11:
      monthName = "November";
      break;
    case 12:
      monthName = "December";
    }

    return monthName;
  }

  /** Print month body */
  public static void printMonthBody(int year, int month) {
    // Get start day of the week for the first date in the month
    int startDay = getStartDay(year, month);

    // Get number of days in the month
    int numberOfDaysInMonth = getNumberOfDaysInMonth(year, month);

    // Pad space before the first day of the month
    int i = 0;
    for (i = 0; i < startDay; i++)
      System.out.print("    ");

    for (i = 1; i <= numberOfDaysInMonth; i++) {
      System.out.printf("%4d", i);

      if ((i + startDay) % 7 == 0)
        System.out.println();
    }

    System.out.println();
  }
  
  /** Get the start day of month/1/year */
  public static int getStartDay(int year, int month)
  {
    int day = 1, h, q, m, j, k,statement;

    q = day;
    if(month == 1 || month == 2)
    {
      month += 12;
      year -= 1;
    }
    m = month;
    j = year / 100;
    k = year % 100;
    
    // Calculate the day of the week
    h = (q + (26 * (m + 1) / 10) + k + k / 4 + j / 4 + 5 * j) % 7;
    
    // Display the day of the week
    switch(h)
    {
      case 0:
        statement = 6; break;
      case 1:
        statement = 0; break;
      case 2:
        statement = 1; break;
      case 3:
        statement = 2; break;
      case 4:
        statement = 3; break;
      case 5:
        statement = 4; break;
      case 6:
        statement = 5; break;
      default:
        statement = -1;
    }
    return statement;
    
  }
  
  /** Get the number of days in a month */
  public static int getNumberOfDaysInMonth(int year, int month) {
    if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12)
      return 31;

    if (month == 4 || month == 6 || month == 9 || month == 11)
      return 30;

    if (month == 2)
      return isLeapYear(year) ? 29 : 28;

    return 0; // If month is incorrect
  }

  /** Determine if it is a leap year */
  public static boolean isLeapYear(int year) {
    return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0);
  }
}

运行效果：

注：编写程序要养成良好习惯
1.文件名要用英文，具体一点
2.注释要英文
3.变量命名要具体，不要抽象（如:a,b,c等等），形式要驼峰化
4.整体书写风格要统一（不要这里是驼峰，那里是下划线，这里的逻辑段落空三行，那里相同的逻辑段落空5行等等）
5.普通变量，方法名要小驼峰，类名要大驼峰，常量要使用全部大写加上下划线命名法
6.要学习相应的代码编辑器的一些常用快捷键，如：快速对齐等等