java中关于输出当前日历的问题

在Java核心中看到的一个关于输出当前日历的问题

 

import java.text.DateFormatSymbols;
import java.util.Calendar;
import java.util.GregorianCalendar;

/**
 * 输出当前日历
 * @author srong
 * @version 1.0 2011-09-07
 */
public class CalendarTest {

 public static void main(String[] args) {

  // construct as current date
  GregorianCalendar d = new GregorianCalendar();

  int today = d.get(Calendar.DAY_OF_MONTH);
  int month = d.get(Calendar.MONTH);

  // set d to start date of the month
  d.set(Calendar.DAY_OF_MONTH, 1);

  int weekday = d.get(Calendar.DAY_OF_WEEK);

  // get first day of week (Sunday in the U.S)
  int firstDayOfWeek = d.getFirstDayOfWeek();

  // determine the require indentation for the first line
  int indent = 0;
  while (weekday != firstDayOfWeek) {
   indent++;
   d.add(Calendar.DAY_OF_MONTH, -1);
   weekday = d.get(Calendar.DAY_OF_WEEK);
  }

  // print weekday names
  String[] weekdayNames = new DateFormatSymbols().getShortWeekdays();
  do {
   System.out.printf("%4s", weekdayNames[weekday]);
   d.add(Calendar.DAY_OF_MONTH, 1);
   weekday = d.get(Calendar.DAY_OF_WEEK);
  } while (weekday != firstDayOfWeek);
  System.out.println();

  for (int i = 1; i <= indent; i++)
   System.out.print("      ");

  d.set(Calendar.DAY_OF_MONTH, 1);
  do {
   // print day
   int day = d.get(Calendar.DAY_OF_MONTH);
   System.out.printf("%5d", day);

   // mark current day with *
   if (day == today)
    System.out.print("*");
   else
    System.out.print(" ");

   // advance d to the next day
   d.add(Calendar.DAY_OF_MONTH, 1);
   weekday = d.get(Calendar.DAY_OF_WEEK);

   // start a new line at the start of the week
   if (weekday == firstDayOfWeek)
    System.out.println();
  } while (d.get(Calendar.MONTH) == month);
  // the loop exits when d is day 1 of the next month

  // print final end of line if necessary
  if (weekday != firstDayOfWeek)
   System.out.println();
 }

}