这是我写的显示某月日历的程序,调试通过,但是运行时总是出现"Exception in thread "main" java.lang.NullpointerException"
import java.util.*;
public class MonthCalendar
{
private static Year year = new Year();
public MonthCalendar()
{
}
private static Calendar cal;
private String log[]={"日","一","二","三","四","五","六"};

public void show(int objYear, int objMonth)
{
cal.set(objYear, objMonth, 1);
int days = 0;
int dayOfWeek = 0;
if (year.isLeapYear(objYear))
{
if (objMonth == 2)
{
days = year.getDaysOfMonth(objMonth) + 1;
dayOfWeek = cal.get(Calendar.DAY_OF_WEEK);
}
}
for ( int i = 0 ; i < 7 ; i++ )
{
System.out.print("\t"+log[i]);
}
System.out.println();
for (int j = 1 ; j <= days ; j++ )
{
for (int i = 0 ; i < dayOfWeek ; i++ )
{
System.out.print("\t");
}
System.out.print("\t"+j);
int flag = dayOfWeek;
flag++;
if (flag % 7 == 0)
{
System.out.println();
}
}
}

public static void main(String[] args)
{
MonthCalendar mc = new MonthCalendar();
int objYear = Integer.parseInt(args[0]);
int objMonth = Integer.parseInt(args[1]);
mc.show(objYear, objMonth);
}
}

[此贴子已经被作者于2006-12-18 12:02:04编辑过]

//关于年的问题
public class Year
{
private int year;
private int day;
private int month;
private int[] daysOfMonth = {31,28,31,30,31,30,31,31,30,31,30,31};
Year()
{
year = 0;
day = 0;
month = 0;
}
//获得月份的天数
public int getDaysOfMonth(int month)
{
return daysOfMonth[month - 1];
}
//判断是否为闰年
public boolean isLeapYear(int year)
{
if (year % 400 == 0||(year % 4 == 0 && year % 100 != 0))
{
return true;
}
return false;
}
//判断该天属于一年中的哪一个月
public int whichMonth(int day, int year)
{
if (isLeapYear(year))
{
daysOfMonth[2] = 29;
int temp = 0;
if (day > 366 || day <= 0)
{
System.out.println("天数超出了范围！");
}
else
{
for (int i = 1 ; i < 12 ; i++ )
{

if (day >=temp && day <= temp + daysOfMonth[i])
{
return i ;
}
else
{
temp = temp + daysOfMonth[i];
}
}
}
}
else
{
int temp = 0;
if (day > 365 || day <= 0)
{
System.out.println("天数超出了范围！");
}
else
{
for (int i = 1 ; i <= 12 ; i++ )
{
if (day >temp && day <= temp + daysOfMonth[i])
{
return i;
}
else
{
temp = temp + daysOfMonth[i];
}
}
}
}
return -1;
}
//判断该天是所在月的第几天
public int whichDay(int day, int year)
{
int manyDay = 0;
int month = 0;
month = this.whichMonth(day, year);
for (int i =1 ; i <= month ; i++ )
{
manyDay = manyDay + daysOfMonth[i - 1];
}
manyDay = day - manyDay;
return manyDay;
}
}

请各位指点一下吧

private static Calendar cal;
这个地方你改成
private static Calendar cal=Calendar.getInstance();
就可以用了

谢谢指点,我已经完成了!!!