1.2.12为SmartDate添加一个方法dayOfTheWeek(),为日期中每周的日返回Monday、Tuesday、Wednesday,Thursday,Friday,Saturday或Sunday中的适当值。你可以假定时间是21世纪。
答：

Code:
public class SmartDate
{
   private final int myYear;
   private final int myMonth;
   private final int myDay;
  
    public SmartDate(int month,int day,int year)
    {
        if(year<0)       throw new IllegalArgumentException("year is error.");
        if(month<1 || month>12)    throw new IllegalArgumentException("month is error.");
        if(day<1 || day>31)    throw new IllegalArgumentException("day is error.");
        if(month%2==0 && day>30)  throw new IllegalArgumentException("day is error.");
        if(year%400!=0 && (!(year%4==0 && year%100!=0)) && month==2 && day>28) throw new IllegalArgumentException("day is error.");
        if((year%400==0 || (year%4==0 && year%100!=0)) && month==2 && day>29) throw new IllegalArgumentException("day is error.");
    
        //
        myYear=year;
        myMonth=month;
        myDay=day;
        //
      }
   
    public int month()
    {
        return myMonth;
    }
   
    public int day()
    {
        return myDay;
    }
   
    public int year()
    {
        return myYear;
    }
   
    public String dayOfTheWeek()
    {
        int w=(myYear%100+(myYear%100)/4+(myYear-myYear%100)/4-2*(myYear-myYear%100)+26*(myMonth+1)/10+myDay-1)%7;
        if(w<0) w=w+7;
        String weekstr="";
        switch(w)
        {
            case 0: weekstr="Sunday"; break;
            case 1: weekstr="Monday"; break;
            case 2: weekstr="Tuesday"; break;
            case 3: weekstr="Wednesday"; break;
            case 4: weekstr="Thursday"; break;
            case 5: weekstr="Friday"; break;
            case 6: weekstr="Saturday"; break;
        }
      return weekstr;
    }
   
    public String toString()
    {
        return month()+"/"+day()+"/"+year();
    }
   
    public static void main(String[] args)
    {
        SmartDate d=new SmartDate(9,18,2016);
        StdOut.printf("Date is:%s,WeekDay is:%s",d.toString(),d.dayOfTheWeek());
    }
}
参考资料：百度百科：蔡勒公式、维基百科英文版：Zeller's congruence、《离散数学及其应用》原书第七版中文版：除法算法。
注意事项：计算机语言中的模运算可以返回负数，而数学定义中的模运算不能返回负数，这里采用数学定义中的模运算。