d.set(Calendar.DAY_OF_MONTH,1)我不明白

Question

public static void main(String [] args)
      {
            GregorianCalendar d=new GregorianCalendar();
            int today=d.get(Calendar.DAY_OF_MONTH);
            int month=d.get(Calendar.MONTH);
            d.set(Calendar.DAY_OF_MONTH,1);//这条语句是把当前的日期设置为这个月的第一天,我不明白,如果说今天是19号,那1-18号怎么算出来?因为下面有一条语句是使日期向后加的.
            int weekDay=d.get(Calendar.DAY_OF_WEEK);
            System.out.println("Sun Mon Tue Wed Thu Fri Sat");
            for(int i=Calendar.SUNDAY;i<weekDay;i++)
            System.out.print(" ");
            do
            {
                  int day=d.get(Calendar.DAY_OF_MONTH);
                  System.out.printf("%3d",day);
                  if(day==today)
                  System.out.print("*");
                  else 
                  System.out.print(" " );
                  if(weekDay==Calendar.SATURDAY)
                  System.out.println();
                  d.add(Calendar.DAY_OF_MONTH,1);//这条语句就是使日期向后加
                  weekDay=d.get(Calendar.DAY_OF_WEEK);
                  
            }while(d.get(Calendar.MONTH)==month);
            if(weekDay!=Calendar.SUNDAY)
            System.out.println();
            
      }

Answer 1

GregorianCalendar这是一个类他有年，月，日之类的变量你new了一个不带参数的就是生成当前系统日期.
d.set(Calendar.DAY_OF_MONTH,1);就相当于
class GregorianCalendar
{
int year,month,day;
GregorianCalendar()
{}
public void setDay(int day)
{
this.day=day;}
}