帮我解释下面关于日历的一些函数什么意思啊？？？

Question

Calendar thisMonth=Calendar.getInstance();
if(month!=null&&(!moth.equals("null"))
thisMonth.set(Calendar.MONTH,Integer.parseInt(month));
if(year!=null&&(!year.equals("null"))
thisMonth.set(Calendar.YEAR,Integer.parseInt(year));
year=String.valueOf(thisMonth.get(Calendar.YEAR));
month=String.valueOf(thisMonth.get(Calendar.MONTH));
thisMonth.setFirstDayOfWeek(Calendar.SUNDAY);
thisMonth.set(Calendar.DAY_OF_MONTH,1);
int firstIndex=thisMonth.get(Calendar.DAY_OF_WEEK)-1;
int maxIndex=thisMonth.getActualMaximum(Calendar.DAY_OF_MONTH);
以上面为列子，帮我解释一下
谢谢