此方法必须返回int类型的结果

Question

I attempt to create a function in Java that checks the days in months from 1582 to 2199. Event if i have return statements of int type i get the following error : This method must return a result of type int

See my sample of code:

/ ------------------------- daysInMonth ---------------------- /

public static int daysInMonth(int year, int month)

{

    //returns the number of days in month of year, or -1 if date is invalid.
    //October 1582 has 16 days (16th-31st)

    if (year < 1582 || year > 2199) 

        return -1;


    else if(month == 1 || month ==3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12)

         return 31;





}   

Answer 1

There is a combination of parameters when no return is called in your method. Javac is smart enough to notice that and it won't compile such code as it can lead to unexpected behavior

Answer 2

Event if i have return statements of int type i get the following error : This method must return a result of type int

Yes you do return int, though if the statements all are false. If we do something like daysInMonth(1000, 100); then the program is confused because it ends up at a point where there is nothing to return.

Simply add a else after the else if or just add a return in the end of the method.

public static int daysInMonth(int year, int month)
{
    //returns the number of days in month of year, or -1 if date is invalid.
    //October 1582 has 16 days (16th-31st)

    if (year < 1582 || year > 2199) 
        return -1;
    else if(month == 1 || month ==3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12)
        return 31;

    return -1;
}  

Answer 3

您忘了在else情况下结束if-else语句。

Answer 4

You should also write a case if none of the two conditions meet. So better write a case for else after else if .

Answer 5

And what should happen if none of the conditions are satisfied?

I would suggest something like this

int ret = 0;
if (year < 1582 || year > 2199) 
    ret = -1;


else if(month == 1 || month ==3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12)

     ret = 31;

return ret;

Answer 6

You should add these lines

else if(month==2)
           {
          if(  year%4==0  )
                 return 29;
             else
                 return 28;
           }


else if( month==4|| month==6|| month==9||                month==11)
  return 30;

 return -1;