How to NOT return value in INT method?

When I run the code it returns:

31 and -1

How can I get rid of -1? Is there a way NOT to return in method INT? I tried to return java.lang.Integer(null) but it gave me an error. I think I used it in a wrong way.

Here is the code:

package com.company;
public class Main {
public static void main(String[] args) {
int y = getDaysInMonth(1, 2020);
System.out.println(y);
}
public static boolean isLeapYear(int year) {
if (year > 1 && year < 9999) {
if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0) {
return true;
}
return false;
}
return false;
}
public static int getDaysInMonth(int month, int year) {
if ((month < 1 || month > 12) && (year < 1 || year > 9999)) {
return -1;
} else if (!isLeapYear(year)) {
switch (month) {
case 1:
System.out.println(31);
break;
case 2:
System.out.println(28);
break;
case 3:
System.out.println(31);
break;
case 4:
System.out.println(30);
break;
case 5:
System.out.println(31);
break;
case 6:
System.out.println(30);
break;
case 7:
System.out.println(31);
break;
case 8:
System.out.println(31);
break;
case 9:
System.out.println(30);
break;
case 10:
System.out.println(31);
break;
case 11:
System.out.println(30);
break;
case 12:
System.out.println(31);
break;
default:
return -1;
}
} else if (isLeapYear(year)) {
switch (month) {
case 1:
System.out.println(31);
break;
case 2:
System.out.println(29);
break;
case 3:
System.out.println(31);
break;
case 4:
System.out.println(30);
break;
case 5:
System.out.println(31);
break;
case 6:
System.out.println(30);
break;
case 7:
System.out.println(31);
break;
case 8:
System.out.println(31);
break;
case 9:
System.out.println(30);
break;
case 10:
System.out.println(31);
break;
case 11:
System.out.println(30);
break;
case 12:
System.out.println(31);
break;
default:
return -1;
}
} else return -1;
return -1;
}

}

I tried almost everything there should be something that I don't know yet.

This is because you are printing first in the method and then printing again the returned value of the method in y.

Instead of printing every time, try returning the values. Also the program you have written could be written much efficiently.
Here is an example :

public static int getDays(int monthNumber, int yearNumber)
{
if (monthNumber == 2 && !isLeapYear(yearNumber))
return 28;
else if (monthNumber==2)
return 29;
else if ((monthNumber >= 1) && (monthNumber <= 7) && (monthNumber % 2 ==1))
return 31;
else if ((monthNumber >= 8) && (monthNumber %2==0))
return 31;
else 
return 30;
}

> Is there a way NOT to return in method INT?

Yea, sure. Throw an exception. For an example:

if (month < 1 || month > 12) {
throw new IllegalArgumentException("month value is out of range");
} 

^{(Hint: I noted a bug in your existing error checking. Look carefully at it when you rewrite it.)}

Throwing an exception causes the method to terminate without returning any result. Even if the method signature says that a result should be returned. You can read about it in the Oracle Java Tutorials: https://docs.oracle.com/javase/tutorial/essential/exceptions/definition.html. That part of the Java Tutorials explains what exceptions are, how and when to throw them, how to catch them, and what happens when you don't catch them.

Having said that, there are a few places where you are returning -1. You need to check each one carefully to decide if it is actually possible. If the inputs to getDaysInMonth are valid, then you should always be able to compute a "days in the month" value. And you should only need to check the arguments in one place.

My recommendation would be check the argument values at the start of the method. The rest of the method can then be coded on the assumption that the arguments are valid.

Finally, if the getDaysInMonth is returning -1 for getDaysInMonth(1, 2020) that indicates that you have a bug in the logic of the method. I recommend you use a debugger to find it ... if you can't spot it by reading and logically analyzing your code.

Looking carefully at your function, as it is now, it will always return -1 and nothing else.

A function declared public static int getDaysInMonth(int month, int year) must return an int value, if it completes successfully.

The only case it may exit without returning a value is by throwing an exception.

So, I am going to assume your function needs to return the number of day in the month and not just print it.

Here is the code for that which will never return -1, or any value other than 28, 29, 30, 31:

public static int getDaysInMonth(int month, int year) throws IllegalArgumentException {
if (year < 1 || year > 9999) {
throw new IllegalArgumentException(String.format("Invalid year %d must be between 1 and 9999", year));
}
switch (month) {
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:        	
System.out.println(31);
return 31;
case 2:
if (isLeapYear(year)) {
System.out.println(29);
return 29;
} else {
System.out.println(28);
return 28;
}
case 4:
case 6:
case 9:
case 11:
System.out.println(30);
return 30;
}
throw new IllegalArgumentException(String.format("Invalid month %d must be between 1 and 12", month));
}

Note the following changes:

• case in Java is "fall through" which means you don't have to write the same code over and over, you can put cases with same result one below the other without the brake statement and they will all work together to give the same result.

• At the beginning of the function we only check if the year is valid. If the month is invalid non of the case statements will execute and the code will go directly to the line after the switch block. At that point we know the month is not valid, so no need to check, just trow the exception.

• Any function that throws an exception must declare that it does so in the function header, by adding throws clause and listing the exceptions separated by commas.

• When you call a function that throws an exception, you must wrap it in try .. catrch block.

Java methods can return only one type of result: https://www.javatpoint.com/method-in-java

> Return Type: Return type is a data type that the method returns. It may have a primitive data type, object, collection, void, etc. If the method does not return anything, we use void keyword.

As a workaround:

Use the wrapper class

You can use the Integer class as a return type and return null. It's not much better than -1 but it indicates that nothing was returned.

Invalid argument exception

Throw this: https://docs.oracle.com/javase/7/docs/api/java/lang/IllegalArgumentException.html it's what it's made for

default:
throw new IllegalArgumentException("Invalid month value - out of range");

Create enum

Create an enum type and have it as a parameter in your method. That way there won't be any way to have an unknown value.

public enum Month {
January(1), February(2), March(3), April(4),May(5),June(6), July(7), August(8), September(9), October(10),  November(11), December(12)
}