Checking if an integer argument is defined for a method

Scenario: I am currently attempting to make a calendar in Java, one of the methods I have created (getMonthSize) is to get the size of a month (as shown). My goal currently is to make the method respond regardless of whether the argument is defined by the user (give the current month size if argument left blank, and give the size of the inputted month if argument is defined).

Efforts: I have given quite a thorough search for an answer on the web, I have come across answers testing to see if the arg.length==0 (although it turns out that works only if the arg is a String), and answers using something along the lines of "arg!==undefined" (which I think may be for javascript), with no avail. I have also learned that if an integer is not initialized, its default value will be 0, so that is what I tried to take advantage of with my code shown, although it seemed that java still expects me to input "0" for the method to work.

Question: I am hoping there is some command that checks if the argument is left blank, allowing me to set 2 cases, and achieve what I want with the method.

package model;
import java.util.Calendar;
public class myCalendar {
private Calendar calendar=Calendar.getInstance();
private int day=calendar.get(Calendar.DAY_OF_WEEK);
private int date=this.calendar.get(Calendar.DAY_OF_MONTH);
private int month=calendar.get(Calendar.MONTH)+1;
private int year=calendar.get(Calendar.YEAR);
public void setCalendar(int Year, int Month, int Date){
this.calendar.set(Year,Month-1,Date);
}
public int getDay(){
return day;
}
public int getDate () {
return date;
}
public int getMonth(){
return month;
}
public int getYear() {
return year;
}
public int getFirstDay () {
Calendar test = this.calendar;
test.set(year, month-1, 1);
return test.get(Calendar.DAY_OF_WEEK)-1;
}
public int getMonthSize (int num) {
int Month=0;
int days=0;
if (num>0){
Month=num;
} else {
Month=month;
}
if(Month==1||Month==3||Month==5||Month==7  
||Month==8||Month==10||Month==12){   
days=31;   
}   
if(Month==4||Month==6||Month==9||Month==11)   
{   
days=30;   
}   
if(Month==2)   
{   
if(((year%4==0)&&(year%100!=0))||(year%400==0))   
{   
days=29;   
}   
else   
{   
days=28;   
}   
}
return days;
}
public String getNow () {
String monthNames[]={"January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"};
String a=monthNames[month-1]+", "+String.valueOf(year);
return a;
}
}

I hope that I understood your problem correctly.

Your problem is that your user can start the method without a valid argument. If you have a frontend than you should catch it there too. But in the backend (java) you can work with a try-block or simply with if-else.

Cast the Integer to a String and then use regex (maybe something like this

String text = num + "";
if (text != null && text.matches("\\d+")) {
//...
}

Or you can cast your Integer to a String and use String.isEmpty().isEmpty returns true if, and only if, length() is 0. (https://docs.oracle.com/javase/7/docs/api/java/lang/String.html#isEmpty()).

Sting temp = num + "";
if (num.isEmpty()) {
//...
}

Or simply a try-catch Block where you cast your input to a new variable.

try {
Month = Integer.parseInt(num);
} catch (NumberFormatException e) {
// handle error
}

And please notice that your variable "Month" should be "month". In most programming communities everyone uses lowercase variables.

There are countless options how you can handle this situation and you can try every one of them. I hope that the ones I listed are all correct.

Edit:
Oh and I checked the other comments and I noticed that you are a beginner. Here are maybe some useful tips, so everyone in your class including the teacher can read easier your code. In most programming languages there is something like "checkstyle". It is hard to explain, but with some google you can find useful tips.

I will make a few "suggestions" to your code, which you can adopt or ignore. Like spaces between some characters some comments, etc.
But don't focus to hard on the quality. In some IDEs you can automatically format your code to "the standard". With Intellij it is. ctrl+alt+l

import java.util.Calendar;
public class myCalendar {
private Calendar calendar = Calendar.getInstance();
private int day = calendar.get(Calendar.DAY_OF_WEEK);
private int date = this.calendar.get(Calendar.DAY_OF_MONTH);
private int month = calendar.get(Calendar.MONTH) + 1;
private int year = calendar.get(Calendar.YEAR);
/**
* Description what the method does.
*
* @param Year  what does the parameter?
* @param Month ""
* @param Date  ""
*/
public void setCalendar(int Year, int Month, int Date) {
this.calendar.set(Year, Month - 1, Date);
}
public int getDay() {
return day;
}
public int getDate() {
return date;
}
public int getMonth() {
return month;
}
public int getYear() {
return year;
}
/**
* Description what the method does.
*
* @return value (short description)
*/
public int getFirstDay() {
Calendar test = this.calendar;
test.set(year, month - 1, 1);
return test.get(Calendar.DAY_OF_WEEK) - 1;
}
/**
* Description what the method does.
*
* @param num (short description)
* @return (short description)
*/
public int getMonthSize(int num) {
int month;
int days = 0; //My IDE tells me that need to be initialized, but why? Are there cases where the method returns 0?
//I think that num=100 returns 0, so maybe you can catch this case with if(num > 12) or something similar
if (num > 0) {
month = num;
} else {
month = this.month;
}
if (month == 1 || month == 3 || month == 5 || month == 7
|| month == 8 || month == 10 || month == 12) {
days = 31;
}
if (month == 4 || month == 6 || month == 9 || month == 11) {
days = 30;
}
if (month == 2) {
if (((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0)) {
days = 29;
} else {
days = 28;
}
}
return days;
}
public String getNow() {
String monthNames[] = {"January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"};
String a = monthNames[month - 1] + ", " + String.valueOf(year);
return a;
}
}