Creating a input method that prints right amount of days in a month/Int can't be converted into string

For my code I am supposed to set a method that will ask for the month for parameters then print out the right amount of days that month had.

I have developed if statements for each day of the month for when the user inputs a month, however, I keep getting a message saying int cannot be converted into string.

I was told I was suppose to return the number of days

[Example January should return 31]

however I am not exactly sure how to do this. Instead of trying to print "31" do I put in "return 31"?

import java.util.*;

public class findDays{
    public static String monthDays(int month){
        Scanner key = new Scanner(System.in);
        System.out.println("Type a month");
        month = key.nextInt();

        if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) 
        {
            System.out.println("31");
        }
        else if(month == 2)
        {
            System.out.println("28");
        }
        else
        {
            System.out.println("30");
        }
   }
}

There would be no need to pass the argument, and using java.time.* library could help substantially.

Here is a suggestion as a replacement of the code above, specifying whether does the method return a number of days of a month in a leap year or not could enhance the functionality as well.
The number of days in Month of February could be different if leap year:

public static String monthDays(boolean isLeapYear) {
    int month;
    Scanner key = new Scanner(System.in);
    System.out.println("Type a month (a number in range of 1 to 12)");
    month = key.nextInt();
    if (month > 12 || month < 1)
        throw new IllegalArgumentException("The month number is invalid.");
    int thisYear = LocalDate.now().getYear();
    YearMonth yearMonth;
    int year = 0;
    if (isLeapYear) {
        if (Year.of(thisYear).isLeap())
            year = thisYear;
        else {
            for (int i = 1; i < 4; i++)
                if (Year.of(thisYear + i).isLeap())
                    year = thisYear + i;
        }
    } else {
        if (Year.of(thisYear).isLeap())
            year = thisYear + 1;
        else
            year = thisYear;
    }
    yearMonth = YearMonth.of(year, month);
    System.out.println("Inserted month: " + yearMonth.getMonth());
    int monthDays = yearMonth.lengthOfMonth();
    System.out.println("Number of days in month: " + monthDays);
    return String.valueOf(monthDays);
}

This is noteworthy that both in your method and the suggested one, it is required to determine the year so that calculating the number of days in a month would be more accurate.

To start, we could set the year as currentYear and based on the leapYear flag and whether this year is a leap year or not, we will find out the number of days in the inserted month.

Hope that helps out.

To return a value from a method, you'll use the return keyword followed by whichever value you want to return, in your case, you are just printing the value for each month, if you want to return it, change it to

if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) 
        {
            System.out.println("31");
            return "31";
        }

This will both print the value, and then return it.
Also you could implement a switch case, it is faster and cleaner than the if statement in cases like that:

switch(month) {
    case 1:
    case 3:
    case 5:
    //.... all your 31 day months here
    return 31;
    
    case 2:
    return 28;
    default: 
    return 30;
}

The default keyword means that, if the value does not match any of the case clauses, the default will be executed, similar to the else in your code.

This article explains the switch statement in more detail if you wanna read more about it.