Problems with Java local variables

import java.util.Scanner;

public class MyCalendar {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        System.out.println("如果您想查看某年的日历，请输入1");
        System.out.println("如果您想查看某天的星期，请输入2");

        int user_input = scanner.nextInt();

        if (user_input == 1) {
            int year = scanner.nextInt();
            //已知2023年一月一日是星期日
            int temp;
            int first_day;

            if (year < 2023) {
                for (int i = year; i < 2023; i++) {
                    int days = 0;
                    if (i % 4 == 0 && i % 100 != 0 || i % 400 == 0) {
                        days += 366;
                    } else {
                        days += 365;
                    }
                    temp = days % 7;
                    first_day = 7 - temp;  //每年的1月1日是星期几
                    if (temp == 0) {
                        first_day = 0;  //将星期日记作0
                    }
                }
            } else if (year == 2023) {
                first_day = 0;  //2023一月一日是星期日
            } else {
                int days = 0;
                for (int i = 2023; i < year; i++) {
                    if (i % 4 == 0 && i % 100 != 0 || i % 400 == 0) {
                        days += 366;
                    } else {
                        days += 365;
                    }
                    temp = days % 7;
                    first_day = temp;
                }
            }

            int day = 1;
            int[] a;
            if (year % 4 == 0 && year % 100 != 0 || year % 400 ==0) {
                a = new int[]{31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
            } else {
                a = new int[]{31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
            }
            
            int m;
            for (m = 1; m <= 12; m++) {
                System.out.println("Sun Mon Tue Wed Thu Fri Sat");
                for (int i = 0; day < a[m]; day++) {
                    for (int j = 0; j < first_day; j++) {

                    }
                }
            }

        } else if (user_input == 2) {
            
        } else {
            System.out.println("请输入有效的数字！");
        }

    }
}

Write an interactive system, users enter the year to see the calendar, enter the date to see the week.

The program is not finished yet, and I'm writing the first part. I defined a first_day to indicate the day of January 1st of the input year. first_day is a local variable, but I assigned it to all if else, but I reported an error in the later loop: first_day might not be initialized yet. What's going on? How do I fix it? Comment if < br / >

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