Программа,которая читает год и первые три буквы месяца(первая буква заглавная) и определяет сколько дней в месяце, учитывая високосный год [дубликат]

import java.util.Scanner;
public class Classwork {
    public static void main(String args[] ) throws Exception {
        Scanner stdin = new Scanner(System.in);
        int year = stdin.nextInt(); 
        String month = stdin.next();
        boolean isLeapYear =
                (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
        if(isLeapYear == true && month == "Feb")
            System.out.print(month + " " + year + " has 29 days");
        else if(isLeapYear == false && month == "Feb") 
            System.out.print(month + " " + year + " has 28 days");
        switch(month){
            case "Jan": System.out.print(month + " " + year + " has 31 days"); break;
            case "Mar": System.out.print(month + " " + year + " has 31 days"); break;
            case "Apr": System.out.print(month + " " + year + " has 30 days"); break;
            case "May": System.out.print(month + " " + year + " has 31 days"); break;
            case "Jun": System.out.print(month + " " + year + " has 30 days"); break;
            case "Jul": System.out.print(month + " " + year + " has 31 days"); break;
            case "Aug": System.out.print(month + " " + year + " has 31 days"); break;
            case "Sep": System.out.print(month + " " + year + " has 30 days"); break;
            case "Oct": System.out.print(month + " " + year + " has 31 days"); break;
            case "Nov": System.out.print(month + " " + year + " has 30 days"); break;
            case "Dec": System.out.print(month + " " + year + " has 31 days"); break;
        }



    }
}

    if(isLeapYear && month.equals("Feb"))
    System.out.print(month + " " + year + " has 29 days");
    else if(!isLeapYear && month.equals("Feb"))
        System.out.print(month + " " + year + " has 28 days");

Соль в том, что строки нужно сравнивать через .equals, а не через ==.

Нет необходимости делать отдельный код для февраля, можно его спокойно встроить в switch:

case "Feb": 
if (isLeapYear) {
System.out.print(month + " " + year + " has 29 days"); 
} else {
System.out.print(month + " " + year + " has 28 days"); 
}
break;

Кроме того, месяцы, имеющие одинаковое количество дней (30 или 31) можно сгруппировать:

case "Jan": 
case "Mar": 
case "May": 
// итд
     System.out.print(month + " " + year + " has 31 days"); 
     break;

Нельзя сравнивать строки с помощью ==

Используйте метод java - .equals("");

Допустим есть строка String str = "Hello, world!";

И нужно сравнить ее:

    `if(str.equals("Hello, world!")){
       System.out.println("Сравнение произошло успешно!");
    }`

Делаем как то так. И все у вас должно будет заработать

Scanner stdin = new Scanner(System.in);
int year = stdin.nextInt(); String month = stdin.next();
boolean isLeapYear = ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
if(isLeapYear && month.equalsIgnoreCase("Feb"))
    System.out.print(month + " " + year + " has 29 days");
else
    if(month.equalsIgnoreCase("Feb"))
        System.out.print(month + " " + year + " has 28 days");
switch(month){
    case "Jan": System.out.print(month + " " + year + " has 31 days"); break;
    case "Mar": System.out.print(month + " " + year + " has 31 days"); break;
    case "Apr": System.out.print(month + " " + year + " has 30 days"); break;
    case "May": System.out.print(month + " " + year + " has 31 days"); break;
    case "Jun": System.out.print(month + " " + year + " has 30 days"); break;
    case "Jul": System.out.print(month + " " + year + " has 31 days"); break;
    case "Aug": System.out.print(month + " " + year + " has 31 days"); break;
    case "Sep": System.out.print(month + " " + year + " has 30 days"); break;
    case "Oct": System.out.print(month + " " + year + " has 31 days"); break;
    case "Nov": System.out.print(month + " " + year + " has 30 days"); break;
    case "Dec": System.out.print(month + " " + year + " has 31 days"); break;
}