来自main的调用方法

Question

I have a problem with calling a method. For example, I want the following program to have two methods:

• the first one is to check if the year is leap or not,
• the second one is to display the number of days in the month

I can't call the methods in main .

import java.util.Scanner;

public class LeapYearCheck {

    public static void main(String[] args) {
        LeapYearCheck ob = new LeapYearCheck();
        ob.isLeapYear();
        ob.daysInMonth();
    }

    static void isLeapYear() {
        Scanner input = new Scanner(System.in);
        int month = input.nextInt();
        System.out.println("Enter a year: ");
        int year = input.nextInt();
        if (year % 4 == 0 || year % 400 == 0) {
            System.out.println(year + " is leap year:");
        } else {
            System.out.println(year + " is not leap year:");
        }
    }

    static void daysInMonth() {
        Scanner input = new Scanner(System.in);
        System.out.println("Enter a month :");
        int month = input.nextInt();
        int year = 0;
        if (month == 2) {
            System.out.println("There are 29 days in February: " + year + " year");
        } else if (month == 1) {
            System.out.println("The are 31 days in January " + year + " year");
        } else if (month == 2) {
            System.out.println("The are 28 days in February " + year + " year");
        } else if (month == 3) {
            System.out.println("The are 31 days in March " + year + " year");
        } else if (month == 4) {
            System.out.println("The are 30 days in April  " + year + " year");
        } else if (month == 5) {
            System.out.println("The are 31 days in May " + year + " year");
        } else if (month == 6) {
            System.out.println("The are 30 days in June  " + year + " year");
        } else if (month == 7) {
            System.out.println("The are 31 days in July  " + year + " year");
        } else if (month == 8) {
            System.out.println("The are 31 days in August " + year + " year");
        } else if (month == 9) {
            System.out.println("The are 30 days in September  " + year + " year");
        } else if (month == 10) {
            System.out.println("The are 31 days in  October " + year + " year");
        } else if (month == 11) {
            System.out.println("The are 30 days in November " + year + " year");
        } else if (month == 12) {
            System.out.println("The are 31 days in December " + year + " year");
        } else {
            System.out.println("Invalid Month, Please enter a number between 1 & 12 Merci: ");
        }
    }
}

Answer 1

The methods are declared "static". In Java, that means the method is available for the class , not for objects of the class.

To be clear: I understand your problem to be that when calling the second method, the year from the first method isn't available anymore, right?

That's because you don't store it in an instance variable, but in a variable local to the method. Local variables are gone once the method has completed. Instead, create an instance variable for your class, eg "private int year;". In the first method, then use "this.year = ..." to assign a value to the variable. In the second method, use "this.year" to access it.

Answer 2

Your code will compile and run (I've just tried it), but you shouldn't call static methods via expressions like this:

LeapYearCheck ob = new LeapYearCheck();
ob.isLeapYear();
ob.daysInMonth();

You should either make these instance methods or call them as static methods, optionally qualifying them with the type name:

LeapYearCheck.isLeapYear(); // Explicit
daysInMonth(); // Implicit

Calling static methods as if they were instance methods leads to confusion - it looks like it depends on the instance, but it doesn't.

The next oddity is here:

static void isLeapYear() {
    Scanner input = new Scanner(System.in);
    int month = input.nextInt();
    System.out.println("Enter a year: ");

Your waiting for user input but without telling the user why - and you're then ignoring the month anyway (which makes sense, as it's irrelevant to working out whether a year is a leap year or not). Just get rid of this line:

int month = input.nextInt();

Additionally, this logic is broken:

if (month == 2) {
    System.out.println("There are 29 days in February: " + year + " year");
} else if (month == 1) {
    System.out.println("The are 31 days in January " + year + " year");
} else if (month == 2) {
    System.out.println("The are 28 days in February " + year + " year");

Then for the daysInMonth method:

• You're not asking about the year
• You're not trying to detect leap years before you report that there are 29 days in February

Basically the code is a bit of a mess at the moment, but your problem of not being able to call methods from main should not be an issue...

EDIT: As noted, your leap year calculations are wrong anyway - I'm assuming that the calendar side of things is part of the goal of the exercise, but normally it would be better to use Calendar (and its subclasses) or Joda Time to start with.

Answer 3

This should run. Maybe not correctly, but it'll run. Perhaps you're referring to the warning you get on the two method calls in your main method. You see, your two methods are static. A static method doesn't require an object instance to be called. Yet you're calling them on a LeapYearCheck instance. This is allowed (it'll delegate to the proper class), just not necessary.