Why does this method always return zero?

I am trying to make a Date() class, however I think my variables may not be working correctly or I am breaking some obscure class rule? One clue I got was during debugging...it directed me to the "thread.Class" and displayed the following code:

private void exit() {
    if (group != null) {
        group.remove(this);
        group = null;
    }

I am not sure what this means. This is my first time creating my own class. I think this might be causing "System.out.println(today.daysTo(yearFromNow));" to return zero. It may also be related to the scope of some of my variables. My professor told us we should use private variables, but he wasn't entirely clear about it, at least to me. Here is the code that I have so far:

public class Date {

private int year;
private int month;
private int day;

public static void main(String[] args) {
    Date today       = new Date(2010,10,22);
    Date today2      = new Date(2010,10,22);
    Date yearFromNow = new Date(2011,10,22);
    System.out.println(today.daysTo(yearFromNow));
    System.out.println(today.getMonth());
    System.out.println(today.getYear());
    System.out.println(today.equals(today2));
    System.out.println(today.compareTo(yearFromNow));
    System.out.println(today.toString());
}

public Date (int year1, int month1, int day1){

    if (year1 <= 0 || month1 <= 0 || month1 > 12 || day1 <= 0 || day1 > 31){
        throw new IllegalArgumentException(开发者_运维百科"Input a valid d/m/y.");
    }       
    year = year1;
    month = month1;
    day = day1;
}

//Modifies current date by adding specified number of days; Not completed yet.
public void addDays (int days){
    int numberOfDays = daysBeforeMonth(this.month, this.year)+this.day+days;
    if (numberOfDays <= 31){
        this.month = 1;
    }
}

//Returns number of days to specified date.
public int daysTo (Date d){
    int presentDate = dayOfYear(this.day, this.month, this.year);
    int futureDate  = dayOfYear(d.day, d.month, d.year);
    return (futureDate - presentDate);
}

//Returns the number of the month in specified Date;
public int getMonth(){
    return this.month;
}

//Returns the number of the year in specified Date;
public int getYear (){
    return this.year;
}

// Reports whether or not this and d represent the same date.
public boolean equals (Date d){
    return this.day == d.day &&
       this.month   == d.month &&
       this.year    == d.year;
}

// Returns negative if this is earlier than d, returns positive
// if this is later than d, and returns zero otherwise.
public int compareTo (Date d){
    if (this.month < d.month || this.day < d.day || this.year < d.year){
        return -1;
    }
    if (this.month > d.month || this.day > d.day || this.year > d.year){
        return 1;
    }
    else return 0;
}

// Converts this Date into string form, using the format
// mm/dd/yyyy, as in 07/22/2006.
public String toString (){
    String stringDate = month+"/"+day+"/"+year;
    return stringDate;
}

// takes an int year and tests whether year is divisble by 4, but
// not 100 OR if it's divisible by 400 => is leap year, else not.
public static boolean isLeapYear(int year) {
    if (year % 4 == 0 && year % 100 != 0) {
        return true;
    }

    else if (year % 400 == 0) {
        return true;
    }

    else {
        return false;
    }
}

//Returns how many days before a certain month, else returns 0.
public static int daysBeforeMonth(int month, int year) {
    int Feb = 28;
    if (isLeapYear(year) == true) {
        month = 29;
    }
    if (month == 1) {
        return 0;
    } else if (month == 2) {
        return 31;
    } else if (month == 3) {
        return Feb + 31;
    } else if (month == 4) {
        return Feb + 61;
    } else if (month == 5) {
        return Feb + 92;
    } else if (month == 6) {
        return Feb + 122;
    } else if (month == 7) {
        return Feb + 153;
    } else if (month == 8) {
        return Feb + 184;
    } else if (month == 9) {
        return Feb + 215;
    } else if (month == 10) {
        return Feb + 245;
    } else if (month == 11) {
        return Feb + 276;
    } else if (month == 12) {
        return Feb + 306;
    }

    else {
        return 0;
    }
}
//Returns how many days have passed in reference to a specific date.
public static int dayOfYear(int day, int month, int year){
    return daysBeforeMonth(month, year) + day;
}

}

If someone could please explain to me where I am wrong in my logic that would be great! Thanks!

Aside:

if (isLeapYear(year) == true) {
    month = 29;
}

should be:

if (isLeapYear(year) == true) {
    Feb = 29;
}

I think.

But I think the primary problem is that you are comparing dayOfYear for the two dates.

Consider dayOfYear for January 1st:

• 2010-01-01: dayOfYear is 1.
• 2011-01-01: dayOfYear is 1.

So the difference is 0.

Here

public int daysTo (Date d){
    int presentDate = dayOfYear(this.day, this.month, this.year);
    int futureDate  = dayOfYear(d.day, d.month, d.year);
    return (futureDate - presentDate);
}

you calculate the day of year for each dates - which will be the same for both, as each date is Oct 22 (or Nov 22?), [update] albeit from different years [/update] - then return the difference, which is obviously 0.

Moreover, here

public static int daysBeforeMonth(int month, int year) {
    int Feb = 28;
    if (isLeapYear(year) == true) {
        month = 29;
    }
    if (month == 1) {
        return 0;
    } else if ... {
    } else if (month == 12) {
        return Feb + 306;
    }
    else {
        return 0;
    }
}

you change month to an invalid value in leap years, thus the method will return 0.

I guess you want to set Feb, not month.

(Btw the Java convention is to start variable names with lowercase.)

In daysTo() you calculate the day number of the current date and the target date. You ony use the years to check for leap years.

You get a result of 0 because your two days in years 2010 and 2011 have the same day number in the year. You should also take into account the year numbers of the dates.

I think the issue is that it rounds down, the year is 364.25 days. Which means your threshold wouldn't be hit until 365 days. Take a look at Joda time.

There is a logic problem here:

Your daysTo() function uses your dayOfYear() function, which is relative to only that year. Two days that are 1 year apart will return the same dayOfYear() value, and subtracting will give 0.

To solve this, you could make a method that gets days from the UNIX epoch (January 1, 1970) and compare those.

Also to programmatically get the current date (rather than setting today manually), use System.getCurrentTimeMillis() to get the milliseconds from the UNIX epoch, then just divide the days and add them to 1/1/1970.