Question

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("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!

Answer 1

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.)