Days Between Dates Java (homework)

Question

This is a couple days off when I run the program. Any advice on what I am doing wrong?

I know there is a simpler way to do it, but for this I'm trying to show all the actual steps in finding the days between.

Homework assignment, so cannot use date-time libraries.

public class DaysBetween {

    public static void main (String []args) {

        long months1 = Long.parseLong(args[0]);
        long days1 = Long.parseLong(args[1]);
        long year1 = Long.parseLong(args[2]);
        long months2 = Long.parseLong(args[3]);
        long days2 = Long.parseLong(args[4]);
        long year2 = Long.parseLong(args[5]);

        long daysbetween = 0; 
        long leapyearcounter = 0;
        boolean leapyear1 = false;
        boolean leapyear2 = false;    
        boolean valid1 = true;
        boolean valid2 = true;
        int earlier = 0;

        // this tests to see which date is earlier
        if (year1 == year2){ 
            if (months1 == months2) {
                if (days1 == days2) {
                    daysbetween = daysbetween;
                } else if (days1 < days2) {
                    earlier = 1;
                }
            } else if (months1 < months2) {
            earlier = 1;
            }
        } else if (year1 < year2 ) {
            earlier = 1;
        }

        // this switches the dates depending on which is earlier
        switch(earlier) { 
        case 1: months1 = Long.parseLong(args[0]);
            days1   = Long.parseLong(args[1]);
            year1   = Long.parseLong(args[2]);
            months2 = Long.parseLong(args[3]);
            days2   = Long.parseLong(args[4]);
            year2   = Long.parseLong(args[5]);
        break;  
        default: 
            months1 = Long.parseLong(args[3]);
            days1   = Long.parseLong(args[4]);
            year1   = Long.parseLong(args[5]);
            months2 = Long.parseLong(args[0]);
            days2   = Long.parseLong(args[1]);
            year2   = Long.parseLong(args[2]);
        break;
        }

        //this section tests if the earlier date is valid
        if((year1 % 4 == 0) && (year1 % 100 != 0) || (year1 % 400 == 0)) {
            leapyear1 = true;
        }

        if ((leapyear1 == true) && (months1 == 2)) {
            if (days1 <= 29 && days1 >=1){
                valid1 = true;
            }
        } else if ((leapyear1 == false) && (months1 == 2)){
            if (days1 <= 28 && days1 >= 1){
                valid1 = true;
            }
        }

        if (months1 != 2) {
            if (months1 < 8 ) {
                if (months1 %2 != 0) {
                    if (days1 <= 31 && days1 >=1){
                        valid1 = true;
                    } else {
                        valid1 = false;
                    }
                } else if (months1 %2 == 0)  {
                    if (days1 <= 30 && days1 >=1){
                        valid1 = true;
                    } else {
                        valid1 = false;
                    }
                }
            } else if (months1 > 8) {
                if (months1 %2 != 0) {
                    if (days1 <= 30 && days1 >=1){
                        valid1 = true;
                    } else {
                        valid1 = false;
                    }
                } else if (months1 %2 == 0)  {
                    if (days1 <= 31 && days1 >=1){
                        valid1 = true;
                    } else {
                        valid1 = false;
                    }
                }
            }
        }

        // this section tests if the later date is valid
        if((year2 % 4 == 0) && (year2 % 100 != 0) || (year2 % 400 == 0)) {
            leapyear2 = true;
        }

        if ((leapyear2 == true) && (months2 == 2)) {
            if (days2 <= 29 && days2 >=1){
                valid2 = true;
            }
        } else if ((leapyear2 == false) && (months2 == 2)){
            if (days2 <= 28 && days2 >= 1){
                valid2 = true;
            }
        }

        if (months2 != 2) {
            if (months2 < 8 ) {
                if (months2 %2 != 0) {
                    if (days2 <= 31 && days2 >=1){
                        valid2 = true;
                    } else {
                        valid2 = false;
                    }
                } else if (months2 %2 == 0)  {
                    if (days2 <= 30 && days2 >=1){
                        valid2 = true;
                    } else {
                        valid2 = false;
                    }
                }
            } else if (months2 > 8) {
                if (months2 %2 != 0) {
                    if (days2 <= 30 && days2 >=1){
                        valid2 = true;
                    } else {
                        valid2 = false;
                    }
                } else if (months2 %2 == 0)  {
                    if (days2 <= 31 && days2 >=1){
                        valid2 = true;
                    } else {
                        valid2 = false;
                    }
                }
            }
        }

        //this adds a day to the total if the earlier date is in january & its a leap year
        if ((months1 == 1) && (leapyear1 == true)) {
            daysbetween = daysbetween +1;
        }

        //this adds the months left in year1
        if (months1 == 1) { 
            daysbetween = daysbetween + 334;
        } else if (months1 == 2) {
            daysbetween = daysbetween + 306;
        } else if (months1 == 3) {
            daysbetween = daysbetween + 275;
        } else if (months1 == 4) {
            daysbetween = daysbetween + 245; 
        } else if (months1 == 5) {
            daysbetween = daysbetween + 214;
        } else if (months1 == 6) {
            daysbetween = daysbetween + 184;
        } else if (months1 == 7) {
            daysbetween = daysbetween + 153;
        } else if (months1 == 8) {
            daysbetween = daysbetween + 122;
        } else if (months1 == 9) {
            daysbetween = daysbetween + 92;
        } else if (months1 == 10) {
            daysbetween = daysbetween + 61;
        } else if (months1 == 11) {
            daysbetween = daysbetween + 31;
        } else { }

        // this adds the extra day if year2 is leap year and if the month is march or later
        if ((months2 >= 3) && (leapyear2 = true)) { 
            daysbetween = daysbetween + 1;
        }

        // this adds the months up to the month in year2
        if (months2 == 1) { 
            daysbetween = daysbetween;
        } else if (months2 == 2) {
            daysbetween = daysbetween + 31;
        } else if (months2 == 3) {
            daysbetween = daysbetween + 59;
        } else if (months2 == 4) {
            daysbetween = daysbetween + 90; 
        } else if (months2 == 5) {
            daysbetween = daysbetween + 120;
        } else if (months2 == 6) {
            daysbetween = daysbetween + 151;
        } else if (months2 == 7) {
            daysbetween = daysbetween + 181;
        } else if (months2 == 8) {
            daysbetween = daysbetween + 212;
        } else if (months2 == 9) {
            daysbetween = daysbetween + 243;
        } else if (months2 == 10) {
            daysbetween = daysbetween + 273;
        } else if (months2 == 11) {
            daysbetween = daysbetween + 304;
        } else  if (months2 == 12) {
            daysbetween = daysbetween + 334;
        } else { }

        //this add the days left in month1
        if (months1 != 2) { 
            if (months1 < 8 ) {
                if (months1 %2 != 0) {
                    daysbetween = daysbetween + (31 - days1);
                } else if (months1 %2 == 0) {
                    daysbetween = daysbetween + (30 - days1);
                }
            } else if (months1 > 8) {
                if (months1 %2 != 0) {
                    daysbetween = daysbetween + (30 - days1);
                } else if (months1 %2 == 0)  {
                    daysbetween = daysbetween + (31 - days1);
                }
            }
        }

        // this add the days left in month1 if its feb
        if ((leapyear1 == true) && (months1 == 2)) { 
            daysbetween = daysbetween + (29 - days1);
        } else {
            daysbetween = daysbetween + (28 - days1);
        }

        daysbetween = daysbetween + days2; // adds the days left in month2
        daysbetween = daysbetween + ((((year2-1)-(year1+1))+1)*365); // adds the days in the years to the total

        for (long i = (year1+1); i < (year2-1) ; i++) { // sees the # of leapyears b/n year1 & year2
            if ((i % 4 == 0) && (i % 100 != 0) || (i % 400 == 0)) {
                leapyearcounter++;
            }
        }

        daysbetween = daysbetween + leapyearcounter; // adds the leapyear days

        if ((valid1 == false) || (valid2 == false)) {
            System.out.println("Sorry! You gave me one or more invalid dates!");
        } else if (daysbetween == 1){
            System.out.println("There is " + daysbetween + " day between those two dates!");
        } else if (daysbetween != 1) {
            System.out.println("There are " + daysbetween + " days between those two dates!");
        } else if (daysbetween == 0) {
            System.out.println("There are no days between those two dates!");
        }
    }
}

Answer 1

Unless this homework or just some self hurting thing you have going can I recommend using the joda time library for all your datetime calculations:

Time to use joda answer: ~22 minutes

Answer 2

Use java.util.Date: (date1.getTime()-date2.getTime())/(1000*60*60*24).

Answer 3

There's a lot that needs to be said here, but I'd recommend that you start by refactoring this monster into lots of smaller methods that you can test individually. Your code is difficult to read, understand, and debug because of the sheer volume. "Decomposition" is the operative word.

Your main method should only have driver or test code in it.

You should have methods like boolean isValid(Date date) and boolean before(Date d1, Date d2) and boolean isLeapYear(int year). Test these individually and start building up to what you want.