The exercise I wanted to solve is from here. Copying from that page:

public static int dayOfYear(int month, int dayOfMonth, int year) {
    if (month == 2) {
        dayOfMonth += 31;
    } else if (month == 3) {
        dayOfMonth += 59;
    } else if (month == 4) {
        dayOfMonth += 90;
    } else if (month == 5) {
        dayOfMonth += 31 + 28 + 31 + 30;
    } else if (month == 6) {
        dayOfMonth += 31 + 28 + 31 + 30 + 31;
    } else if (month == 7) {
        dayOfMonth += 31 + 28 + 31 + 30 + 31 + 30;
    } else if (month == 8) {
        dayOfMonth += 31 + 28 + 31 + 30 + 31 + 30 + 31;
    } else if (month == 9) {
        dayOfMonth += 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31;
    } else if (month == 10) {
        dayOfMonth += 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30;
    } else if (month == 11) {
        dayOfMonth += 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31;
    } else if (month == 12) {
        dayOfMonth += 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 31;
    }
    return dayOfMonth;
}

And I wanted to refactor this "smelly" example, and here is my implementation in Java.

Months are hardcoded with number of days they have:

public enum Month {

    JAN(1, 31, 31),
    FEB(2, 28, 29),
    MAR(3, 31, 31),
    APR(4, 30, 30),
    MAY(5, 31, 31),
    JUN(6, 30, 30),
    JUL(7, 31, 31),
    AUG(8, 31, 31),
    SEP(9, 30, 30),
    OCT(10, 31, 31),
    NOV(11, 30, 30),
    DEC(12, 31, 31);

    private final int monthIndex;
    private final int numberOfDaysInNonLeapYear;
    private final int numberOfDaysInLeapYear;

    Month(int monthIndex, int numberOfDaysInNonLeapYear, int numberOfDaysInLeapYear) {
        this.monthIndex = monthIndex;
        this.numberOfDaysInNonLeapYear = numberOfDaysInNonLeapYear;
        this.numberOfDaysInLeapYear = numberOfDaysInLeapYear;
    }

    public int getNumberOfDaysInNonLeapYear() {
        return numberOfDaysInNonLeapYear;
    }

    public int getNumberOfDaysInLeapYear() {
        return numberOfDaysInLeapYear;
    }

    public int getMonthIndex() {
        return monthIndex;
    }

    static Month getMonthForIndex(final int index) {
        final Month[] months = Month.values();
        for (final Month month : months) {
            if (month.monthIndex == index) {
                return month;
            }
        }
        return null;
    }
}

A Month has more meaning when it is a YearMonth:

public class YearMonth {

    private final Month month;
    private final boolean isLeapYear;
    private final int yearValue;

    public YearMonth(final int yearValue, final Month month) {
        this.yearValue = yearValue;
        this.month = month;
        if (yearValue % 4 == 0) {
            isLeapYear = true;
        } else {
            isLeapYear = false;
        }
    }

    public final int numberOfDays() {
        if (isLeapYear) {
            return month.getNumberOfDaysInLeapYear();
        }
        return month.getNumberOfDaysInNonLeapYear();
    }

    public final int dayOfYear(final int dayOfMonth) {
        int dayOfYear = 0;
        int monthIndex = month.getMonthIndex() - 1;

        while (monthIndex != 0) {
            final Month previousMonth = Month.getMonthForIndex(monthIndex);
            final YearMonth yearPreviousMonth = new YearMonth(yearValue, previousMonth);
            dayOfYear = dayOfYear + yearPreviousMonth.numberOfDays();
            monthIndex--;
        }

        return dayOfYear + dayOfMonth;
    }
}

Two sample test cases:

public class TestClass {
    public static void main(String[] args) {
        final Month january = Month.getMonthForIndex(1);
        final YearMonth january2016 = new YearMonth(2016, january);
        final int dayOfYearJanuary2016Day1 = january2016.dayOfYear(1);
        System.out.println("Day of Year 2016 for January Day 1: " + dayOfYearJanuary2016Day1);

        final Month march = Month.getMonthForIndex(3);
        final YearMonth march2014 = new YearMonth(2014, march);
        final int dayOfYearMarch2014Day2 = march2014.dayOfYear(2);
        System.out.println("Day of Year 2014 for March Day 2: " + dayOfYearMarch2014Day2);
    }
}

Output will be:

Day of Year 2016 for January Day 1: 1
Day of Year 2014 for March Day 2: 61

This looks like a trivial example but it took me some time to follow the 'best practices' such as immutable objects, SOLID principles, etc.. And I am not sure if I was successful in following them.

Any comments on code smells or fragility are welcome.