16.- SerialDate refactor

Chapter 16 is a case study on the org.jfree.date.SerialDate class from the JCommon library. The author first makes it work correctly, then refactors it in depth, applying a broad catalogue of clean code heuristics.

First Step: Make It Work

Running the existing tests against SerialDate reveals several failures. Analysis uncovers two problems:

• A boundary error in getFollowingDayOfWeek: the adjustment condition was incorrect for certain days of the week.
• The weekInMonthToString and relativeToString methods returned error strings instead of throwing IllegalArgumentException.

After fixing these bugs, all JCommon tests pass.

Second Step: Do It Right

The class is then walked through from top to bottom, applying improvements.

Remove the change history [C1]

The lengthy block of version-history comments is a relic of the past. Modern version control systems already store that information.

Rename SerialDate → DayDate [N1, N2]

The name “SerialDate” describes a concrete implementation (serial-number representation), but the class is abstract. An abstract name such as DayDate is more appropriate for a base class.

Replace MonthConstants with a Month enum [J2]

Inheriting from an interface to obtain constants is a bad Java trick. It is replaced with a dedicated enum:

public static enum Month {
    JANUARY(1), FEBRUARY(2), ..., DECEMBER(12);
    public final int index;
    public static Month make(int monthIndex) { ... }
}

This eliminates isValidMonthCode and all manual month-code validation [G5].

Convert other constant sets to enums [J3]

• WeekInMonth: FIRST, SECOND, THIRD, FOURTH, LAST
• DateInterval: CLOSED, CLOSED_LEFT, CLOSED_RIGHT, OPEN (clearer mathematical nomenclature [N3])
• WeekdayRange: LAST, NEXT, NEAREST

Move constants to the correct level [G6]

EARLIEST_DATE_ORDINAL and LATEST_DATE_ORDINAL are only used by SpreadsheetDate, so they are moved there. MINIMUM_YEAR_SUPPORTED and MAXIMUM_YEAR_SUPPORTED are likewise moved to the subclass.

Introduce DayDateFactory [G7]

A base class should not know about its derived classes. The ABSTRACT FACTORY pattern is introduced:

public abstract class DayDateFactory {
    private static DayDateFactory factory = new SpreadsheetDateFactory();
    public static DayDate makeDate(int ordinal) { return factory._makeDate(ordinal); }
    public static int getMinimumYear()          { return factory._getMinimumYear(); }
    // ...
}

Extract Day to its own file [G13]

The Day enum is large enough and independent enough from DayDate to warrant its own source file.

Move methods to the right place [G14, Feature Envy]

• monthCodeToQuarter → quarter() method on the Month enum
• monthCodeToString / weekdayCodeToString → toString() and toShortString() methods on their respective enums
• stringToMonthCode → Month.parse(String s)
• stringToWeekdayCode → Day.parse(String s)

Additional improvements

• isLeapYear is rewritten more expressively using intermediate variables [G16]
• leapYearCount is moved to SpreadsheetDate where it is actually used [G6]
• addDays is converted from a static method to an instance method [G18]
• Redundant Javadocs and stale comments are removed [C2, C3]
• final on arguments and local variables is removed (it adds noise without benefit) [G12]

Key Rules

Summary

The refactoring of SerialDate is a complete example of how to transform a functional but improvable class into a clean one. The “make it work first, then do it right” strategy allows changes to be applied with confidence, backed at every step by the tests.