Project 3 Design and Implementation



Project 3 Design and Implementation

During the refactoring pass of the previous assignment, you likely noticed a fair amount of duplication
(both in terms of instance variables and methods) when breaking Entity into multiple separate classes.
For this assignment, you will eliminate this duplication by refactoring the duplication into
new abstract classes. The entity classes will then inherit the variables and methods, as appropriate, to
restore the functionality.
• Why do this? Code is easier to maintain when there is less duplication. A change only has to be
made in one place, rather than in all duplicates.
• Why abstract classes? Instances of these new classes will not be meaningful in the context of
the program. Declaring the class abstract prevents instantiation, but still allows for inheritance.
• Why use inheritance? Some in the design community prefer composition to inheritance, but that
principle regards inheriting and then extending/modifying behavior. Our use of inheritance is in
the reverse direction: we are lifting commonality to a single point of maintenance.
Task Overview
With a focus on “entities” (all classes implementing the concept of an Entity from the original, provided
implementation), examine the set of classes to identify duplication. Refactor the duplicate
variables/methods into a hierarchy of abstract classes. The goal (and, to be both explicit and
pedantic, the requirement for this assignment) is to eliminate all such duplication. This may seem
excessive, but it is often illustrative to apply a principle to the extreme to see if it continues to provide
benefits and to get a sense for when it might fall short.
Completion of this assignment will consist of
• updated UML document (named Inheritance.graphml)
• refactored code base
Duplicate Variables
Such duplication is most obvious for instance variables (for this project, you can ignore any duplication
of static variables, like rand), since you may be able to identify duplication directly from the names and
(matching or closely related) types (though there are times when two classes use different names for
conceptually duplicate variables).
Identifying duplicate methods will require careful examination of the code within the potentially duplicate
methods (the names and parameters must generally match to start). You will find that the initial set of
methods to consider are those specified by the interfaces introduced in the previous assignment.
Your refactoring should not abuse inheritance with regard to methods. In particular, assume you have
identified method foo as common among three classes (A, B, and C), but only two of them (B and C)
define the method identically. You should not promote one implementation of foo (e.g., from B and C) to
the new parent class only to then redefine it in some subclasses (e.g., in A). Instead, define
an abstract class (named Abby for this example) with an abstract method foo. A can
extend Abby and define its implementation of foo. Another new abstract class can also
extend Abby to define the single implementation of foo used by B and C (which then extend this new
Take Care — No Incidental Additions, Access Violations
At no point should a class inherit instance variables or methods due to this refactoring that it did not
already possess prior to the refactoring. If such happens, then your refactoring either lifted something too
high in the class hierarchy or it needs an additional intermediate class.
As before, all data should remain private. If a subclass needs to access/change this data, then it can
do so via accessor or mutator methods. If such methods do not exist, then you can add them
(as protected methods) to the abstract class.
You are strongly encouraged to begin this refactoring by updating your UML document first (referencing
the code, of course, for method implementations). Doing so allows you to leverage the graphical aspects
of UML to immediately identify duplication and to quickly refactor by injecting intermediate classes.
Experience with this project indicates that the majority of duplication can be identified and refactored
quickly through this approach, which then gives you have a design guideline for the code modifications.
(Focus on the most obvious duplications first.)
Source Code Refactoring
Your refactoring should mirror the work that you do for your design document (UML diagram). Your
refactoring must not add or remove any program functionality. The resulting program must work as
Additional Tips on Refactoring Methods
Some of the various entity classes contain identical methods (of course they do, these duplicates were
added in the previous assignment). But there are also some methods from the original given code that
are less obvious duplicates. In addition, there are some methods that are not full duplicates, but that do
contain significant amounts of code duplication (your IDE might even be able to help identify these if you
use the code review tools).
Consider the following tips for refactoring the less obvious duplications. This should be done after
refactoring (both in UML and in the code base) the more obvious duplications so that there is less code to
• For two methods that appear to be doing roughly the same thing, but that differ slightly in their
implementation: examine the code to determine if the code can be rewritten to match. This does
require careful consideration for what each method does (and does not) to avoid introducing
• Some methods may have the same general structure (and match identically in significant
portions), but differ in some segments. For such methods, the general structure and identical
portions can be refactored into a parent class. This parent class will declare new protected
abstract method(s) that each subclass then implements to define the unique behavior (as done
in the calculator lab).
Assignment Submission
Your submission must include
• Inheritance.graphml (due 11/2 by 10PM for 40%) minimum requirements is given.
• all source files (even those that were unchanged). Your grader should be able to build
your project based on the files submitted. (You do not need to submit the image files, the
image list, or the world save file.) An explicit list of files is not given because you are
creating new files for this assignment, so verify that you have submitted everything
• Updated UML diagram
Due 11/10 by 10PM for 100%
11/11 by 10PM for 70%


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