CSC/ECE 517 Fall 2011/ch6 6b mm

Subclassing

Introduction

Subclassing, or inheritance, in object-oriented programming grants the programmer the ability to extend and refine the functionality of existing classes. This principle has many advantages that can make the programmers life much more simple. Principles and techniques have been defined that support and oppose the use of subclassing, making for a great debate. Here, some of these principles will be discussed to help determine when it is a good idea to utilize subclassing in a program.

Reasons For Subclassing

There are many different reasons out there for why subclassing is helpful:

• Specialization - If an existing class has the basic functionality needed (ex: a circle class) but is missing some small details (say a method to determine the center of the circle), then subclassing is very useful. It allows the programmer to get most of the functionality needed and provides a means to specialize the superclass to fit the needs of the program.<ref>Skrien 2009</ref>

• Generalization - Two classes that are very closely related to each other could have their common functionality pulled out into a superclass and then changed to implement that superclass. This provides the programmer with less code duplication and makes it easier if a change is needed to that common functionality later on.<ref>Skrien 2009</ref>

• Polymorphism - Polymorphism is basically the utilization of a superclass type for a variable to hold a reference to an object whose class can be the supertype or any of its subtypes.<ref>Venners 1998</ref> This is a great advantage since storing a subtype into that variable would still allow the program to function correctly.

• Dynamic Binding - This feature of subclassing allows the code executor (ex: JVM for Java) to determine at runtime which version of an overridden method to execute. This is needed because sometimes the method to execute cannot be determined at compile-time.

• Code Reuse - When a superclass is extended, the subclass inherits all of the methods and properties of the superclass. Code duplication is held to a minimum since the code is reused so effectively.

• Overriding and Overloading - Overriding and overloading functionality in a subclass is a major draw of subclassing. These abilities allow the programmer to tweak specific methods of the superclass that are close to the desired functionality in order to get the functionality exactly right.

Reasons Not To Subclass

• Inherits Too Much - Sometimes a programmer only needs a subset of the functionality of class. Subclassing requires that all functionality be carried down to the subclass though, often resulting in a class that has a lot of unneeded functionality. If this is done more than once in a hierarchy, then subclasses can become too bulky very quickly.

• Superclass Fragility - This issue arises because sometimes a small change in a superclass can sometimes have a ripple effect that forces changes to be made elsewhere in the code. This is usually due to poorly designed superclasses where there is not a clean separation between interface and implementation.<ref>Venners 1998</ref>

• Weak Encapsulation - This idea crops up when a subclass utilizes functionality of the superclass as it is inherited, essentially creating a case where the code from the superclass is reused. However, if changes are made to that functionality in the superclass, it could break code that directly utilizes an instance of the subclass.<ref>Venners 1998</ref>

• Forces Is-A Relationships - There is a time and a place for is-a relationships, but there are some in the field that believe "almost every is-a relationship would be better off re-articulated as a has-a relationship"<ref>Sumption 2011</ref>. Instances where two subclass functionalities need to be combined make for a hard implementation using is-a relationships.

Subclassing Principles

Liskov Substitution Principle

Probably the most well-known subclassing principle is the Liskov Substitution Principle (LSP). It states:

If S is a subtype of T, then objects of type T may be replaced with objects of type S (i.e., objects of type S may be substitutes for objects of type T) without altering any of the desirable properties of that program (correctness, task performed, etc.).<ref>Wikipedia - LSP</ref>

In other words, superclasses should be replaceable by their subclasses without altering the system in any way. This principle borrows ideas from the design by contract methodology by imposing restrictions on the conditions for which the methods of a class will function. It also restricts how subclass methods can be designed in terms of their return types (covariance) and method arguments (contravariance). These restrictions provide a very good ruleset to determine whether subclassing should be utilized or not.

Design by Contract

The Design by Contract principle has a direct application to subclassing in its definitions of pre- and postconditions that must exist on methods. Its main goal is to provide a method to define "formal, precise and verifiable interface specifications for software components" by utilizing the idea of preconditions, postconditions and invariants<ref>Wikipedia - DbC</ref>. When used, it helps to guarantee the functionality of any software component.

Following this principle allows a programmer to weaken preconditions and strengthen postconditions and invariants, but not vice versa. Utilizing these rules comes very close to mimicking the rules applied in the Liskov substitution principle.

References

Citation Notes

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Full References

Sumption, Bernie. "Inheritance is evil, and must be destroyed". http://berniesumption.com/software/inheritance-is-evil-and-must-be-destroyed/. 2011

Skrien, Dale. "Object-Oriented Design Using Java". Boston: McGraw-Hill, 2009.

Venners, Bill. "Inheritance versus composition: Which one should you choose?" http://www.javaworld.com/javaworld/jw-11-1998/jw-11-techniques.html. 1998

Wikipedia. "Design by Contract". http://en.wikipedia.org/wiki/Design_by_Contract

Wikipedia. "Liskov substitution principle". http://en.wikipedia.org/wiki/Liskov_substitution_principle

External Links

http://en.wikipedia.org/wiki/Liskov_substitution_principle