CSC 205/206/405
Professor Stearns
Design Issue - Inheritance Vs. Object Composition

First some definitions:

Inheritance is a mechanism for implementing generalization, in which the behavior of a superclass is shared by all its subclasses. Sharing of behavior is justifiable only when a true generalization relationship occurs, that is, only when it can be said that this subclass IS A form of the superclass. Operations of the subclass that override the corresponding operation of the superclass have a responsibility to provide the same services. (Rumbaugh, pg. 244)

Object composition is assembling or composing objects to get more complex behavior.

When reusing code, should the designer use inheritance (to reuse the class code) or object composition (to add new functionality by composing two or more objects into a single object)?

Example: There is a Student class in our design; we need to add a GradStudent.

Inheritance:

class GradStudent extends Student

class GradStudent {
    Student s;
    ......


• A static operation - easier to understand than composition.
• The programmer can modify methods by overriding them.
• You don't need the source code to create a subclass.
• Collections can be heterogeneous (polymorphism).

Arguments for object composition

• Object structures remain hidden - inheritance exposes internal structures.
• Exposing internal structures creates a variety of [future] testing problems.
• The interface models the exact behavior needed; an inherited class has all the behavior of its superclass.
• Programmer can't modify operations by overriding them; interface remains stable.