csci-383 object-oriented programming & design lecture 15

CSCI-383

Object-Oriented Programming & Design

Lecture 15

Chapter 8

Inheritance and Substitution

Adapted From: An Introduction to Object Oriented Programming, 3rd Edition, by Timothy Budd

Abstract Idea of Inheritance

We motivated the idea of inheritance with a hierarchy of categories:


Practical Meaning of Inheritance

Data members in the parent are part of the child Behavior defined in the parent are part of the child Note that private aspects of the parent are part of

the child, but are not accessible within the child class


Private, Public and Protected

There are now three levels of visibility modifiers: private: accessible only within the class definition

(but memory is still found in the child class, just not accessible)

public: accessible anywhere protected: accessible within the class definition or

within the definition of child classes


Inheritance is both Extension and Contraction

Because the behavior of a child class is strictly larger than the behavior of the parent, the child is an extension of the parent (larger)

Because the child can override behavior to make it fit a specialized situation, the child is a contraction of the parent (smaller)

This interplay between inheritance and overriding, extension and contraction, is what allows object-oriented systems to take very general tools and specialize them for specific projects. This interplay is ultimately the source of a great deal of the power of OOP


The is-a Rule

Our idealization of inheritance is captured in a simple rule-of-thumb

Try forming the English sentences “An A is-a B”. If it “sounds right” to your ear, then A can be made a subclass of B A dog is-a mammal, and therefore a dog inherits from

mammal A car is-a engine sounds wrong, and therefore

inheritance is not natural but a car has-a engine


Reuse of Code, Reuse of Concept

Why do we use inheritance? Basically there are two major motivations: Reuse of code. Methods defined in the parent can be

made available to the child without rewriting. Makes it easy to create new abstractions

Reuse of concept. Methods described in the parent can be redefined and overridden in the child. Although no code is shared between parent and child, the concept embodied in the definition is shared


Syntax for Inheritance

Languages use a variety of different syntax to indicate inheritance

class Wall : public GraphicalObject -- c++

class Wall extends GraphicalObject -- Java class Wall : GraphicalObject -- C# (defclass Wall (GraphicalObject)()) -- CLOS type Wall = object (GraphicalObject) -- Object Pascal class Wall < GraphicalObject -- Ruby


Trees vs Forests

There are two common views of class hierarchies: All classes are part of a single large class hierarchy.

Thus, there is one class that is the original ancestor of all other classes. Smalltalk, Java and Delphi Pascal do this

Classes are only placed in hierarchies if they have a relationship - results in a forest of many small hierarchies, but no single ancestor. C++, Objective-C, and Apple Object Pascal do this


A portion of the Little Smalltalk Hierarchy


An Argument for Substitution

Consider the following argument: Instances of the subclass must possess all data areas

associated with the parent class Instances of the subclass must implement, through

inheritance at least (if not explicitly overridden) all functionality defined for the parent class (they can also define new functionality, but that is unimportant for the present argument)

Thus, an instance of a child class can mimic the behavior of the parent class. It therefore seems reasonable that a variable declared as a parent, should be able to hold a value generated from the child class


Principle of Substitution

If we have two classes, A and B, such that class B is a subclass of class A, it should be possible to substitute instances of class B for instances of class A in any situation and with no observable effect

Note: The principle of substitutability is sometimes called Liskov substitutability, since one of the first people to describe the idea was Barbara Liskov, of MIT.


Subclass vs Subtype

Of course, the problem with this argument is that a child class can override a method and make arbitrary changes. It is therefore useful to define two separate concepts To say that A is a subclass of B merely asserts that A

is formed using inheritance To say that a is a subtype of B asserts that A

preserves the meaning of all the operations in B It is possible to form subclasses that are not

subtypes; and (in some languages at least) form subtypes that are not subclasses

The two “subs”

A class A is a subtype of a class B if the principle of substitution holds for the relationship between the classes

A class A is a subclass of a class B is the substitution principle may or may not hold for the relationship between the classes


Interfaces and Abstract Classes

An interface is similar to a class, but does not provide any implementation. A child class must override all methods. A middle ground is an abstract class. Here some methods are defined, and some (abstract methods) are undefined. A child class must fill in the definition for abstract methods

An interface is like an abstract class in which all methods are abstract. In C++ an abstract method is called a pure virtual method

abstract class Window {

...

abstract public void paint(); // child class must redefine

...

}


Forms of Inheritance

Many types of inheritance are given their own special names. We will describe some of these specialized forms of inheritance Specialization Specification Construction Generalization or Extension Limitation Variance


Specialization Inheritance

By far the most common form of inheritance is for specialization

Each child class overrides a method inherited from the parent in order to specialize the class in some way


Specification Inheritance

If the parent class is abstract, we often say that it is providing a specification for the child class, and therefore it is specification inheritance (a variety of specialization inheritance)


Inheritance for Construction

If the parent class is used as a source for behavior, but the child class has no is-a relationship to the parent, then we say the child class is using inheritance for construction An example might be subclassing the idea of a Set

from an existing List class Generally not a good idea, since it can break the

principle of substituability, but nevertheless sometimes found in practice


Inheritance for Generalization or Extension

If a child class generalizes or extends the parent class by providing more functionality, but does not override any method, we call it inheritance for generalization

The child class doesn't change anything inherited from the parent, it simply adds new features


Inheritance for Limitation

If a child class overrides a method inherited from the parent in a way that makes it unusable (for example, issues an error message), then we call it inheritance for limitation


Inheritance for Variance

Two or more classes that seem to be related, but it is not clear who should be the parent and who should be the child Example: Mouse and TouchPad and JoyStick

Better solution, abstract out common parts to new parent class, and use subclassing for specialization


Benefits of Inheritance

Software Reuse Code Sharing Improved Reliability Consistency of Interface Rapid Prototyping Information Hiding


Cost of Inheritance

Execution speed Program size Message Passing Overhead Program Complexity

This does not mean you should not use inheritance, but rather than you must understand the benefits, and weigh the benefits against the costs

csci-383 object-oriented programming & design lecture 15

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