Inheritance

CMPS 2143

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Learning OOP

1. Requires learning how to organize interaction of loosely coupled software components

2. Requires learning how to organize classes into a hierarchical structure

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Inheritance

Property that instances of a child class (subclass, derived class) can access both data and behavior (methods) associated with a parent class (superclass).

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Intuitive example

•Fred (the Florist)▫Exhibits all the behaviors of a florist

e.g., arranges flower, delivers them▫BUT Fred is also a shopkeeper

Request money for transactions, gives receipts, etc.

•Florist is a specialized form of a Shopkeeper

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Inheritance

•Behavior and data associated with a child class are an extension of the properties of a parent class

•A subclass▫Has all the properties of the parent class▫+ expanded additional properties ▫BUT since it is specialized, may also be a restricted form

Example: A Stack is a List, but it only has LIFO access

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The is-a test

•Rule of thumb only▫A florist is a shopkeeper.▫A double room is a hotel room.▫A square is a rectangle.▫A rectangle is a parallelogram.▫A bird is NOT a mammal.▫An apple pie is not an apple.

•Sometimes inheritance is used when the is-a test fails, but not usually.

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Inheritance is transitive.

•A dog is a mammal.•A mammal is an animal.• A dog is an animal.

•An freshman is an undergraduate.•An undergraduate is a student.• A freshman is a student.

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Reasons to use Inheritance

1. REUSE CODE

2. REUSE CONCEPTS

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Code reuse

•Do NOT rewrite code for a child class that is identical to code in the parent class!!!

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Concept reuse

•Do NOT declare/define a new method for a child class for the same behavior

OVERRIDE it.Change the implementation!

•NO code is shared, although child and parent share the behavior

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C++ Exampleclass Shape { public: virtual void getArea ( ) const { boring or nonexistant code; }};

class Square : public Shape { public: virtual double getArea() const { return Math.pow (length, 2); }};

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protected access modifier

•Allows child classes to access the member data directly

•BUT, if parent class implemented a different way, you will have to re-write child classes as well.

• I SUGGEST NEVER using protected!!!

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C++ Example:class Parent { private: int three; protected: int two; public: int one; Parent ( ) {one=two=three= 42;} //other methods};

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C++ Example:class Child: public Parent { public: void inChild() { one = 1; two = 2; //legal

three = 3; //illegal }};

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C++ Example:void main (){ Child c; cout << c.one; //legal cout << c.two; //illegal cout << c.three; //illegal}

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Inheritance in various Languages•Reminder: Java, C#, Smalltalk, Objective-C, and Delphi

Pascal require every class to inherit from an existing parent class.▫Object in Smalltalk,Objective-C, Java, C#▫TObject in Delphi Pascal

•C++ and Apple Pascal do NOT. ▫They support, but do NOT enforce OOP.▫Doesn’t mean we can do inheritance in these languages.

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Insistence that all classes inherit from a class…

•Advantage: single root class that is an ancestor to all objects.▫Nice if we have an array of references to various types of

objects•Disadvantage: a SINGLE large inheritance tree

combining all classes into a tightly coupled unit.▫Languages that #include libraries have to bring everything

in.▫ another reason for import.

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Other languages syntax•C++ class Square : public Shape {…};• Java class Square extends Shape { …}

•C# class Square : Shape { ….}•Python class Square (Shape): def __init__ (self): :•Ruby class Square < Shape : end

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Subclass, Subtype, and Substitution

• Idealized view of subclasses▫Instances of child classes possess all data members

associated with the parent class▫Instances of child classes possess all functionality (that is,

methods) of the parent class▫Thus, a instance of a child class can mimic behavior of the

parent class AND is indistinguishable from an instance of a parent class if substituted in a similar situation.

•Not always valid, but when it is – it is said to support the principle of substitution

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Principle of Substitution

• “If class B is a subclass of class A (perhaps several times removed), it should be possible to substitute instances of class B for instances of class A in any situation with no observable effect.”

•The term subtype is used to refer to a subclass that satisfies the principle of substitution (versus the more general term subclass which may or may not satisfy it.)

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Example of Subtype (C++)//assume you have a linked list of shapesShape * sptr = shapesList;

while (sptr != null) { (sptr->shape).draw(); sptr = sptr->next;}

sptr nullcircle circle oval

shape next

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Substitution and strong typing

•Statically typed languages (C++, Java, C#, Pascal) place more emphasis on principle of substitution than dynamically typed languages (Smalltalk, Perl, Obj-C).▫Statically typed languages characterize objects by their

class▫Dynamically typed languages characterize objects by their

behavior (so as long as the it can respond to the message, it is okay)

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Overriding and Virtual Methods•Child classes might find it necessary to modify or

override the behavior they inherit from their parent class

•Means the child will have a method with the same name as one in the parent class (as well as signature), but the implementation will be different▫In this case we want to ignore the method from the

parent and execute the method in the child

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Override syntax

• Java and Smalltalk – occurs naturally when a child class declares a method with the same name/signature

•C++ and C# require you be explicit, using keywords to designate this is permitted ▫C++ - virtual and C# - override▫Parent class gives permission using keyword▫Child class indicates they are indeed doing so▫Or both

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ExampleC++ .h file C#

class Shape { public: virtual void draw ();};

class Circle : public Shape { public: virtual void draw (); };

class Shape { public: virtual void draw () { : }}

class Circle : Shape { public override void draw () { : }}

C++ .cpp file

virtual void Shape::draw() {…}};

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ExampleJava - implicit

class Shape { public void draw () { : }}

class Circle extends Shape { public void draw () () { : }}

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Benefits of Inheritance

•Software reusability –▫do not have to rewrite code that is inherited

•Code sharing▫Several classes can inherit from the same parent class

•Consistency of interface▫Inheriting methods means if one class understood, the

parent or child class will be too.•Software components▫Can create libraries for a collection of classes

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Benefits of Inheritance

•Rapid Prototyping▫Focus on new/unusual portions of the system▫Reuse old/common/usual portions

•Polymorphism and Frameworks▫Easier to generate high-level reusable components

tailored to fit different applications by change low-level components

• Information Hiding▫Only need to understand nature of component and its

interface, so if super methods are called or overridden, client code need not worry about it.

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Costs of Inheritance•Execution speed▫Run-time needs to figure out what method is actually

called▫BUT programmer time MORE expensive

•Program size ▫Reusing code means you might bring in more than what

you actually need▫BUT reusing code means less time debugging new code

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Costs of Inheritance•Message-passing overhead▫Method call chaining▫May really only be a few more assembly instructions▫Total time penalty +10%

Higher in dynamically bound languages like Lisp•Program Complexity▫Overuse can be confusing▫Most people can deal with up to 3 levels

Have to do multiple up and down scans of code or inheritance graph (yo-yo problem)