1. Inheritance and Polymorphism
An Introduction
Copyright © Curt Hill

2. Biology Example Class is a borrowed word from biology
One line from an inheritance tree
Animals (a kingdom)
Vertebrates (a phylum)
Mammals (a class)
Rodents (an order)
Squirrels (a family)
Thirteen line ground squirrel (a specie)
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3. Relationship
Characteristics are preserved as we go down the line of inheritance
The prior list shows an is a relationship
A thirteen line ground squirrel is a squirrel
A squirrel is a rodent
A thirteen line ground has all the characteristics of any of the above
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4. Biology Example Again
Mammals are warm blooded, have fur and feed young milk
Rodents have 1 pair of upper incisor teeth and are mammals
Squirrels have furry tails and are rodents
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5. A specific instance A thirteen line ground squirrel is a squirrel
Does a thirteen line ground squirrel have
warm blood?
1 pair of upper incisors?
furry tail?
Yes: a thirteen line ground squirrel is a squirrel, rodent, mammal, vertebrate and animal
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6. Inheritance Deriving new classes from old classes
New class may retain any properties or methods of old
New class may add new properties, new methods
New class may replace/remove old methods
Remember the platypus, which is an egg laying mammal
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7. Copying and Deriving What is the difference?
After the copy, the two classes are separate and no longer related
After a derivation the derived class can be changed by changing the original
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8. Inheritance example Person has a name and age
Student is a person with GPA and major
Employee is a person with a wage
Grad is a student with degree
The characteristics of a person are still contained in a grad
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9. person set_name employee student set_wage set_major grad set_degree
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10. Inherited properties and methods
All classes have properties (eg. name and age) and methods (eg. set_name)
Person explicitly declares it
All the rest inherit it
Grad has degree, as well as GPA and major
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11. Terminology of inheritance
Person is the base class or superclass
Student and Employee are derived classes
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12. Displaying the contents
Each class needs a display method
How would that work?
Each class would call the display of its ancestor
Then display the things that are unique to it
Much easier that re-doing the work of the older classes
If the ancestral class changes the display routine the descendants would take benefit without any change
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13. Polymorphism Literally: many shapes
Classes that are related are interchangable
In C++ only the pointers are interchangable
In Java any container, such as an array can have related contents
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14. Effects of Polymorphism
Do a display on any class derived from person
Not know until run-time, which kind it is
Determining the function to use is called binding
Dynamic binding vs static binding
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15. Binding Most programming languages use static (or early) binding
At compile or link time determine the function needed
Examples: Pascal, C, COBOL
Some programming languages use dynamic (or late) binding
Determine function needed at run time
Examples: LISP and Java
C++ uses both, depending on situation
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16. Inherited functions
Execute the set_name function against any variable of these types derived from person
The same effect occurs
Defined in the base type, person
Variable is upcast, that is converted to base type
This is not polymorphism
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17. Polymorphism
Execute the display function against any variable of these four types
The different effects occur based on the actual type of the variable
Calls display knowing that all person derived classes have a display function
They inherit it or redefine it
A function can accept a person type and receive either a person, student, employee or grad without problem
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18. Virtual Functions
If two classes in the same inheritance tree have the same function they may be virtual
Such as display in the person hierarchy
Calling a virtual function with any class in the tree and you get that classes function, without knowing until run-time which class you have
This is the basis of polymorphism
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19. Requirements of Polymorphism
The class must be accessed via pointer
Only way in Java
The pointer must be of a base class of any possible polymorphic methods
The method in question must be virtual
It must have same signature
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20. Rodents in North America
Rodents are an order consisting of several families
Squirrels
Pocket gophers
Pocket Mice and Kangeroo Rats
Beaver
Mice, Rats, Voles, Lemmings
Old world Rats and Mice
Jumping Mice
Porcupines
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21. Natural inheritance trees
In nature the inheritance tree consists only of leaves
No examples of rodents that do not belong to one of the rodent families
No generic rodent that is not a squirrel, not a ...
Generic rodent is group of characteristics not an animal
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22. Object inheritance trees
Programming language object hierarchies may have objects that are branches and leaves
We may instantiate a person or a student
An ancestral object may be more than a group of characteristics
Objects may also be groups of characteristics
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23. Abstract Base Type A type that is not instantiatable
Only used to define the interface (the characteristics) for derived classes
Consider a generic rodent
Has the characteristics of rodent but not a member of any of the subordinate families
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24. Abstract Base Class Example
Consider a Shape class
Want to be able to:
Move a shape
Draw a shape
The Shape class then has descendents:
Circle
Square
The Shape class may implement move but cannot implement draw
The Shape class forces all of its descendents to implement draw
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25. Pure Virtual Functions
Only interface if specified
Such as draw
Every abstract base class has one or more pure virtual functions
No class with pure virtual functions may be instantiated
An instantiable function overrides the pure virtual with a callable function
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26. Conclusion Two ideas: Reuse as much code as possible
Maximize the flexibility
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