Inheritance, Polymorphism and Dynamic Binding

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Inheritance, Polymorphism and Dynamic Binding
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Inheritance

• Basically, inheritance defines a relationship
  among classes, wherein one class shares the
  structure or behaviour defined in one or more
  classes (called single inheritance and multiple
  inheritance, respectively). Booch
• Inheritance is therefore an implementation
  construct

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Inheritance and Human Reasoning

• Inheritance is an implementation language
  construct that maps to cognitive processing
• we think of new abstractions in terms of ones we
  already understand
• In Analysis we look for Generalisation-Specialisat
  ion hierarchies which can be implemented with
  inheritance.

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How to think about Inheritance

• Inheritance is the Is-a-Kind-of relationship
• Kerry is a Border Collie which is a kind of a
  dog
• Kerry is an instance of the class Border Collie
  which is a subclass of the Class dog

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Implications of Inheritance

• We should use inheritance to extend the behaviour
  of an existing class

Bank Account
Credit ( ) Debit ( )
Savings Account
Interest( )
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Design Principle Liskov Substitution

• What is wanted here is something like the
  following substitution property If for each
  object o1 of type S there is a an object o2 of
  type T such that for all programs P in terms of
  T, the behavior of P is unchanged when o1 is
  substituted for o2 then S is a subtype of T
  Barbara Liskov Data abstraction and hierarchy,
  SIGPLAN Notices, May 1988
• In plain English, only place a class in an
  inheritance hierarchy if one of its objects can
  be substituted for an object of its base class
  without changing the expected behavior of the
  program
• Beware - this can be subtle!

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Inheritance in a Simple Graphics System

• Place the following classes in an inheritance
  hierarchy (The application is a graphics system
  in which shapes can be drawn, moved, rotated and
  stretched along a horizontal axis, or a vertical
  axis.)
• Point, Line, Spline, Elliptical curve, Ellipse,
  Square, Graphical Item, Circle, Rectangle
• Hint Add the abstract class ClosedShape to your
  hierarchy

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A Solution?
ClosedShape
Line
Point
abstract
Where do Square, Rectangle, Circle and Ellipse go?
Hstretch(factor) Vstretch(factor)
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Sidebar Why is Multiple Inheritance a Problem?

• Answer It isnt - Repeated Inheritance is

Helicopter
Jet Plane
Fly ( )
Fly ( )
Helijet
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Other problems with inheritance

• Programming languages do not constrain you to
  inherit only on behaviour
• Interface inheritance is not cleanly separated
  from implementation inheritance (except in Java)
• The Yo-yo problem
• Overuse of inheritance limits reuse!!
• Often composition and delegation are a better
  choice for extending behaviour

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Polymorphism

• Literally many forms
• In OO means that given the same message, two
  different (but usually related) classes will
  respond with different implementations of the
  same abstract behaviour
• e.g., a circle class and a polygon class will use
  different implementations to draw themselves

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Polymorphic Interfaces

• Polymorphic interfaces are fundamental to the
  Object paradigm
• Separation of interface and implementation
  enables Responsibility Driven Design
• where objects are considered to take
  responsibility for implementing their own
  behaviour
• Leads to easier maintenance
• IF statement considered harmful!

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Typed and Untyped Polymorphism

• Typed polymorphism (due to classes being in the
  same inheritance hierarchy) leads to overriding
  of superclass methods by subclasses
• e.g., Debit() method of SavingsAccount overrides
  the implementation in BankAccount
• Untyped polymorphism is when classes share the
  same interface, but have no other relationship

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Overloading versus Overriding

• Do not confuse overloading with overriding
• An overriden method means the same semantics are
  being implemented in different ways
• An overloaded method means the same method name
  has two different meanings
• usually has slightly different signature
• but not always (remember MichaelAngelo and
  BillyTheKid?)

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Dynamic Binding

• Polymorphic methods are resolved at run time (a
  message is bound to an implementation of the
  method it invokes)
• Permits the elimination of case-tests in client
  programs
• Maximises the flexibility of the software
• BUT it means the run-time structure of an OO
  program looks nothing like its static structure
  (debugging can be painful!)