Introduction to Polymorphism

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Introduction to Polymorphism
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Outline

• Prerequisites
• Functions with Memory
• Local
• Basic OO in Scheme
• Objectives
• Understand Polymorphism

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Background

• We set out to explore the Object Oriented
  Paradigm.
• We claimed that it would allow us to have
  superior
• Encapsulation
• Reusability
• Adaptability
• But as of now that may not be quite so apparent.
• A list might hold a collection of Student
  Records, but to hold anything else it will
  require some a significant amount of modification
• We need another tool to allow us to make
  collections that will hold anything!

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Scenarios

• A veterinarian's algorithm might have a list of
  animals, but each one needs different food or
  care we want ONE information system to track all
  of this without complex logic for each individual
  kind of animal.
• A car dealership sells many different types of
  cars with different features, but each has a
  price and quantity in stock.
• A registration system might treat in-state
  students differently from out-of-state students,
  graduate students differently from
  undergraduates, etc.

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Motivation

• Wed like to be able to manage different kinds of
  classes.
• Since different classes often share common
  methods and attributes, wed like to make use of
  this fact to make our algorithms simpler.

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Polymorphism

• Polymorphism means taking many or multiple forms.
• Refers to the ability to tell an object to
  perform a method without knowing exactly how that
  method is going to get invoked.
• May initially seem somewhat magical
• Well, actually it is...

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Processing Collections

• One of the main benefits of polymorphism is the
  ability to easily process collections with common
  methods.
• We will consider a collection (list) of vehicles
  in the next example. . .

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Different Vehicles
car
motorcycle
truck
semi
transporter
car
car
car
car
car
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A Collection of Vehicles

• Imagine a simulation needs to manipulate all of
  the different types of vehicles.
• Rather than maintain multiple separate lists, one
  each for cars, motorcycles, trucks, semis,
  transporters etc
• We can maintain only one list of vehicles as long
  as they all share the appropriate methods.

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Polymorphic Vehicles

• the vehicle list is polymorphic
• It is holding vehicles of many forms.
• Each of the vehicles is within the family of
  vehicles because they share the same method
  names.
• Each one may actually implement the methods
  differently.

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polymorphism.scm
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Summary

• Polymorphism allows objects to represent
  instances of its own class and any of its
  subclasses.
• Polymorphic collections are useful for managing
  objects since all objects are descendants of
  class Object
• No matter what the reference type the actual
  object knows what type it is
• When there is a choice dynamic binding goes to
  the lowest level.

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Questions?
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