Introduction to Polymorphism

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Introduction to Polymorphism
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Recall

• We set out to explore a new language, Java, which
  uses 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.
• The LinkedList class that we made held a linked
  list of StudentRecords but to hold anything else
  it will require some not insignificant 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 objects of
  different kinds of classes.
• Since classes within a class hierarchy 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 many or multiple forms.
• Refers to the ability to tell an object to
  perform a method without knowing exactly which
  method is going to get invoked.
• May initially seem somewhat magical
• Well, actually it is...

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A Class Hierarchy
Animal
Dog
Cat
Fish
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A Polymorphic Example
Animal
Dog
Dog myDog myDog new Dog() Animal
myAnimal myAnimal myDog
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Extends

• Note that when Java uses inheritance the keyword
  is extends
• This is because literally making a subclass
  object in a sense also makes a superclass
  object!!!
• Huh?
• When we say
• myDog new Dog()
• the Dog constructor gets called.
• It, in turn, must call the Animal constructor
• And heres the big secret

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Everything is an Object!

• Thats right!
• There is a class Object already defined
• When you dont extend anything by default you
  extend Object
• Thus the Animal constructor calls the Object
  constructor
• Looking at an object in memory it would look
  something like this

Object
reference
Dog Object
Animal
myDog
Dog
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Polymorphism Explained

• The rule is very simple
• A reference can refer to an object which is
  either
• The same type as the reference
• Has a superclass of the same type as the
  reference
• So all of the following are legal
• Dog d new Dog()
• Animal a new Animal()
• Object o new Object

Object
reference
Dog Object
Animal

Dog
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Object
Dog
Animal
Object
Dog d d new Dog()
Dog d d new Animal()
Dog d d new Object()
Object
REF
REF
REF
Animal a a new Dog()
Animal a a new Animal()
Animal a a new Object()
Object
REF
REF
REF
Reference
Object o o new Dog()
Object o o new Animal()
Object o o new Object()
Object
REF
REF
REF
Could this be some form of Matrix?
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An Illegal Example

• We are able to assign an object of a sub-class
  into an object of a super-class as in
• Animal MyAnimal new Dog()
• But the reverse is not true. We cant assign a
  superclass object into a sub-class object.
• Dog MyDog new Animal() // illegal

All dogs are animals but not all animals are dogs
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Method Calls and Polymorphism

• Assume the Dog class extends the Animal class,
  redefining the makeNoise method.
• Consider the following
• Animal MyAnimal new Dog()
• MyAnimal.makeNoise()
• Note The Animal reference is referring to a Dog
  object. And its the Dogs makeNoise method that
  gets invoked!

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Dynamic Binding

• Very simple rule.
• No matter what the reference type is, Java will
  search the object and execute the lowest
  occurence of a method it finds.
• class Object has a toString method
• Assume that both Animal and Dog have overridden
  the toString method

Object toString()
Object o
Animal a
A Dog Object
Animal toString()
Dog d
Dog toString()
o.toString() a.toString() d.toString()
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Dynamic Binding

• Very simple rule.
• No matter what the reference type is, Java will
  search the object and execute the lowest
  occurrence of a method it finds.
• class Object has a toString method
• Even if Animal has no toString method!

Object toString()
Object o
Animal a
A Dog Object
Animal
Dog d
Dog toString()
o.toString() a.toString() d.toString()
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Polymorphism vs. Inheritance

• Inheritance is required in order to achieve
  polymorphism (we must have class hierarchies).
• Re-using class definitions via extension and
  redefinition
• Polymorphism is not required in order to achieve
  inheritance.
• An object of class A acts as an object of class B
  (an ancestor to A).

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Processing Collections

• One of the main benefits of polymorphism is the
  ability to easily process collections.
• We will consider a collection (queue) of Objects
  in the next example. . .

Note This means we rewrite our Queue class (and
all associated classes) to hold Objects.
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The Banking Class Hierarchy
Object
BankAccount
SavingsAccount
CheckingAccount
DeluxeSavings
NOWAccount
MoneyMarketAccount
CDAccount
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A Collection of Bank Accounts

• Imagine a bank needs to manage all of the
  different types of accounts.
• Rather than maintain seven separate queues, one
  each for BankAccounts, SavingsAccounts,
  DeluxeSavings, CDAccounts, CheckingAccounts,
  NOWAccounts, and MoneyMarketAccounts
• We can maintain only one queue of Objects.

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Polymorphic Banking

• // Assume accounts of various kinds
• CheckingAccount johnAaccount
• DeluxeSavings paulAccount
• CDAccount paulOtherAccount
• NOWAccount georgeAccount
• MoneyMarket ringoAccount
• // Then put them all in a single structure
• // which is a Queue which holds Objects
• Queue acctQueue
• acctQueue.enqueue(johnAccount)
• acctQueue.enqueue(paulAccount)
• acctQueue.enqueue(paulOtherAccount)
• acctQueue.enqueue(georgeAccount)
• acctQueue.enqueue(ringoAccount)

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Polymorphic Banking

• accountQueue is polymorphic
• It is holding accounts of many forms.
• Each of the accounts is within the family of
  the class hierarchy of bank accounts.
• Each one will have its own set of capabilities
  via inheritance (extension, and/or redefinition).

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Example of Polymorphic Banking

• With polymorphism, our main algorithm doesnt
  care what kind of account it is processing
• double sum
• . . .
• sum 0.0
• while(! acctQueue.isEmpty())
• sum
• ((BankAccount)(acctQueue.dequeue())).getBalan
  ce()
• System.out.println
• (Sum of the balances is sum )

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Resolving Polymorphic Method Calls

• The Java compiler is our friend
• Since we said the Queue was a Queue which was
  written to hold Objects what comes out is an
  Object reference
• class Object has no getBalance() method so the
  compiler would give us a friendly error message
  politely refuse to compile until we cast the
  object to a BankAccount object
• As annoying as this may seem its for our own
  good!
• Again, Java wants to catch as many problems at
  compile time to avoid errors at run time!

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Polymorphism

• This is the magic of polymorphismit keeps
  track of family members within the inheritance
  hierarchy for you.
• Without it, wed have lots of code sprinkled
  through out our algorithm choosing among many
  options
• if( its a Checking_Account )
• call Checking_Account Calc_Interest
• else if( its a Super_Savings )
• call Super_Savings Calc_Interest
• else if( its a CD_Account )
• call CD_Account Calc_Interest
• else if( its a NOW_Account )
• call NOW_Account Calc_Interest
• . . .

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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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