CSE 501N Fall 06 13: Polymorphism

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CSE 501NFall 0613 Polymorphism

• 16 Oct 2006
• Rohan Sen

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Lecture Outline

• Lab 3 questions
• Polymorphism
• Mid-semester review

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Polymorphism

• Polymorphism is an object-oriented concept that
  allows us to create versatile software designs
• We will focus on
• defining polymorphism and its benefits
• using inheritance to create polymorphic
  references
• using interfaces to create polymorphic references
• using polymorphism to implement sorting and
  searching algorithms

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Polymorphism

• The term polymorphism literally means "having
  many forms"
• A polymorphic reference is a variable that can
  refer to different types of objects at different
  points in time
• The method invoked through a polymorphic
  reference can change from one invocation to the
  next
• All object references in Java are potentially
  polymorphic

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Binding

• Consider the following method invocation
• obj.doIt()
• At some point, this invocation is bound to the
  definition of the method that it invokes
• If this binding occurred at compile time, then
  that line of code would call the same method
  every time
• However, Java defers method binding until run
  time -- this is called dynamic binding or late
  binding
• Late binding provides flexibility in program
  design

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Polymorphism

• Suppose we create the following reference
  variable
• Occupation job
• Java allows this reference to point to an
  Occupation object, or to any object of any
  compatible type
• This compatibility can be established using
  inheritance or using interfaces
• Careful use of polymorphic references can lead to
  elegant, robust software designs

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Polymorphism

• Interface variable holds reference to object of a
  class that implements the interfaceMeasurable
  xNote that the object to which x refers
  doesn't have type Measurable the type of the
  object is some class that implements the
  Measurable interface

x new BankAccount(10000)x new Coin(0.1,
"dime")
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Polymorphism

• You can call any of the interface methods
• Which method is called?

double m x.getMeasure()
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Polymorphism

• Depends on the actual object. 
• If x refers to a bank account, calls
  BankAccount.getMeasure
• If x refers to a coin, calls Coin.getMeasure
• Behavior can vary depending on the actual type of
  an object

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Polymorphism

• Called late binding resolved at runtime
• Different from overloading overloading is
  resolved by the compiler (early binding)

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Polymorphism via Interfaces

• An interface name can be used as the type of an
  object reference variable
• Speaker current
• The current reference can be used to point to any
  object of any class that implements the Speaker
  interface
• The version of speak that the following line
  invokes depends on the type of object that
  current is referencing
• current.speak()

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Polymorphism via Interfaces

• Suppose two classes, Philosopher and Dog, both
  implement the Speaker interface, providing
  distinct versions of the speak method
• In the following code, the first call to speak
  invokes one version and the second invokes
  another
• Speaker guest new Philospher()
• guest.speak()
• guest new Dog()
• guest.speak()

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References and Inheritance

• An object reference can refer to an object of its
  class, or to an object of any class related to it
  by inheritance
• For example, if the BankAccount class is used to
  derive a class called SavingsAcct, then a
  BankAccount reference could be used to point to a
  SavingsAcct object

BankAccount acct acct new SavingsAcct()
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References and Inheritance

• Assigning a child object to a parent reference is
  considered to be a widening conversion, and can
  be performed by simple assignment
• Assigning an parent object to a child reference
  can be done also, but it is considered a
  narrowing conversion and must be done with a cast
• The widening conversion is the most useful

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Polymorphism

• As we have seen, in Java, type of a variable
  doesn't completely determine type of object to
  which it refers
• Method calls are determined by type of actual
  object, not type of object reference

BankAccount aBankAccount new SavingsAccount(1000
) // aBankAccount holds a reference to a
SavingsAccount
BankAccount anAccount new CheckingAccount()
anAccount.deposit(1000) // Calls "deposit"
from CheckingAccount
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Polymorphism

• Compiler needs to check that only legal methods
  are invoked

Object anObject new BankAccount()
anObject.deposit(1000) // Wrong!
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Polymorphism

• Polymorphism ability to refer to objects of
  multiple types with varying behavior
• Polymorphism at work
• Depending on types of amount and other, different
  versions of withdraw and deposit are called

public void transfer(double amount, BankAccount
other) withdraw(amount) // Shortcut for
this.withdraw(amount) other.deposit(amount)

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Polymorphism via Inheritance

• It is the type of the object being referenced,
  not the reference type, that determines which
  method is invoked
• Suppose the BankAccount class has a method called
  withdraw, and the SavingsAcct class overrides it
• Now consider the following invocation
• acct.withdraw()
• If acct refers to a BankAccount object, it
  invokes the BankAccount version of withdraw if
  it refers to a SavingsAcct object, it invokes the
  SavingsAcct version

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Polymorphism via Inheritance

• Consider the following class hierarchy

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Polymorphism via Inheritance

• Now let's look at an example that pays a set of
  diverse employees using a polymorphic method
• Example on the board

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Conclusion

• Exam on Wednesday
• Try to be here at exactly 530 so we can start on
  time
• Closed book, closed notes