OOP

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OOP

• Chapter 11 Overview

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Todays Lesson Plan

• Discuss labs, quizzes, and TDA topics (10 min)
• One more week left so please keep up!
• Class Activities covers Chapter 11 (120 min)
• Discuss a PICA (Polymorphic, Inheritance, and
  Composition Architecture)
• Discuss and visually show the structure, syntax
  and execution of an Inheritance application.
• Discuss and visually show the structure, syntax
  and execution of a Polymorphic Application.
• Discuss and visually show the structure, syntax
  and execution of a Composition application.
• NOTE All of these applications could actually be
  one application with a PICA.
• Coding Exercises break (90 min)
• Code Ex 1 p 594 11.3.8a 8b
• Code Ex 2 p 601 11.4.5
• Code Ex 3 p 601 11.4.5

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Objectives

• Additional Class Features
• Class Inheritance Polymorphism
• Reference Variables as Class Members
• The this reference
• Memberwise assignment

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Each object has its own copy of the Class
instance variables

• Each object maintains its own set of instance
  variables
• Permits each object to have its own clearly
  defined state
• When an object is created, a distinct area of
  memory is set aside for its instance variables

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The this Reference

• Member methods
• Unlike data storage
• Only one copy of each member method is maintained
  in memory per class
• Requires providing a means of identifying which
  specific object a member method should be
  operating on
• Use the this reference to specify the current
  instance

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Memberwise Assignment

• Using the operator to assign objects results in
  a shallow copy
• Memberwise assignment
• Also called deep copy
• Assigns each instance variable a value
  specifically

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Memberwise Assignment (continued)

• Cascading method call
• Example
• a.setEqual(b.setEqual(c))
• Method must return same data type as parameter
• Can use the this reference in a return statement

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Class Inheritance Polymorphism

• Inheritance
• Constructing one class from another
• E.g., constructing a Sphere from a Circle
  class
• Polymorphism
• Provides the ability to redefine how methods of
  related classes operate based on the object being
  referenced

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Inheritance

• Capability of deriving one class from another
  class
• Base class or Original class
• Also called Parent or Superclass

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Derived Class Properties

• Derived class
• Derived from base
• Also called
• Child or Subclass
• Completely new class
• Incorporates all the variables and methods of the
  base class
• Adds new data and method members
• Can override any base class method

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Kinds of Inheritance

• Simple inheritance
• Each derived type has only one immediate base
  type
• Multiple inheritance
• Derived type has two or more base types
• Not supported in Java
• Class hierarchies
• Illustrate the order in which one class is
  derived from another

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Inheritance Diagrams

• Illustrate the relationship between a base class
  and one or more derived classes
• Base class is always drawn at the top of the
  diagram (top-down)
• Derived classes are drawn under the base class

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Inheritance Syntax Idioms

• Header line syntax for derived class
• public class derivedClassName extends
  baseClassName
• protected visibility
• Behaves identically to private visibility within
  a class
• But it permits this restriction to be inherited
  by any derived class
• super reserved word
• Refers to a method or variable in the parent
  class

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Abstract Classes and Interfaces

• Abstract class
• Can only be used as a base class for another
  class
• Can not be instantiated
• Created using the abstract reserved word in the
  class header
• Typically contains one or more abstract methods
• Method does not contain a body, only a header
• Derived classes must define abstract methods
  implementation
• Class can also include non-abstract methods

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Abstract methods cannot be

• Abstract methods cannot be declared as
• static or final
• final not allowed because it doesnt allow
  over-riding by definition
• static because you can only hide a static
  method
• Static means one per class, not one for each
  object no matter how many instance of a class
  might exist. This means that you can use them
  without creating an instance of a class.Static
  methods are implicitly final, because overriding
  is done based on the type of the object, and
  static methods are attached to a class, not an
  object. A static method in a superclass can be
  shadowed by another static method in a subclass,
  as long as the original method was not declared
  final. However, you can't override a static
  method with a nonstatic method. In other words,
  you can't change a static method into an instance
  method in a subclass.

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Interfaces

• An Interface consists of constants and abstract
  methods only
• Syntax
• interface interfaceName
• constant declarations
• abstract method declarations

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Interfaces (continued)

• Interface
• Methods must be public and abstract
• Constants must be public and final
• No need to actually use these modifiers in method
  and constant definitions since theyre understood
  for an interface!

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Implementing an interface

• Implementing an interface syntax create a new,
  derived class, as per the following
• class className implements interfaceName
  override defns of all abstract methods

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Polymorphism

• Permits the same method name to invoke
• One response in objects of a base class
• Another response in objects of a derived class
• The determination of which overloaded method is
  actually called is made at run time
• Known as run-time binding
• Object being operated on ultimately determines
  the appropriate method to be called

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Overriding or Overloading Polymorphism?

• Polymorphism translates from Greek as many forms
  ( poly many, morph - forms)
• Method overloading is the primary way
  polymorphism is implemented in Java
• Overloaded methods appear in a subclass with same
  name but different parameter lists and return
  types
• Late-binding (or run-time binding) also supports
  overriding
• Overriding allows a subclass to re-define a
  method it inherits from it's superclass
• An over-ridden method in a subclass has the same
  signature (name and parameter list) and also the
  same return type (different from C!)

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Reference Variables as Class Members

• Be aware of object assignment rules using the
  operator
• Suppose two classes contain a String data member
• To copy value of data from one class to another
  class
• Use StringBuffer or perform a deep copy

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Summary

• For each class
• Only one copy of each member method is created
• Each object uses the same methods!
• Location of the objects data members is provided
  to the member method by using a reference
  argument named this
• Shallow copy occurs when two reference variables
  both locate one and the same object
• Deep copy occurs when two reference variables
  locate two distinct objects, where both objects
  contain the same information
• Inheritance
• Capability of deriving one class from another
• Polymorphism
• Permits the same method to invoke different
  responses based on the object making the method
  call

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Additional Applications

• Complete simulation
• Using two separate classes
• Gas pump and Customer

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Application 1 A Single-Class Gas Pump Simulation

• Involves two distinct object types
• Person and Gas pump
• Pseudocode for top-level tasks
• Put the pump in service
• Display values
• Pump an amount of gas

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Application 1 A Single-Class Gas Pump Simulation
(continued)

• Attributes of interest for pump
• Amount of gallons in the supply tank and Price
  per gallon
• Required operations include
• Supplying the initial values for the pumps
  attributes
• Interrogating the pump for attribute values
• Satisfying the request for gas

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Application 2 A Multiclass Gas Pump Simulation

• Customer class
• No attributes are needed
• Operation arrival() provides a random arrival
  time between 1 and 15 minutes
• Operation gallons() provides a random request of
  between 3 and 20 gallons of gas

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Application 2 A Multiclass Gas Pump Simulation
(continued)

• Each interaction between Customer and Pump
• Obtain Customer arrival time
• Obtain Customer request for gas
• Activate Pump with request