Extending Classes

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Extending Classes

• B.Ramamurthy

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Introduction

• In the last lectures we studied encapsulation
  (class) concept.
• In this discussion we will look at extending
  classes.
• Inheritance by extending a concrete class,
  abstract class and implementing an interface will
  be examined.

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Inheritance

• Inheritance is the act of deriving a new class
  from an existing one.
• A primary purpose of inheritance is to reuse
  existing software.
• Original class used to derive a new class is
  called super class and the derived class is
  called sub class.
• Inheritance represents is a relationship
  between the superclass and the subclass.

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Syntax

• class subclass extends superclass
• class definition
• Example
• class Windstar extends FordCar // meaning it
  inherits from class Fordcar ....
• Windstar MyCar
• In this example, FordCar is the super-class and
  Windstar is a sub-class and MyCar is an object
  reference to Windstar class.
• MyCar new Windstar(green) // object
  instantiated

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Representing the Relationship
BankClass
has a
has a
has a
Account
MortgageSVC
BrokerageSVC
is a
is a
Savings
Checking
is a use inheritance has a use aggregation,
or membership
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Example Animal Hierarchy
is a
Animal
type
Dog
constructor
name breed
show
constructor (name) constructor (name , breed)
In the code, note the call to super-class construc
tor super
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Modifers

• Modifiers are special words that are added in
  front of methods, data fields, classes to specify
  their nature.
• Visibility modifiers private, public, protected
• Protected modifier is used when the entity needs
  to be available to the subclass but not to the
  public.

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Need for protected modifier
Employee public name private wage_rate and hours
uses
super class
Application uses Employee class,
Paid_Employee class
Paid_Employee
uses
public compute_wages()
sub class
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Need for protected modifier

• Employee is a super class, Paid_Employee is a sub
  class derived from it.
• If we make wage_rate and hours public they are
  available to the whole world.
• If you make them private then they are not
  accessible to the sub class unless there are set
  and get functions.
• Protected access control provides a mechanism for
  hiding details from the world but making it
  available to sub classes.

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Attributes (Data) Modifiers

• Data with no modifier is visible to sub-class
  within the same package but not to sub-class
  outside the package.
• Private data is available only within the class
  in which it is defined.
• Protected is available to the class and all its
  sub-classes.
• Public is available to all class.

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Polymorphism

• Polymorphism is ability of a function to assume
  several different forms and the automatic
  resolution of one of the forms depending on the
  (run time) type of the object.

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Abstract Class (ABC) and methods

• An abstract class is a class in which one or more
  methods are declared but not defined.
• Declaration of the methods can be omitted since
  it make no sense to implement them in the
  superclass.
• Am abstract method has abstract modifier in
  front of it.
• ABC cannot be instantiated.
• Sole purpose of ABC is to provide an appropriate
  super class from which classes may inherit.
• Classes from which objects can be instantiated
  are called concrete classes.

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Example
Food
Abstract super class
Pizza
Hamburger
HotDog
subclasses
Think of method eat of these classes.. eat() is
polymorphic.
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Example for Superclass Framework

• public abstract class EPA
• \\ different data
• public abstract void emission_control()
• class CA_EPA extends EPA
• void emission_control () //implementation
• class NY_EPA extends EPA
• void emission_control () // implementation

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Interface

• An interface essentially takes the abstract class
  concept to the extreme.
• When a class contains all abstract methods and/or
  constants, it can be implemented using an
  interface.
• It contains just constants, abstract methods or
  both.
• Many different implementations can be realized
  from a single interface.
• The concept of interface-implementation is the
  core of the JDK event model.

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Animal interface and implementations
implementation1
dog
spaniels
show()
sound().
Special details added on to the general dog
characteristics
interface
others
animal
Other implementations
show() sound()
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Example

• GUI and listerners

EVENT LISTERNERS
UPDATE GUI
GUI
EVENT HANDLERS
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Summary

• Inheritance implements a very useful relationship
  between classes.
• Inheritance lets you extend and reuse existing
  classes / library of classes.
• Interface provides a framework for a system of
  classes.
• A class may implement more than one interface.