Extending Classes

1
Extending Classes

• Inheritance and Polymorphism

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What is Inheritance?

• Represents the is a relationship
• Super Class parent, very generic
• Derived Class superset of Super class
• all of the components of the Super Class PLUS
  components specific to the new Class
• You Can Inherit in a Hierarchy

Animal
is a
is a
Dog
Cat
is a
Boxer
3
Super Class Animal

• public class Animal
  
• protected int age
• public Animal( ) //default Constructor
  
• age 0
• public Animal( int a ) //overloaded
  Constructor
• age a
• public void setAge( int years )
• age years
• public int getAge( )
• return age
• public void speak( )
• System.out.println(I do not know what
  noise to make)

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Dog - Derived Class from Animal

• public class Dog extends Animal
• private int numberOfFleas
  //attribute specific to Dog
• public Dog( )
• super( )
• public Dog( int inAge )
• super( inAge )
• public void setNumberOfFleas( int myFleas )
• numberOfFleas myFleas
• public int getNumberOfFleas( )
• return numberOfFleas
• public void speak( )
  //overridden

5
Cat - Derived Class from Animal

• public class Cat extends Animal
• private int numberOfLives
  //attribute specific to Cat
• public Cat( )
• super( )
• public Cat( int inAge )
• super( inAge )
• public void setNumberOfLives( int myLives )
• numberOfLives myLives
• public int getNumberOfLives( )
• return numberOfLives
• public void speak( )
  //overridden

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What is Polymorphism?

• Changing the implementation of Inherited Methods
  to be more specific for a Derived Class.
• Allows objects of different classes related by
  inheritance to respond differently to the same
  message.
• Example
• Using the same message speak( ) Make a dog
  bark and a cat meow.
• Accomplished by overriding the implementation
  of a method.

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Method Overriding

• Super Class
• Provides method interface
• Has simple ( or no ) method definition
• Derived Class
• uses exact same method interface, and rewrites
  the method definition to accommodate the specific
  needs of the derived class objects.

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Using Polymorphism

• public class AnimalDriver
• public static void main( String args )
• Animal animalRef
  //declare a generic reference to an Animal
• Dog fido new Dog( 5 ) //create a
  Dog object with 5 fleas
• Cat boots new Cat( 10 ) //create a
  Cat object with 10 lives
• animalRef fido //point
  the animalRef reference to the Dog
• //run the Dog version of speak( )
• System.out.println(Fido says
  animalRef.speak( ) )
• animalRef boots
  //point the animalRef reference to the Cat
• //run the Cat version of speak( )
• System.out.println(Boots says
  animalRef.speak( ) )

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Abstract Methods

• No method definition (code) in super class
• The base class only provides the interface
  (function signature).
• Code for the method is written in each derived
  class
• Subclass implementation meets specific subclass
  needs.
• Using a reference to the super class, you can
  treat all objects the same
• because the super class DOES provide the
  interface for the method.
• A class that has at least one abstract method is
  considered to be an abstract class.

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Abstract Class Animal

• public abstract class Animal
  
• protected int age
• public Animal( ) //default Constructor
  
• public Animal( int inYears ) //overloaded
  Constructor
• setAge( inYears )
• public void setAge( int years )
• age years
• public int getAge( )
• return age
• 
  //abstract method only has signature
• public abstract void speak( ) //in super
  class

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• Concrete Classes
• Classes from which objects can be instantiated
• Abstract Classes
• Contains at least one abstract method
• No objects can be instantiated from abstract
  classes

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Javas Universal Superclass

• Every Class is a subclass of Javas Object
  class.
• The Object class provides basic implementation
  for useful methods.
• These methods are available in every class in
  Java, regardless of whether you created it or
  not. They are inherited from the Object class.

• toString( ) returns a String representation of
  an object.
• equals( ) provides comparison logic.
• These inherited methods should be over-ridden so
  that they behave appropriately for your class.

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Partial Java Class Hierarchy
Object Parent Class for All Java
Classes toString() writes the hashcode and
Class Name to the screen
by default.
Super Classes You Create toString() You
override to print contents of
data members - or other
information of your choice.
Java Classes toString() - Inherit the parent
class implementation
Sub Class You can override Here.
Sub Class You can override Here, too.
Sub Class
Sub Class
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• public abstract class Animal
  
• protected int yearsOld
• protected int animalYears
• public Animal( ) //default Constructor
  
• setYearsOld( 0 )
• public Animal( int inYears ) //overloaded
  Constructor
• setYearsOld( inYears )
• public void setYearsOld( int years )
• yearsOld years
• .
• .
• .
• 
  //abstract method only has signature
• public abstract void speak( ) //in super
  class

15
Lab

• Write Cat and Dog classes that extend Animal.

16
Project 2

• Define an abstract base class Shape that includes
  2 protected data members of your Point type and a
  public abstract method show( ) to output a the x
  y values for a shape.
• Derive subclasses for Lines and Rectangles. You
  can represent a Line as two points. You can
  represent a Rectangle as two points on diagonally
  opposite corners. Implement the toString( )
  method for each class.
• Write a driver program to test your classes.
  Instantiate one of each type (Line and
  Rectangle), use the show( ) method to display
  each objects Point values.
• Turn in a hard copy of all classes Shape,
  Point, Line, Rectangle. A copy of your output,
  and your disk