Inheritance

1
Lecture 12

• Inheritance

2
Inheritance

• Objectives
• Derive new classes from existing ones
• How does inheritance supports software reuse
• How to add and modify methods in child classes
• Extend simple class derivations into
  well-designed class hierarchies
• Discuss its use in java graphics frame

3
Introduction

• Inheritance is the act of deriving a new class
  from an existing one
• Derived class is a class created from an existing
  one
• Eg SavingsAccount is the derived class of
  Account class
• The key idea here is the software reuse
• Inheritance is the is-a relationship
• Eg Spinach is-a vegetable

4
Terminology

• The original class is called
• Parent class or
• Superclass or
• Base class
• The derived class is called
• Child class or
• subclass

5
An Example - the base class book

• class Book
• protected int pages 1500
• //---------------------------------------------
  -------------------
• // Prints a message concerning the pages of
  this book.
• //---------------------------------------------
  -------------------
• public void pageMessage ()
• System.out.println ("Number of pages "
  pages)

6
The derived class dictonary

• class Dictionary extends Book
• private int definitions 52500
• //---------------------------------------------
  --------------------
• // Prints a message using both local and
  inherited values.
• //---------------------------------------------
  --------------------
• public void definitionMessage ()
• System.out.println ("Number of definitions
  " definitions)
• System.out.println ("Definitions per page
  " definitions/pages)

7
BOOK
Dictionary
8
The Protected Modifier

• Not all variables and methods are inherited in
  derivation
• The visibility modifier (public, protected,
  private) decides which gets inherited
• Protected variables retains the encapsulation
  properties but derived class will inherit it

9
The super reference

• Constructors in base class are not inherited by
  the derived class, even if they have the public
  visibility
• super is a general reference to a parent class
• The parents constructor can be invoked using the
  super reference
• It the super reference is included in the childs
  constructor it must be the first line of the
  constructor

10
Example - book2

• class Book2
• protected int pages
• //---------------------------------------------
  -------------------
• // Sets up the book with the specified number
  of pages.
• //---------------------------------------------
  -------------------
• public Book2 (int pages)
• this.pages pages
• //---------------------------------------------
  -------------------
• // Prints a message concerning the pages of
  this book.
• //---------------------------------------------
  -------------------
• public void pageMessage ()
• System.out.println ("Number of pages "
  pages)

11
Example - Dictionary 2

• class Dictionary2 extends Book2
• private int definitions
• //---------------------------------------------
  --------------------
• // Sets up the dictionary with the specified
  number of pages
• // (maintained by the Book parent class) and
  defintions.
• //---------------------------------------------
  --------------------
• public Dictionary2 (int pages, int
  definitions)
• super (pages)
• this.definitions definitions
• //---------------------------------------------
  --------------------
• // Prints a message using both local and
  inherited values.
• //---------------------------------------------
  --------------------
• public void definitionMessage ()
• System.out.println ("Number of definitions
  " definitions)
• System.out.println ("Definitions per page
  " definitions/pages)

12
Multiple inheritance

• Javas approach to inheritance is called single
  inheritance
• Languages like C allow multiple inheritance but
  Java does not
• PickUpTruck is-a car and a truck
• If parents classes have similar methods, which
  ones the child supports?
• However the use of interface provides some of the
  abilities of multiple inheritance
• A java class can implement many interfaces
• It allows to interact with a particular class
  while inheriting most crucial information

13
Overriding methods

• If a child class defines a method with the same
  name and signature, the child class overrides the
  parents method from the parent
• Eg Message class contains a method that
  instantiate two objects
• One from Thought class
• One from Advice class

14
Examples

• class Messages
• //---------------------------------------------
  --------------------
• // Instatiates two objects a invokes the
  message method in each.
• //---------------------------------------------
  --------------------
• public static void main (String args)
• Thought parked new Thought()
• Advice dates new Advice()
• parked.message ()
• dates.message () // overridden

15
Class hierarchies

• Multiple classes can be derived from a single
  parent.
• Eg animal class (parent), Rabbit, Cow, Cat
  (children)
• Two children of the same parent are called
  siblings
• Common features should be kept as high as
  possible in the class hierarchy
• Inheritance mechanism is transitive
• There are no single best hierarchy organization
  for all situations

16
The object class

• In Java all classes are ultimately derived from
  the object class.
• If the class definition doesnt uses the extends
  clause, then it is automatically derived from the
  object class.
• class Thing
• class Thing extends Object
• Are equivalent

17
Object class methods

• boolean equals (Object obj)
• Returns true if the objects are the same(aliases)
• String toString()
• Returns a string representation of this object
• Object clone()
• Creates and returns a copy of this object

18
Abstract Classes

• An abstract class is a generic concept in a class
  hierarchy
• Abstract class cannot be instantiated
• Abstract class is similar to an interface
• Any class that contains one or more abstract
  methods is declared to be abstract
• Eg public abstract class StaffMember
• Eg Vehicle (abstract), derive car, Van,.