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 The derived 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{
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 child’s 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
Java’s 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 doesn’t 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); String toString();
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,….