Inheritance

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Inheritance

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Class hierarchies often have to be extended and modified to keep up with changing needs * Single vs. Multiple Inheritance Java supports single inheritance, ... – PowerPoint PPT presentation

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Title: Inheritance

1
Inheritance

Inheritance is a fundamental object-oriented
technique is which enhances software design and
promotes reuse
We will focus on
deriving new classes
creating class hierarchies
the protected modifier
polymorphism via inheritance
inheritance used in graphical user interfaces

2
The Three Pillars of OOP

The Three Pillars of OOP
Encapsulation
Inheritance
Polymorphism
Every class in Java extends some other class.
If you don't explicitly specify the class that
your new class extends, it will automatically
extend the class named Object.

3
Encapsulation

By creating a class e.g. Student - and storing
in that class all the variables and methods
associated with a student in our program we are
Encapulating within the Student class all data
pertaining to a student
Thus later changes to a student profile can all
be done in one place in class

4
Class Hierarchy

Java uses a class hierarchy to organize its
classes.
Thus, all classes in Java exist in a class
hierarchy where the class named Object forms the
root of the hierarchy.
Some classes extend Object directly, while other
classes are
subclasses of Object further down the hierarchy.

5
Inheritance

Inheritance allows a software developer to derive
a new class from an existing one
The existing class is called the parent class, or
superclass, or base class
The derived class is called the child class or
subclass.

6
UML OR Unified Modeling Language is used in to
model an applicationS structures, behavior and
even business processes
PARENT
Child
PARENT CHILD RELATIONSHIP
7
Inheritance

As the name implies, the child inherits
characteristics of the parent.
That is, the child class inherits the methods and
data defined in the parent class
This means that all child classes have access
to the methods and data.

8
Inheritance

Inheritance relationships are shown in a UML
class diagram as solid arrow with an unfilled
triangular arrowhead pointing to the parent class

Proper inheritance creates an is-a relationship,
meaning the child is a more specific version of
the parent

9
Inheritance

Class Car is a subclass of Vehicle and it
inherits state and behavior and from its parent
An objects state is in the form of instance
variables and
its behavior in the form of methods.
The child inherits both from all of its
ancestors.

10
Class Vehicle extends Object // data //
methods ( functions) Class Car extends
Vehicle // data - // methods (
functions) //some data and methods can be
inherited from Vehicle
11
Inheritance
ClassVehicle Method 1 Method 2 Method 3
Class Car Inherits all of the above methods and
can add methods of its own
12
Inheritance

A programmer can tailor a child class as needed
by adding new variables or methods,
or by modifying the inherited ones
Software reuse is the fundamental benefit of
inheritance

13
CODE REUSE

By using existing software components to create
new ones,
we capitalize on all the effort that went into
the design, implementation,
and testing of the existing software

14
Class Account deposit()
Class SavingsAccount Inherits deposit ()
Class SeniorAccount Inherits deposit()
15
Inheritance

This leads to smaller and easier to understand
systems.
WHY?
Class components can be reused, promoting the
concept of code reusability.

16
Inheritance

Inheritance cuts redundancy as descendant classes
only implement functionality that is particular
to them.
///MOST IMPORTANT
When changes are made on the parent information,
all child classes automatically are changed also.
.

17
CODE REUSE

Thus the models that are created are
easier to modify and
easier to implement

18
Concepts to Review
WHAT IS AN ISA Relationship? What is
Encapsulation? What are the benefits of
Inheritance? What does Code Reuse mean in
terms of efficiency? What effect does a code
change in the parent class have on a child
class?
19
Sub or Child Classes

In Java, the reserved word extends is used to
establish an inheritance relationship
To declare a subclass you would write
class subclass extends SuperClass . . .
or,
class Dictionary extends Book
A Java class can have only one direct superclass.

20
Sub or Child Classes

class ParentClass // the super class
protected int num // instance variable
public ParentClass(int num) // constructor
this. num num

class ParentClass
protected int num // instance variable
public ParentClass (int num) // constructor
this. num num
class ChildClass extends ParentClass
private float num1 // instance variable
public ChildClass(int newnum, float num1) //
Constructor
super(newnum) // calls parent class
constructor
this.num1 num1

22
INHERITANCE

TO create an instance of the Childclass we use
Childclass child new Childclass( 20, 30)
This code call the parent class constructor and
sends it two parameters which are integers that
will be stored in newnum and
num1
( header for the Parent Class constructor)
public ParentClass(int newnum, float num1)

23
Deriving Subclasses

In Java, we use the reserved word extends to
establish an inheritance relationship
class Car extends Vehicle
// class contents

24
Controlling Inheritance

Visibility modifiers determine which class
members get inherited and which do not
Variables and methods declared with public
visibility are inherited, and
those with private visibility are not

25
PROTECTED VISIBILITY MODIFIER

But public instance variables violate our goal
of encapsulation
(DO NOT USE THEM IN THIS COURSE!!!)
SO -------
There is a third visibility modifier that helps
in inheritance situations protected

26
The protected Modifier

The protected visibility modifier allows a member
of a base class to be inherited into the child
class
The protected visibility modifier provides more
encapsulation than public does.
Encapsulation requires that we protect as much as
possible our instance variables from outside
access.

27
Class Book

//------------------------------------------------
-------------------
// Class Book serves as a parent class for
class Dictionary
//------------------------------------------------
-------------------
class Book
protected int pages 1500
//
// Prints a message using the value of an
instance variable.
//
public void page_message ()
System.out.println ("Number of pages "
pages)
// method page_message
// class Book

//------------------------------------------------
-------------------
class Dictionary extends Book
private int definitions 52500
//
// Prints an instance variable declared in this
class and
// an instance variable one that it inherited
from Book
//
public void definition_message ()
System.out.println (" of definitions "
definitions)
System.out.println ("Definitions per page
"
definitions/pages)
// method definition_message
// class Dictionary

29
Class Words

//
// Instantiates a child class and invokes its
inherited methods.
//
public static void main (String args)
// create object of Dictionary class(child
of Word Class)
Dictionary webster new Dictionary ()//
webster.page_message() // method defined
in Book class
webster.definition_message() //method in
Dictionary class
// method main
OUTPUT
Number of pages 1500
of definitions 52500/1500 Definitions
per page "

30
Default Constructor

In the previous slide, we created an object of
the Dictionary class.
Dictionary webster new Dictionary ()
But the Dictionary class has no coded
constructor.
How can this be? All classes must have a
constructor
Answer The default constructor is called if a
class has no coded constructor. This happens
when instance variables already have a value.

31
The super Reference

Constructors are not inherited, even though they
have public visibility !!!!
Yet we often need to call the parent's
constructor to give values to the instance
variables in the parent class
The super reference refers to the parent class,
It calls the parent class constructor.

//------------------------------------------------
----------
// Class Book s constructor initializes pages
//------------------------------------------------
------------
class Book 2
protected int pages // instance variable
//
// Sets up a book with the specified number
of pages.
//
public Book 2(int num_pages)
pages num_pages // sets number of
pages
// constructor Book
// class Book2

////
//prints out the number
of pages in a book
//
public void page_message ()
System.out.println ("Number of pages "
pages)
// method page_message
// Close class Book2

//
// Class Dictionary demonstrates the interaction
between the constructors of parent and child
classes.
//
class Dictionary2 extends Book 2
private int definitions
//
// CONSTRUCTOR USES the super reference to
call parent class
Book's constructor.
//
public Dictionary2 (int num_pages, int
num_definitions)
// super calls parent class constructor and
sends the number of pages
super (num_pages)
definitions num_definitions // sets of
definitions
// constructor Dictionary

35
Dictionary2 class

//
// Prints an instance variable declared in this
class and
// an instance variable one that it inherited
from Book
//
public void definition_message ()
System.out.println (" of definitions "
definitions)
System.out.println ("Definitions per page
"
definitions/pages)
// method definition_message
NOTE that pages is inherited from the parent
class

36
//------------------------------------------------
----------// Driver class that creates an
object of the Dictionary2 class.
//-----------------------------------------------
-------------

class Words2
public static void main (String args)
/// Dictionary2 is child of class Book2
Dictionary2 webster new Dictionary2 (1500,
52500)
//webster calls method from the parent Book
class
webster.page_message()
// webster calls method from Dictionary
class
webster.definition_message()
// method main
// class Words2

37
Review

I want to create an object of the child class
Dictionary2
Dictionary2 inherits the number of pages from its
parent Book 2
HOW Did I GIVE A VALUE TO the inherited variable
Num_Pages?
How do I create an object of Dictionary2?
How many parameters do I use for the constructor
?

38
Class Diagram for Words
39
The Super Reference - Rules

In Words2.java, The Dictionary2 class uses the
super reference to call the parent constructor so
that the number of pages can be set.
Java requires that a child class must call the
parent class constructor using super IF and
ONLY IF
the parent class constructor has one or more
parameters.

40
SUPER and Constructors

This insures that the parent classs instance
variables are given values.
If the parent class constructor has no
parameters,
the child class is not required to call it.
A default constructor is called for the parent
class instead.

41
Key Terms to Review

Visibility modifiers protected
The Super Reference

42
Super Reference - A review

When we create the constructor for a child class,
we invoke the constructor for the super class.
Do this in the first line of the sub class
constructor
The parameter list must match the parameters in
the argument list of the superclass
constructor.e.g.
super(num_pages) // initializes the of
pages in parent class

OVERLOADED CONSTRUCTORS
We might have a constructor for a Rectangle class
with one parameter
public Rectangle(int width)
.. Initialize instance variables

44
Overloaded Constructors

Another constructor that could be defined by
Rectangle with two parameters
public Rectangle(int length, int width)
this.length length // shortcut assignment
this.width width //
Both constructors share the same name, Rectangle,
but they have different parameter lists.

44
45
OVERLOADED CONSTRUCTORS

The compiler determines which constructors to use
based on the number and types of the parameters
to the constructor. E.g
// uses constructor with one parameter
Rectangle rect new Rectangle(20)
Rectangle rect2 new Rectangle(20,30)
Each Rectangle object, rect and rect2, call a
different constructor

46
Overloading Methods similar to overloaded
constructors

Method overloading is the process of using the
same method name for multiple
methods.
The signature of each overloaded method must be
unique
The signature is based on the number, type, and
order of the parameters.

46
47
Method Overloading

The compiler must be able to determine which
version of the method is being invoked by
analyzing the parameters.
The return type of the method is not part of the
signature

48
Overloading Methods
48
49
Overloaded Methods

The println method is overloaded
println (String s)
println (int i)
println (double d)
etc.
The following lines invoke different versions of
the println method
System.out.println ("The total is")
System.out.println (total)

49
50
Overriding Methods

A child class can override the definition of an
inherited method in favor of its own.
This is different from the overloading that we
used in constructors.
A child can redefine a method it inherits from
its parent.

51
Overriding Methods

The new method must have the same signature as
the parent's method,
but can have different code in the body
Same signature same number and type of
parameters etc.

//
// Contains a method that is overridden in a
child class.
//
class Thought
public void message()
System.out.println ("I feel like I'm
diagonally parked " "in a parallel
universe.")
// method message
// class Thought

//
// Class Advice overrides the message method.
//but keeps same method name and number of
parameters
//
class Advice extends Thought
// This method overrides the parent version,
public void message()
System.out.println ("Warning Dates in
calendar are " "closer than they appear.")
// method message
// class Advice

///
// Driver class that creates objects of parent
and child //classes.
//
class Messages
public static void main (String args)
Thought parked new Thought() // create
Thought object
Advice dates new Advice()// create
Advice object
parked.message() // calls method in
Thought class
dates.message() // calls method in Advice
class
// method main
// class Messages

55
Overloading vs. Overriding

Don't confuse the concepts of overloading and
overriding
Overloading deals with multiple methods in the
same class with the same name but
different signatures
Overriding deals with two methods, one in a
parent class and one in a child class, that have
the same signature

56
Overloading VS Overriding

Overloading lets you define a method
in different ways for different data
in the same class
Overriding lets you define a method in a child
class
That has the same signature as the parent class
version but
Implements it differently from the parent class

57
Overriding Methods

MOST IMPORTANT!!!!
To overload a method, you must duplicate the
method name, but use a different argument list.
To override a method, you must match the entire
method signature - same argument list

58
Class Hierarchies

Two children of the same parent are called
siblings.
Good class design puts all common features as
high in the hierarchy as is possible.

59
Single vs. Multiple Inheritance

Java supports single inheritance, meaning that a
derived class can have only one parent class
Multiple inheritance allows a class to be derived
from two or more classes, inheriting the members
of all parents
This causes Collisions, such as the same
variable name in two parents, have to be resolved

60
The Object Class

The class Object is defined in the java.lang
package of the Java standard class library
All classes are derived from the Object class
The Object class is therefore the
ultimate root of all class hierarchies

61
The Object Class

The Object class contains a few useful methods,
which are inherited by all classes, e.g.
The toString method in the object class prints
out the name of the class and its memory address
We override the toString method in OUR classes to
output the instance variables in OUR class

62
Class Object

The class Object defines the following methods
clone()
equals(Object obj) -- we use
finalize()
getClass() - we use
hashCode() - sometimes
notify()
notifyAll()
toString() -- we use
wait()
wait(long timeout)
wait(long timeout, int nanos)
As you can see, this list includes three
overloaded versions of the method named wait
(same name, different formal argument lists).

63
Class Object

Because every class is either a direct or
indirect subclass of Object, every class in Java
inherits these eleven methods.
Many of these eleven methods are intended to be
overridden for various purposes.
However, some of them, such as getClass, notify,
and wait, are intended to be used directly
without overriding.

64
The super Reference Revisited

A method in the parent class can be invoked
explicitly using the super reference
To call a method in the parent class, the syntax
is
super.method(parameters)

public class Student
protected String Firstname, Lastname
protected int numCourses
//---------------------------------------------
--------------------
// Sets up a student with a name and number of
courses.
//---------------------------------------------
--------------------
public Student (String Firstname, String
Lastname, int numCourses)
this.Firstname Firstname
this. Lastname LastName
this.numCourses numCourses
(cond)

66
To String method

//------------------------------------------------
-----------------
// Returns information about this student as
a string.
// Used in System.out.println calls typical
toString
//---------------------------------------------
--------------------
public String toString ()
String result "Student name "
Firstname Lastname "\n"
result "Number of courses "
numCourses
return result
(cond)

//method to compare two objects
// Uses String class equals method to do
comparison
// String class overrides Object class equals
method
public boolean equals(Object other)
boolean result false
if (LastName.equals(((Student)other).LastName)
)
result FirstName.equals(((Student)other
).FirstName)
return result

public class GradStudent extends Student
private String source // instance variables
private double rate
//-----------------------------------------------
-----------
// Sets up the gradate student using the
specified
// information. Parameter include values for
// parent class instance variable
//------------------------------------------------
-------------
public GradStudent (String Fastname, String
LastName ,int numCourses, String source, double
rate)
// call parent class constructor to
initialize its instance variables
super (Firstname, Lastname, numCourses)
// initalize this classs instance variables
this.source source
this.rate rate

69
ToString method

//------------------------------------------------
-----------------
// Returns a description of this graduate
student as a string. uses super to call parent
class toString()
//------------------------------------------------
-----------------
public String toString ()
// call parent class toString method and
store in result
String result super.toString()
result "\nSupport source " source
"\n "Hourly pay rate " rate
return result

class Academia
//-----------------------------------------------
-----
// Creates objects of two student types, prints
some //information about them, then checks them
for equality.
//-----------------------------------------------
------
public static void main (String args)
Student susan new Student ("Susan", James,
4)
GradStudent Paul new GradStudent
("Paul",McCartney , 1,Govt, 10.00)
System.out.println (susan) // uses
toString method in Student
System.out.println (Paul) /// uses
toString method in GradStudent
// calls equals method in Student which compares
last and first names -
if (!susan.equals(Paul))
System.out.println ("These are
two different students.")

71
REVIEW

What is overloading - for Constructors, for
class methods
What is overriding
How do they differ
What does equals method in Object class do.
What does equals method in your class do
What does toString method in Object class return
What does toString method implemented in your
class return

72
Polymorphism

An object reference can refer to an object of its
class, or to an object of any class related to it
by inheritance
For example, if the Holiday class has a child
class called Christmas,
then a Christmas reference could actually be
declared to be a Holiday object. CONFUSING?

73
References and Inheritance

I can do
Holiday day new Holiday()
Christmas day new Christmas()

OR Holiday day day new Christmas()
OR Holiday day new Christmas()
74
References and Inheritance

Assigning a child object to an ancestor reference
is considered to be a widening conversion,
performed by simple assignment
Assigning an ancestor(parent) object to child
reference can also be done, but it is considered
to be a narrowing conversion and must be done
with a cast
The widening conversion is the most useful

75
Widening VS Narrowing Conversion

The only direct legal way to do a conversion is a
widening conversion.
A child object may be stored in an parent object
as this is a widening conversion
A parent object may not be stored in a child as
it is a narrowing conversion
Legal parent child
Error child parent

76
Conversion

Legal student
gradstudent(child)
Error gradstudent
student(parent)
(It is possible to store a child in a parent
under certain circumstances which we will address
in the homework).

77
Polymorphism

What is polymorphism?
The meaning of the word polymorphism is something
like one name, many forms.
How does Java implement polymorphism?
Polymorphism manifests itself in Java in the form
of multiple methods having the same name.
Also, assigning a child class object to a parent
class object

78
Polymorphism

Some forms of polymorphism
Method overloading
Method overriding through inheritance
Widening conversions
Polymorphism through inheritance

79
Polymorphism via Inheritance

A polymorphic reference is one which can refer to
different types of objects at different times.
Inheritance can be used as a basis of
polymorphism
An object reference can refer to one object at
one time,
then it can be changed to refer to another object
(related by inheritance) at another time

80
Class Account
balance double
deposit() double
withdraw() double
Class Senior Savings
deposit() double Overrides parent version
Inherits other parent methods and variables
81

public class Accounts
// instance variables
// two methods
// public void deposit( double amount)
public void withdraw (double amount)
// close class
class SeniorSavings class extends Accounts
// overrides deposit in parent class
public void deposit (double amount)

We create an array of Accounts objects.
Accounts customer new Accounts10
E.g.
We create an object of the Senior_Savings class
Senior_Savings Sam new Senior_Savings(Sam)
We store Sam , a Senior_Savings object in the
Accounts array
customer5 Sam

83
Polymorphism

We create an Accounts object called Ricky and
store it in the array
Accounts Ricky new Accounts(Ricky)
customer3 Ricky
So the array of class Accounts holds both
Senior-Savings and regular Accounts because the
two classes are related by inheritance.
This is a polymorphic assignment

public class Bank
public static void main(String args)
Account customer new Account10
for (int i0 ilt 3 i)
System.out.print ("Name of customer)
String name scan.next()
System.out.print ("Type of Account S for
Senior, R for regular ")
String type scan.next()
if(type.charAt(0) 'S')
customeri new Senior_Savings(Name,
acctype,accountnumber)
else if (acctype 'R)
customeri new Account(Name, acctype,
accountnumber)
// close for

When we call the deposit method in a loop ,
the deposit method that is called depends on
whether the object in the array
belongs to the Account class or the
SeniorSavings class.

86
Polymorphism

if(acctype 'S')
customeri new SeniorSavings(name,
acctype, accnum)
customeri.deposit(45) // SeniorSavings
version called
else if (acctype
'R)
customer i new Account(name, acctype,
acctnum)
customeri.deposit(75) // Account version
called

Lets look at another example of polymorphism.
Recall the classes we did earlier,
classes Advice and Thought.
Class Advice extends class Thought and
overrides the message method

88
Class Thought

// Class Thought has one method
/
class Thought
public void message()
System.out.println ("Some people get lost
in thought "
"because it's
unfamiliar territory.")
// method message
// class Thought

89
Class Advice extends Thought

/
// class Advice is a child class of Thought and
over rides the parent version of Advice
/
class Advice extends Thought
public void message()
System.out.println ("Avoid cliches like the
plague.")
// method message
// class Advice

class Messages2
public static void main (String args)
Thought Territory new Thought()
Advice Cliche new Advice()
// call the message method in each class
Territory.message() //
Cliche.message()
// Territory Cliché point to same address
they are aliases
Territory Cliche
// the message method from the Advice class is
printed
Territory.message() //Territory now refers
to the Cliché class
// method main
// class Messages

91
Final Methods

A final method cannot be overridden or
extended.
Declaring a method final prevents the derived
class from erroneously redefining a class method.
Declaring a method final allows the compiler to
perform inline optimization.

92
Class Hierarchies