Java programming -Object-Oriented Thinking- Inheritance

JAVA PROGRAMMING
Dr R Jegadeesan Prof-CSE
Jyothishmathi Institute of Technology and Science,
Karimnagar

SYLLABUS
Object-Oriented Thinking- A way of viewing world – Agents and Communities,
messages and methods, Responsibilities, Classes and Instances, Class Hierarchies-
Inheritance, Method binding, Overriding and Exceptions, Summary of Object-Oriented
concepts. Java buzzwords, An Overview of Java, Data types, Variables and Arrays,
operators, expressions, control statements, Introducing classes, Methods and Classes,
String handling.
Inheritance– Inheritance concept, Inheritance basics, Member access, Constructors,
Creating Multilevel hierarchy, super uses, using final with inheritance, Polymorphism-ad
hoc polymorphism, pure polymorphism, method overriding, abstract classes, Object class,
forms of inheritance specialization, specification, construction, extension, limitation,
combination, benefits of inheritance, costs of inheritance. 2

UNIT 1 : OBJECT ORIENTED THINKING
Topic Name : Object Oriented Thinking
Topic : Object Oriented Thinking and Inheritance
Aim & Objective : To make the student understand the concepts on object oriented
concepts.
Principle & Operation/ Detailed Explanation : java data data types, methods and
classes, Inheritance
Application With Example : Example java program using object oriented concepts.
Limitations If Any :
Reference Links
• ftp://ftp.cs.orst.edu/pub/budd/java/toc.pdf:
• Understanding Object-Oriented Programming with Java, updated edition, T. Budd,
Pearson Education.
• Java The complete reference, 9th edition, Herbert Schildt, McGraw Hill Education
(India) Pvt. Ltd.
• http://www.freejavaguide.com/corejava.htm
• http://www.oracle.com/technetwork/java/newtojava/java8book-2172125.pdf
•Video Link details
https://www.youtube.com/watch?v=4XRy-TdfU0I
3

Tutorial Problems : Forms of Inheritance
Universities & Important Questions :
• Explain the need for OOP paradigm.
• Explain the roles of agents, community and messages amongst agents in OOP paradigm.
• Distinguish between object-oriented programming and object-based programming.
• Explain the two Paradigms of Programming.
• Explain the Characterists of OOP.
• Explain classes and instances.
• Explain Objects.
• Write the general form of a class.
• Explain Class Hierarchies (Inheritance).
• Explain all types of Inheritance.
• Explain the uses of Inheritance.
• Explain Method Overriding with an example.
• Explain Method Binding.
• Explain any two forms of inheritance.
4

▪ A way of viewing world
▪ Overview of java
▪ Introducing classes
▪ Methods and Classes
▪ String Handling
▪ Inheritance Concepts
▪ Polymorphism
▪ Forms of Inheritance
▪ Benefits and costs of inheritance
UNIT – I
CONTENTS

Hierarchical Abstraction
• An essential element of object-oriented programming is
abstraction.
• Humans manage complexity through abstraction. For
example, people do not think of a car as a set of tens of
thousands of individual parts. They think of it as a well-
defined object with its own unique behavior.
• This abstraction allows people to use a car without being
overwhelmed by the complexity of the parts that form
the car. They can ignore the details of how the engine,
transmission, and braking systems work.
• Instead they are free to utilize the object as a whole.

Class Hierarchy
• A child class of one parent can be the parent of
another child, forming class hierarchies
Animal
Reptile Bird Mammal
Snake Lizard Bat
Horse
Parrot
 At the top of the hierarchy there’s a default
class called Object.

Class Hierarchy
• Good class design puts all common features as
high in the hierarchy as reasonable
• The class hierarchy determines how methods are
executed
• inheritance is transitive
–An instance of class Parrot is also an
instance of Bird, an instance of Animal, …,
and an instance of class Object

Base Class Object
• In Java, all classes use inheritance.
• If no parent class is specified explicitly, the
base class Object is implicitly inherited.
• All classes defined in Java, is a child of Object
class, which provides minimal functionality
guaranteed to e common to all objects.

Base Class Object
(cont)
Methods defined in Object class are;
1.equals(Object obj) Determine whether the
argument object is the same as the receiver.
2.getClass() Returns the class of the receiver, an object
of type Class.
3.hashCode() Returns a hash value for this object. Should
be overridden when the equals method is changed.
4.toString() Converts object into a string value. This
method is also often overridden.

Base class
1) a class obtains variables and methods from
another class
2) the former is called subclass, the latter super-
class (Base class)
3) a sub-class provides a specialized behavior
with respect to its super-class
4) inheritance facilitates code reuse and avoids
duplication of data

• One of the pillars of object-orientation.
• A new class is derived from an existing class:
1) existing class is called super-class
2) derived class is called sub-class
• A sub-class is a specialized version of its super-class:
1) has all non-private members of its
super-class
2) may provide its own implementation of
super-class methods
• Objects of a sub-class are a special kind of objects of a
super-class.

extends
 Is a keyword used to inherit a class from
another class
 Allows to extend from only one class
class One
{
int a=5;
}
class Two extends One
{
int b=10;
}

• One baseobj;// base class object.
• super class object baseobj can be used to refer its sub
class objects.
• For example, Two subobj=new Two;
• Baseobj=subobj // now its pointing to sub class

Subclass, Subtype and
Substitutability
• A subtype is a class that satisfies the principle of
substitutability.
• A subclass is something constructed using
inheritance, whether or not it satisfies the principle
of substitutability.
• The two concepts are independent. Not all
subclasses are subtypes, and (at least in some
languages) you can construct subtypes that are
not subclasses.

Subclass, Subtype, and Substitutability
• Substitutability is fundamental to many of the
powerful software development techniques in
OOP.
• The idea is that, declared a variable in one
type may hold the value of different type.
• Substitutability can occur through use of
inheritance, whether using extends, or using
implements keywords.

Subclass, Subtype, and Substitutability
When new classes are constructed using inheritance, the
argument used to justify the validity of substitutability is as
follows;
• Instances of the subclass must possess all data fields
associated with its parent class.
• Instances of the subclass must implement, through
inheritance at least, all functionality defined for parent class.
(Defining new methods is not important for the argument.)
• Thus, an instance of a child class can mimic the behavior of
the parent class and should be indistinguishable from an
instance of parent class if substituted in a similar situation.

Subclass, Subtype, and
Substitutability
➢The term subtype is used to describe the relationship
between types that explicitly recognizes the principle of
substitution. A type B is considered to be a subtype of A if
an instances of B can legally be assigned to a variable
declared as of type A.
➢The term subclass refers to inheritance mechanism
made by extends keyword.
➢Not all subclasses are subtypes. Subtypes can also be
formed using interface, linking types that have no
inheritance relationship.

Subclass
• Methods allows to reuse a sequence of statements
• Inheritance allows to reuse classes by deriving 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.
• As the name implies, the child inherits characteristics of the
parent(i.e the child class inherits the methods and data
defined for the parent class

Subtype
• Inheritance relationships are often shown
graphically in a class diagram, with the
arrow pointing to the parent class
Animal
weight : int
+ getWeight() : int
Bird
+ fly() : void

Substitutability (Deriving Subclasses)
• In Java, we use the reserved word extends to establish
an inheritance relationship
class Animal
{
// class contents
int weight;
public void int getWeight() {…}
}
class Bird extends Animal
{
// class contents
public void fly() {…};
}

Defining Methods in the Child Class:
Overriding by Replacement
• A child class can override the definition of an inherited method in
favor of its own
– that is, a child can redefine a method that it inherits from its parent
– the new method must have the same signature as the parent's method,
but can have different code in the body
• In java, all methods except of constructors override the methods of
their ancestor class by replacement. E.g.:
– the Animal class has method eat()
– the Bird class has method eat() and Bird extends Animal
– variable b is of class Bird, i.e. Bird b = …
– b.eat() simply invokes the eat() method of the Bird class
• If a method is declared with the final modifier, it cannot be
overridden

Forms of Inheritance
Inheritance is used in a variety of way and for a variety of different
purposes .
• Inheritance for Specialization
• Inheritance for Specification
• Inheritance for Construction
• Inheritance for Extension
• Inheritance for Limitation
• Inheritance for Combination
One or many of these forms may occur in a single case.

(- Inheritance for Specialization -)
Most commonly used inheritance and sub classification is for
specialization.
Always creates a subtype, and the principles of substitutability
is explicitly upheld.
It is the most ideal form of inheritance.
An example of subclassification for specialization is;
public class PinBallGame extends Frame {
// body of class
}

Specialization
• By far the most common form of inheritance is for specialization.
– Child class is a specialized form of parent class
– Principle of substitutability holds
• A good example is the Java hierarchy of Graphical components in
the AWT:
• Component
• Label
• Button
• TextComponent
– TextArea
– TextField
• CheckBox
• ScrollBar

(- Inheritance for Specification -)
This is another most common use of inheritance. Two different
mechanisms are provided by Java, interface and abstract, to make
use of subclassification for specification. Subtype is formed and
substitutability is explicitly upheld.
Mostly, not used for refinement of its parent class, but instead is
used for definitions of the properties provided by its parent.
class FireButtonListener implements ActionListener {
// body of class
}
class B extends A {
// class A is defined as abstract specification class
}

Specification
• The next most common form of inheritance
involves specification. The parent class
specifies some behavior, but does not
implement the behavior
– Child class implements the behavior
– Similar to Java interface or abstract class
– When parent class does not implement actual
behavior but merely defines the behavior that will be
implemented in child classes
• Example, Java 1.1 Event Listeners:
ActionListener, MouseListener, and so on specify
behavior, but must be subclassed.

(- Inheritance for Construction -)
Child class inherits most of its functionality from parent,
but may change the name or parameters of methods
inherited from parent class to form its interface.
This type of inheritance is also widely used for code
reuse purposes. It simplifies the construction of newly
formed abstraction but is not a form of subtype, and often
violates substitutability.
Example is Stack class defined in Java libraries.

Construction
• The parent class is used only for its
behavior, the child class has no is-a
relationship to the parent.
– Child modify the arguments or names of
methods
• An example might be subclassing the idea
of a Set from an existing List class.
– Child class is not a more specialized
form of parent class; no substitutability

(- Inheritance for Extension -)
Subclassification for extension occurs when a child
class only adds new behavior to the parent class and
does not modify or alter any of the inherited attributes.
Such subclasses are always subtypes, and
substitutability can be used.
Example of this type of inheritance is done in the
definition of the class Properties which is an
extension of the class HashTable.

Generalization or Extension
• The child class generalizes or extends the parent
class by providing more functionality
– In some sense, opposite of subclassing for
specialization
• The child doesn't change anything inherited from
the parent, it simply adds new features
– Often used when we cannot modify existing
base parent class
• Example, ColoredWindow inheriting from Window
– Add additional data fields
– Override window display methods

(- Inheritance for Limitation -)
Subclassification for limitation occurs when the
behavior of the subclass is smaller or more
restrictive that the behavior of its parent class.
Like subclassification for extension, this form
of inheritance occurs most frequently when a
programmer is building on a base of existing
classes.
Is not a subtype, and substitutability is not
proper.

Limitation
• The child class limits some of the behavior of
the parent class.
• Example, you have an existing List data type,
and you want a Stack
• Inherit from List, but override the methods
that allow access to elements other than top
so as to produce errors.

(- Inheritance for Combination -)
This types of inheritance is known as multiple inheritance in
Object Oriented Programming.
Although the Java does not permit a subclass to be formed be
inheritance from more than one parent class, several
approximations to the concept are possible.
Example of this type is Hole class defined as;
class Hole extends Ball implements
PinBallTarget{
// body of class
}

Combimnation
• Two or more classes that seem to be related,
but its not clear who should be the parent
and who should be the child.
• Example: Mouse and TouchPad and JoyStick
• Better solution, abstract out common parts to
new parent class, and use subclassing for
specialization.

Summary of Forms of Inheritance
• Specialization. The child class is a special case of the parent class;
in other words, the child class is a subtype of the parent class.
• Specification. The parent class defines behavior that is implemented
in the child class but not in the parent class.
• Construction. The child class makes use of the behavior provided by
the parent class, but is not a subtype of the parent class.
• Generalization. The child class modifies or overrides some of the
methods of the parent class.
• Extension. The child class adds new functionality to the parent class,
but does not change any inherited behavior.
• Limitation. The child class restricts the use of some of the behavior
inherited from the parent class.
• Variance. The child class and parent class are variants of each other,
and the class-subclass relationship is arbitrary.
• Combination. The child class inherits features from more than one
parent class. This is multiple inheritance and will be the subject of a
later chapter.

The Benefits of Inheritance
• Software Reusability (among projects)
• Increased Reliability (resulting from reuse and sharing of
well-tested code)
• Code Sharing (within a project)
• Consistency of Interface (among related objects)
• Software Components
• Rapid Prototyping (quickly assemble from pre-existing
components)
• Polymorphism and Frameworks (high-level reusable
components)
• Information Hiding

The Costs of Inheritance
• Execution Speed
• Program Size
• Message-Passing Overhead
• Program Complexity (in overuse of
inheritance)

Types of inheritance
 Acquiring the properties of an existing
Object into newly creating Object to
overcome the redeclaration of properties
in deferent classes.
 These are 3 types:
1.Simple Inheritance
SUPER
SUB
SUPER
SUB 1 SUB 2
extends
extends

2. Multi Level
Inheritance
3. Multiple
Inheritance
SUPER
SUB
SUB SUB
SUPER 1
SUPER 2
extends
extends
implements
SUB
SUPER 1 SUPER 2
implements
SUB
extends

Member access rules
• Visibility modifiers determine which class members
are accessible and which do not
• Members (variables and methods) declared with
public visibility are accessible, and those with
private visibility are not
• Problem: How to make class/instance variables visible
only to its subclasses?
• Solution: Java provides a third visibility modifier that
helps in inheritance situations: protected

Modifiers and Inheritance
(cont.)
Visibility Modifiers for class/interface:
public : can be accessed from outside the class definition.
protected : can be accessed only within the class definition in
which it appears, within other classess in the same package, or
within the definition of subclassess.
private : can be accessed only within the class definition in
which it appears.
default-access (if omitted) features accessible from inside the
current Java package

The protected Modifier
• The protected visibility modifier allows a member of a base class
to be accessed in the child
– protected visibility provides more encapsulation than
public does
– protected visibility is not as tightly encapsulated as private
visibility
Book
protected int pages
+ getPages() : int
+ setPages(): void
Dictionary
+ getDefinitions() : int
+ setDefinitions(): void
+ computeRatios() : double

Example: Super-Class
class A {
int i;
void showi() {
System.out.println("i: " + i);
}
}

Example: Sub-Class
class B extends A {
int j;
void showj() {
System.out.println(“j: " + j);
}
void sum() {
System.out.println("i+j: " + (i+j));
}
}

Example: Testing Class
class SimpleInheritance {
public static void main(String args[]) {
A a = new A();
B b = new B();
a.i = 10;
System.out.println("Contents of a: ");
a.showi();
b.i = 7; b.j = 8;
System.out.println("Contents of b: ");
subOb.showi(); subOb.showj();
System.out.println("Sum of I and j in b:");
b.sum();}}

Multi-Level Class Hierarchy
The basic Box class:
class Box {
private double width, height, depth;
Box(double w, double h, double d) {
width = w; height = h; depth = d;
}
Box(Box ob) {
width = ob.width;
height = ob.height; depth = ob.depth;
}
double volume() {
return width * height * depth;
}
}

Adding the weight variable to the Box class:
class BoxWeight extends Box {
double weight;
BoxWeight(BoxWeight ob) {
super(ob); weight = ob.weight;
}
BoxWeight(double w, double h, double d, double m) {
super(w, h, d); weight = m;
}
}

Adding the cost variable to the BoxWeight class:
class Ship extends BoxWeight {
double cost;
Ship(Ship ob) {
super(ob);
cost = ob.cost;
}
Ship(double w, double h,
double d, double m, double c) {
super(w, h, d, m); cost = c;
}}

class DemoShip {
Ship ship1 = new Ship(10, 20, 15, 10, 3.41);
Ship ship2 = new Ship(2, 3, 4, 0.76, 1.28);
double vol;
vol = ship1.volume();
System.out.println("Volume of ship1 is " + vol);
System.out.print("Weight of ship1 is”);
System.out.println(ship1.weight);
System.out.print("Shipping cost: $");
System.out.println(ship1.cost);

vol = ship2.volume();
System.out.println("Volume of ship2 is " + vol);
System.out.print("Weight of ship2 is “);
System.out.println(ship2.weight);
System.out.print("Shipping cost: $“);
System.out.println(ship2.cost);
}
}

“super” uses
 ‘super’ is a keyword used to refer to hidden
variables of super class from sub class.
super.a=a;
 It is used to call a constructor of super class
from constructor of sub class which should
be first statement.
super(a,b);
 It is used to call a super class method from
sub class method to avoid redundancy of
code
super.addNumbers(a, b);

Super and Hiding
• Why is super needed to access super-class members?
• When a sub-class declares the variables or methods with the same
names and types as its super-class:
class A {
int i = 1;
}
class B extends A {
int i = 2;
System.out.println(“i is “ + i);
}
• The re-declared variables/methods hide those of the super-class.

Example: Super and Hiding
class A {
int i;
}
class B extends A {
int i;
B(int a, int b) {
super.i = a; i = b;
}
void show() {
System.out.println("i in superclass: " + super.i);
System.out.println("i in subclass: " + i);
}
}

Example: Super and Hiding
• Although the i variable in B hides the i
variable in A, super allows access to the
hidden variable of the super-class:
class UseSuper {
B subOb = new B(1, 2);
subOb.show();
}
}

Using final with inheritance
• final keyword is used declare constants which can not change its
value of definition.
• final Variables can not change its value.
• final Methods can not be Overridden or Over Loaded
• final Classes can not be extended or inherited

Preventing Overriding with final
• A method declared final cannot be overridden in any sub-
class:
class A {
final void meth() {
System.out.println("This is a final method.");
}
}
This class declaration is illegal:
class B extends A {
void meth() {
System.out.println("Illegal!");
}
}

Preventing Inheritance with final
• A class declared final cannot be inherited – has no
sub-classes.
final class A { … }
• This class declaration is considered illegal:
class B extends A { … }
• Declaring a class final implicitly declares all its
methods final.
• It is illegal to declare a class as both abstract and
final.

Polymorphism
• Polymorphism is one of three pillars of object-
orientation.
• Polymorphism: many different (poly) forms of objects
that share a common interface respond differently when
a method of that interface is invoked:
1) a super-class defines the common interface
2) sub-classes have to follow this interface (inheritance),
but are also permitted to provide their own
implementations (overriding)
• A sub-class provides a specialized behaviors relying on
the common elements defined by its super-class.

Polymorphism
• A polymorphic reference can refer to different types of objects at
different times
– In java every reference can be polymorphic except of references
to base types and final classes.
• It is the type of the object being referenced, not the reference type,
that determines which method is invoked
– Polymorphic references are therefore resolved at run-time, not
during compilation; this is called dynamic binding
• Careful use of polymorphic references can lead to elegant, robust
software designs

Method Overriding
• When a method of a sub-class has the
same name and type as a method of the
super-class, we say that this method is
overridden.
• When an overridden method is called
from within the sub-class:
1) it will always refer to the sub-class
method
2) super-class method is hidden

Example: Hiding with Overriding 1
class A {
int i, j;
A(int a, int b) {
i = a; j = b;
}
void show() {
System.out.println("i and j: " + i + " " + j);
}
}

class B extends A {
int k;
B(int a, int b, int c) {
super(a, b);
k = c;
}
void show() {
System.out.println("k: " + k);
}
}

• When show() is invoked on an object of type B, the
version of show() defined in B is used:
class Override {
B subOb = new B(1, 2, 3);
subOb.show();
}
}
• The version of show() in A is hidden through
overriding.

Overloading vs. Overriding
• Overloading deals with
multiple methods in the
same class with the same
name but different
signatures
• Overloading lets you
define a similar operation
in different ways for
different data
• Overriding deals with two
methods, one in a parent
class and one in a child class,
that have the same signature
o Overriding lets you define a
similar operation in different
ways for different object
types

Abstract Classes
• Java allows abstract classes
– use the modifier abstract on a class header to declare an
abstract class
abstract class Vehicle
{ … }
• An abstract class is a placeholder in a class hierarchy
that represents a generic concept
Vehicle
Car Boat Plane

Abstract Class: Example
public abstract class Vehicle
{
String name;
public String getName()
{ return name; } method body
abstract public void move();
no body!
}
 An abstract class often contains abstract
methods, though it doesn’t have to
⚫ Abstract methods consist of only methods
declarations, without any method body

Abstract Classes
• An abstract class often contains abstract methods, though it
doesn’t have to
– Abstract methods consist of only methods declarations, without any
method body
• The non-abstract child of an abstract class must override the
abstract methods of the parent
• An abstract class cannot be instantiated
• The use of abstract classes is a design decision; it helps us
establish common elements in a class that is too general to
instantiate

Abstract Method
• Inheritance allows a sub-class to override the methods of its
super-class.
• A super-class may altogether leave the implementation
details of a method and declare such a method abstract:
• abstract type name(parameter-list);
• Two kinds of methods:
1) concrete – may be overridden by sub-classes
2) abstract – must be overridden by sub-classes
• It is illegal to define abstract constructors or static methods.

Java programming -Object-Oriented Thinking- Inheritance

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