Java Day-3

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www.peoplestrategists.com
1

Day 3
Inheritance
Advance Class Features
Abstraction
2

Inheritance
3

Inheritance Basic
• In Java, the classes can be derived from other classes.
• The derived class can inherit the fields and methods of base class.
• A class that is derived from another class is called a subclass.
• The class from which the subclass is derived is called as superclass.
• The syntax to derive a class:
class <classname> extends <superclassname>
4

Inheritance Basic (Contd.)
• An example to derive a class:
class Base{
}
class Sub extends Base{
}
• In the preceding code snippet, Base is the super class and Sub is the
sub class.
• Java supports following types of inheritance:
• Single inheritance
• Multilevel inheritance
• Hierarchical inheritance
5

Class A
Class B
Class A
Class B
Class C
Class A
Class B
Class D Class E
Class C
Class F
Single Inheritance Multilevel Inheritance Hierarchical Inheritance
6

• An example to implement single inheritance:
public class Base {
int baseA;
Base(){
baseA=20;
}
void printBase(){
System.out.println("baseA value="+baseA);
}
}
7

public class Sub extends Base{
int subA;
Sub(){
subA=67;
}
void printSub(){
System.out.println("baseA value="+baseA);
System.out.println("subA value="+subA);
}
}
8

public class InheritanceDemo {
public static void main(String args[]){
Sub obj=new Sub();
obj.printSub();
obj.printBase();
}
}
• The preceding code output will be:
baseA value=20
subA value=67
baseA value=20
9

• In the preceding code, the base class object is not created. However,
from the output you can notice that base class constructor was
invoked.
• When a sub class object is created the sub class constructor is
invoked. From the sub class constructor the JVM internally invokes
the super class constructor.
• You can also notice that the sub class objects are able to access the
super class members.
10

• An example to implement multilevel inheritance:
public class Circle {
double radius;
public Circle() {
radius = 1.0;
}
public Circle(double r) {
radius = r;
}
public double getRadius() {
return radius;
}
public double getArea() {
return radius*radius*3.14;
}
}
11

public class Cylinder extends Circle {
private double height;
public Cylinder() {
height = 1.0;
}
public Cylinder(double r, double h) {
setRadius(r);
height = h;
}
public double getHeight() {
return height;
}
public void setHeight(double height) {
this.height = height;
}
public double getVolume() {
return getArea()*height;
}
}
12

Cylinder cy1 = new Cylinder();
System.out.println("Radius is " + cy1.getRadius()
+ " Height is " + cy1.getHeight()
+ " Base area is " + cy1.getArea()
+ " Volume is " + cy1.getVolume());
Cylinder cy2 = new Cylinder(5.0, 2.0);
System.out.println("Radius is " + cy2.getRadius()
+ " Height is " + cy2.getHeight()
+ " Base area is " + cy2.getArea()
+ " Volume is " + cy2.getVolume());
}
}
13

Radius is 1.0 Height is 1.0 Base area is 3.14 Volume
is 3.14
Radius is 5.0 Height is 2.0 Base area is 78.5 Volume
is 157.0
14

• An example to implement hierarchical inheritance:
public class Shapes {
int noOfSides;
void display(){
System.out.println("No of sides:"+noOfSides);
}
}
15

public class Rectangle extends Shapes {
Rectangle(){
noOfSides=4;
}
}
16

public class Pentagon extends Shapes {
public Pentagon() {
noOfSides=5;
}
}
17

public class Hexagon extends Shapes {
public Hexagon() {
noOfSides=6;
}
}
18

Shapes sObj=new Shapes();
Rectangle rObj =new Rectangle();
Pentagon pObj=new Pentagon();
Hexagon hObj=new Hexagon();
sObj.display();
rObj.display();
pObj.display();
hObj.display();
}
}
19

• The output of the preceding code will be:
No of sides:0
No of sides:4
No of sides:5
No of sides:6
20

• Consider the following code snippet:
class Shapes {
int noOfSides;
void displayShapes(){
}
}
class Rectangle extends Shapes {
Rectangle(){
noOfSides=4;
}
void displayRectangle(){
System.out.println("Rectangle");
}
}
21

Shapes sObj=new Rectangle();
sObj.displayShapes();
}
}
• In the preceding code, the reference variable sObj, refers to the
object of Rectangle class. Therefore, the
sObj.displayShapes();statement will raise a compilation
error.
22

Use of super Keyword
• The super keyword is used to refer an instance of immediate parent class
object.
• An example to work with super keyword:
public class Base {
int val;
Base(){
val=20;
}
void printBase(){
System.out.println("Value="+val);
}
}
23

Use of super Keyword (Contd.)
int val;
Sub(){
val=67;
}
void printSub(){
System.out.println("Base class
Value="+super.val);
System.out.println("Sub class value="+val);
}
}
24

Sub sObj=new Sub();
sObj.printBase();
sObj.printSub();
}
}
Value=20
Base class Value=20
Sub class value=67
25

• When a sub class constructor does not explicitly calls the super class
constructor, then the compiler will insert a super() statement.
• This statements invokes the super class constructor without parameter.
• Consider the following code:
class Base{
int baseX;
Base(){
this(10);
System.out.println("Base()");
}
26

Base(int x){
baseX=x;
System.out.println("Base(int)");
}
}
27

class Sub extends Base{
int subX;
Sub(){
subX=20;
System.out.println("Sub()");
}
}
public class SuperDemo {
Sub sObj=new Sub();
}
}
28

Base(int)
Base()
Sub()
• Now, in the preceding code if you remove the Base(){} constructor
definition, an compilation error will be raised in the Sub(){}
constructor definition.
• To rectify this error, the Base class constructor should be explicitly
called from Sub class constructor. This can be achieved by adding the
following code snippet to the Sub() constructor:
super(7);
29

• The super() statement must be the first statementinside the sub class
constructor.
30

Overriding
• Overriding is feature that enables a sub class to redefine a super class
method.
• The overriding method has the same name, number and type of
parameters, and return type as the method that it overrides.
• In Java, runtime polymorphism is achieved through overriding.
• Runtime polymorphism means that the exact method that is bound
to the object is not known at compile time.
31

Overriding (Contd.)
• An example to work with overridden methods:
public class Base {
int val;
Base(){
val=20;
}
void display(){
}
}
32

Overriding (Contd.)
int val;
Sub(){
val=67;
}
void display(){
System.out.println("Base class
Value="+super.val);
System.out.println("Sub class value="+val);
}
}
33

Overriding (Contd.)
Base bObj=new Base();
Sub sObj=new Sub();
bObj.display();
sObj.display();
}
Value=20
Base class Value=20
Sub class value=67
34

Overriding (Contd.)
class Shapes {
int noOfSides;
public void displayShapes(){
}
}
Rectangle(){
noOfSides=4;
}
35

Overriding (Contd.)
}
}
• The preceding code will generate a compilation error, because the
base class method displayShape() access control level is more than
the overridden method.
• The following code is a valid overridden method example :
class Shapes {
int noOfSides;
}
}
36

Overriding (Contd.)
Rectangle(){
noOfSides=4;
}
public void displayShapes(){
}
}
37

Overriding (Contd.)
class Shapes {
int noOfSides;
}
}
Rectangle(){
noOfSides=4;
}
38

Overriding (Contd.)
}
}
Shapes sObj=new Rectangle();
sObj.displayShapes();
}
}
39

Overriding (Contd.)
Rectangle
• The behavior demonstrated in the preceding code is referred
as virtual method invocation.
• The JVM calls the appropriate method for the object that is
referred by the reference variable.
40

Advance Class Features
41

Static Variables
• Variables that have the static modifier are called static variables or
class variables.
• Static variable belongs to the class and not to object.
• A single copy static variable is shared by all instances of the class.
• A static variable can be accessed outside the class by using the class
name.
• The syntax to access a static variable:
<classname>.<staticvariablename>
42

Static Variables (Contd.)
• An example to work with static variable:
public class StaticVarDemo {
static int count;
StaticVarDemo(){
count++;
}
void display(){
System.out.println("The count="+count);
}
43

Static Variables (Contd.)
StaticVarDemo obj1=new StaticVarDemo();
obj1.display();
StaticVarDemo obj2=new StaticVarDemo();
obj2.display();
obj1.display();
}
}
The count=1
The count=2
The count=2
44

Static Methods
• Methods that have the static modifier are called static methods.
• Static methods belongs to the class and not to object.
• A static method can access only static data.
• A static method cannot refer to this or super keywords.
• A static method can be overloaded. However, they can not be
overridden.
• A static method can be accessed outside the class by using the class
name.
45

Static Methods (Contd.)
• The syntax to access a static method:
<classname>.<methodname>
• Example:
class Test{
static void display(){
System.out.println("display()");
}
}
public class StaticMethodDemo {
static void show(){
StaticMethodDemo o=new StaticMethodDemo();
Test.display();
}
}
46

• An example of to work static method:
public class StaticMethodDemo {
static void display(){
System.out.println("Static display method");
}
public static void main(String args[]) {
display();
}
}
Static display method
47

void display(){
}
static void show(){
display();
}
• The preceding code snippet will generate a compilation error because
a non static method can not be called from a static method. However,
you can create an instance of the class and call the non static method
using the reference variable.
48

Static Block
• A static block is a block of code enclosed in braces, { }, and preceded
by the static keyword.
• The code in static blocks is executed when the class is loaded by JVM.
• A static block is used to initialize the static variables.
49

Static Block (Contd.)
• An example to work with static block:
public class StaticBlockDemo {
static int val;
static
{
val=10;
}
StaticBlockDemo(){
val++;
}
void display(){
}
50

Static Block (Contd.)
StaticBlockDemo obj=new StaticBlockDemo();
obj.display();
}
}
Value=11
51

Static Import
• Static import allows to import static members of class and use them,
as they are declared in the same class.
• The syntax for static import:
import static <packagename>.<classname>.*;
• An example for static import:
import static java.lang.Math.*;
52

Static Import (Contd.)
• An example to work with static import:
import static java.lang.Math.*;
public class StaticImportDemo {
public static void main(String args[]) {
double circleArea=PI*pow(4, 2);
System.out.println("Area of
circle:"+circleArea);
}
}
Area of circle:50.26548245743669
53

Abstraction
54

Abstract Class
• An abstract class is a class that is declared abstract.
• Abstract classes cannot be instantiated, but they can be sub classed.
• The syntax to define abstract class:
<accessmodifier> abstract class <classname>{
}
• An example to define abstract class:
abstract class Instrument {
……….
}
55

Abstract Method
• An abstract method is a method that is declared without an
implementation.
• The syntax to declare an abstract method:
<accessmodifier> abstract
<returntype><methodname>(parameterlist);
• An example to declare an abstract method:
abstract public void play();
56

Abstract Class and Method (Contd.)
• An example to work with abstract class and method:
abstract class Instrument {
protected String name;
abstract public void play();
}
abstract class StringedInstrument extends Instrument {
protected int numberOfStrings;
}
public class ElectricGuitar extends StringedInstrument {
public ElectricGuitar() {
super();
this.name = "Guitar";
this.numberOfStrings = 6;
}
57

public class ElectricGuitar extends StringedInstrument {
public ElectricGuitar() {
super();
}
public ElectricGuitar(int numberOfStrings) {
super();
this.numberOfStrings = numberOfStrings;
}
public void play() {
System.out.println("An electric " + numberOfStrings + "-string "
+ name
+ " is rocking!");
}
}
58

public class ElectricBassGuitar extends StringedInstrument {
public ElectricBassGuitar() {
super();
this.name = "Bass Guitar";
}
public ElectricBassGuitar(int numberOfStrings) {
super();
this.name = "Bass Guitar";
this.numberOfStrings = numberOfStrings;
}
public void play() {
System.out.println("An electric " + numberOfStrings + "-string " + name
+ " is rocking!");
}
}
59

public class Execution {
public static void main(String[] args) {
ElectricGuitar guitar = new ElectricGuitar();
ElectricBassGuitar bassGuitar = new ElectricBassGuitar();
guitar.play();
bassGuitar.play();
guitar = new ElectricGuitar(7);
bassGuitar = new ElectricBassGuitar(5);
guitar.play();
bassGuitar.play();
}
}
60

An electric 6-string Guitar is rocking!
An electric 4-string Bass Guitar is rocking!
An electric 7-string Guitar is rocking!
An electric 5-string Bass Guitar is rocking!
61

• An abstract class can have abstract and/or non abstract methods.
• An abstract method can be declared only inside an abstract class or
an interface.
• An abstract class can extend another abstract class.
• When an abstract class extends another abstract class, it may or may
not override the abstract methods of the abstract base class.
• However, when a non abstract sub class inherits an abstract class,
then the sub class must override all the abstract methods.
62

Final Class
• Final class is class defined with final keyword.
• If a class should not be sub classed then a final class should be
created.
• The syntax to define a final class:
<access modifier> final class <class name>{
}
• An example to define a final class:
public final class Mobile {
}
63

Final Class (Contd.)
public final class Mobile {
String mobileMakerName, modelName;
int price;
public Mobile(String mobileMakerName, String
modelName, int price) {
this.mobileMakerName = mobileMakerName;
this.modelName = modelName;
this.price = price;
}
}
64

public String getMobileMakerName() {
return mobileMakerName;
}
public void setMobileMakerName(String mobileMakerName) {
this.mobileMakerName = mobileMakerName;
}
public String getModelName() {
return modelName;
}
public void setModelName(String modelName) {
this.modelName = modelName;
}
public int getPrice() {
return price;
}
public void setPrice(int price) {
this.price = price;
}
}
65

• In the preceding code a final class Mobile is created.
class MobileTest extends Mobile{
}
• The preceding code, will generate a compile time error as the Mobile
class can not be sub classed.
66

Final Method
• A final method is defined using the final keyword.
• A method which should not be overridden by its sub class are defined
as final method.
• The syntax to define final method:
<accessmodifier> final
<returntype><methodname>(parameterlist){
}
• An example to define final method:
final void displayShape(String shapeName){
System.out.println("Shape Name="+shapeName);
}
67

Final Method (Contd.)
• An example to work with final method:
public abstract class Shape
{
public static float pi = 3.142f;
protected float height;
protected float width;
abstract float area() ;
final void displayShape(String shapeName){
System.out.println("Shape Name="+shapeName);
}
}
68

public class Square extends Shape{
Square(float h, float w)
{
height = h;
width = w;
}
float area()
{
return height * width;
}
}
69

public class Rectangle extends Shape
{
Rectangle(float h, float w)
{
height = h;
width = w;
}
float area()
{
return height * width;
}
}
70

public class Circle extends Shape
{
float radius;
Circle(float r)
{
radius = r;
}
float area()
{
return Shape.pi * radius *radius;
}
} © People Strategists - Duplication is strictly prohibited -
71

class FinalMethodDemo
{
public static void main(String args[])
{
Square sObj = new Square(5,5);
Rectangle rObj = new Rectangle(5,7);
Circle cObj = new Circle(2);
sObj.displayShape("Square");
System.out.println("Area: " + sObj.area());
rObj.displayShape("Rectangle");
System.out.println("Area: " + rObj.area());
rObj.displayShape("Circle");
System.out.println("Area: " + cObj.area());
}
}
72

• In the preceding code, if the sub classes, Square, Rectangle, or Circle
try to override the displayShape() method compile time error will be
raised.
73

Final Variables
• A final variables are defined using the final keyword.
• The final variables can not be reinitialized.
• The syntax to create final variable:
final <data type> <variable name>=<value>;
• An example to create final variable:
final int maxValue=100;
74

Interface
• An interface is a group of related methods with empty bodies.
• An interface is declared using interface keyword.
• The variable inside interface are by default public, static and final.
• The methods inside interface are by default public and abstract.
• The syntax to define an interface:
<access modifier> interface <interface name>{
}
75

Interface
• An example to define an interface:
public interface Printable {
}
• A class can implement an interface and it can implement any number
of interfaces.
• The class that implements an interface should override all the
abstract methods declared inside the interface.
• The syntax to implement an interface:
class <class name> implements <interface name>
76

Interface (Contd.)
• An example to implement an interface:
class A4Paper implements Printable{
}
• An interface can extend one or more interfaces.
• A class can extend another class and implement one or more
interfaces.
• The class that implements the interface should override all the
abstract methods of the interface.
77

Interface (Contd.)
• An example to work with interface:
public interface Printable {
public void print();
}
public class A4Paper implements Printable{
public void print(){
System.out.println("A4Paper Print");
}
}
78

Interface (Contd.)
public class A6Paper implements Printable{
public void print(){
System.out.println("A6Paper Print");
}
}
public class InterfaceDemo {
A4Paper obj1=new A4Paper();
A6Paper obj2=new A6Paper();
obj1.print();
obj2.print();
}
}
79

Interface (Contd.)
A4Paper Print
A6Paper Print
80

Abstract Class vs Interface
• Java provides and supports the creation of abstract classes and
interfaces. Both implementations share some common features, but
they differ in the following features:
• All methods in an interface are implicitly abstract. On the other hand, an
abstract class may contain both abstract and non-abstract methods.
• A class may implement a number of Interfaces, but can extend only one
abstract class.
• In order for a class to implement an interface, it must implement all its
declared methods. However, a class may not implement all declared methods
of an abstract class. Though, in this case, the sub-class must also be declared
as abstract.
• Abstract classes can implement interfaces without even providing the
implementation of interface methods.
81

Abstract Class vs Interface (Contd.)
• Variables declared in a Java interface is by default final. An abstract class may
contain non-final variables.
• Members of a Java interface are public by default. A member of an abstract
class can either be private, protected or public.
• An interface is absolutely abstract and cannot be instantiated. An abstract
class also cannot be instantiated, but can be invoked if it contains a main
method.
82

Summary
• In this topic, you have learnt that:
• In Java, the classes can be derived from other classes.
• The derived class can inherit the fields and methods of base class.
• A class that is derived from another class is called a subclass.
• Java supports single, multilevel, and hierarchical inheritance.
• The super keyword is used to refer an instance of immediate parent class
object.
• Overriding is feature that enables a sub class to redefine a super class
method.
• Variables and methods that have the static modifier are called static variables
and methods.
• A single copy static variable is shared by all instances of the class.
83

Summary (Contd.)
• The code in static blocks is executed when the class is loaded by JVM.
• Static import allows to import static members of class and use them, as they
are declared in the same class.
• Abstract classes cannot be instantiated, but they can be sub classed.
• A class that should not be sub classed should created as a final class.
• A method which should not be overridden by its sub class are defined as final
method.
• An interface is a group of related methods with empty bodies.
84

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