Core Java by Mahika Tutorials

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Title: Core Java by Mahika Tutorials

1
Core Java
checked exception
2
Programming Languages
Low-Level Languages (Machine dependent)
High Level Languages (Machine independent)
Assembly language
Machine(Binary) language
3

Java
Java was created by James Gosling at Sun
Microsystems (which is now acquired by Oracle
Corporation) and released in 1995.
Java is a high level, object-oriented programming
language.

4
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5
OOPs Concepts
6

Encapsulation
Binding (or wrapping) code and data together into
a single unit.
A java class is the example of encapsulation.

Abstraction
Hiding internal details and showing functionality
only.

WithoutAbstraction
With Abstraction
8

Inheritance
A mechanism by which a class acquires all the
properties and behaviours of an existing class.
Provides code reusability.
Used to achieve runtime polymorphism.

Polymorphism
Ability to take multiple forms.

Example- int a10,b20,c cab
//addition String firstnameSachin,
lastnameTendulkar, fullname fullnamefirstname
lastname//concatenation
10
Java supports primitive data types , hence it is
a fully OOPL but not pure OOPL. int i20
//primitive type Integer anew Integer(20)
//Class type
11
Core Java
12

Java Version History
JDK Alpha and Beta (1995)
JDK 1.0 (23rd Jan, 1996)
JDK 1.1 (19th Feb, 1997)
J2SE 1.2 (8th Dec, 1998)
J2SE 1.3 (8th May, 2000)
J2SE 1.4 (6th Feb, 2002)
J2SE 5.0 (30th Sep, 2004)
Java SE 6 (11th Dec, 2006)
Java SE 7 (28th July, 2011)
Java SE 8 (18th March, 2014)
Java SE 9 (September 2017)

13
Signature of main()
14
public static void main(String args)
.. ..
15
returnType methodName(arguments)
int m3(int n) .. return nn
void m1() .. ..
int m2() .. return 2
16
public static void main(String args)
.. ..
17
Invoking Member Funtions Of A Class
class Test public static void myStatic()
----- -----
----- public void myNonStatic()
-----
class Sample public void CallerFunction()
// Invoking static function
Test.myStatic() // Invoking non-static
function Test t new Test()
t.myNonStatic()
18
Invoking main()
class Test public static void
main(String args) -----
----- -----
// Incase of static main()
Test.main()
// Incase of non-static main() Test
tnew Test() t.main()

Since the main method is static Java virtual
Machine can call it without creating any instance
of a class which contains the main method.

19
Thank You
20

Secured
Java is secured because
No explicit pointer
Java Programs run inside virtual machine sandbox
Exception handling concept

Valid java main method signature
public static void main(String args)
public static void main(String args)
public static void main(String args)
public static void main(String... args)
static public void main(String args)
public static final void main(String args)
final public static void main(String args)
final strictfp public static void main(String ar
gs)

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23

JVM
JVM (Java Virtual Machine) is an abstract
machine. It is a specification that provides
runtime environment in which java bytecode can be
executed.
JVM is the engine that drives the java code
The JVM performs following main tasks
Loads code
Verifies code
Executes code

JRE
JRE is an acronym for Java Runtime Environment.

JDK
JDK is an acronym for Java Development Kit.
Includes a complete JRE (Java Runtime
Environment) plus tools for developing,
debugging, and monitoring Java applications.

26
Internal Architecture of JVM
Memory areas allocated by JVM
27
Internal Architecture of JVM
Memory areas allocated by JVM
28
Java Keywords

Words that cannot be used as identifiers in
programs.
There are 50 java keywords.
All keywords are in lower-case.
Each keyword has special meaning for the
compiler.
Keywords that are not used in Java so far are
called reserved words.

29
List of Java Keywords
Category Keywords
Access modifiers private, protected, public
Class, method, variable modifiers abstract, class, extends, final, implements, interface, native,new, static, strictfp, synchronized, transient, volatile
Flow control break, case, continue, default, do, else, for, if, instanceof,return, switch,while
Package control import, package
Primitive types boolean, byte, char, double, float, int, long, short
Exception handling assert, catch, finally, throw, throws, try
Enumeration enum
Others super, this, void
Unused const, goto

Points to remember
const and goto are resevered words.
true, false and null are literals, not keywords.

30
Java Identifiers

Names given to programming elements like
variables, methods, classes, packages and
interfaces.

Rules for Naming Java Identifiers

Allowed characters for identifiers areA-Z,
a-z,0-9, (dollar sign) and _
(underscore).
Should not start with digits(0-9).
Case-sensitive, like id and ID are different.
Java keyword cannot be used as an identifier.

31
Java Naming conventions
Name Convention
class name should start with uppercase letter and be a noun e.g.Shape, String, Color, System, Thread etc.
interface name should start with uppercase letter and be an adjective e.g. Runnable, Clonable etc.
method name should start with lowercase letter and be a verb e.g. compute(), print(), println() etc.
variable name should start with lowercase letter e.g. firstName, lastName etc.
package name should be in lowercase letter e.g. java, lang, sql, util etc.
constants name should be in uppercase letter. e.g. BLUE, MAX_PRIORITY etc.
32
Core Java
33

Class
A template that describes the data and behavior.

Object
An entity that has state and behavior is known as
an object e.g. chair, bike, table, car etc.
state represents data (value) of an object.
behavior represents the functionality of an
object such as deposit, withdraw etc.

34
Data members id name
Student
class
Methods setId() setName() getId() getName()
35
Java class example
public class Student private int id
private String name public int getId()
return id public void setId(int
id) this.id id public
String getName() return name
public void setName(String name)
this.name name
36
Instantiation/ Object Creation- Classname
referenceVariablenew Classname() e.g.- Student
snew Student() Method Invocation/Call- Non-sta
tic method referenceVariable.methodname() e.g.-
s.show() static method Classname.methodname()
e.g.- Sample.display()
37

Scanner class
A class in java.util package
Used for obtaining the input of the primitive
types like int, double etc. and strings
Breaks its input into tokens using a delimiter
pattern, which by default matches whitespace.
To create an object of Scanner class, we usually
pass the predefined object System.in, which
represents the standard input stream.
We may pass an object of class File if we want to
read input from a file.

38
Creating instance of Scanner class To read
from System.in Scanner sc new
Scanner(System.in) To read from File Scanner
sc new Scanner(new File("myFile"))
39
Commonly used methods of Scanner class
Method Description
public String next() it returns the next token from the scanner.
public byte nextByte() it scans the next token as a byte.
public short nextShort() it scans the next token as a short value.
public int nextInt() it scans the next token as an int value.
public long nextLong() it scans the next token as a long value.
public float nextFloat() it scans the next token as a float value.
public double nextDouble() it scans the next token as a double value.
40
Thank You
41
Types of Variable
local
instance
static
42

public class Student
int rn //instance variable
String name //instance variable
static String college //static variable
void m1()
int l //local variable
.

43
class Student   int id
String name static String
collegeSVC    public static void ma
in(String args)      Student s1new Studen
t()      Student s2new Student()

Class Area
collegeSVC
id2name Anil
s2
id1name Hriday
s1
Stack
Heap
44
Thank You
45
class Student   int id   String name      vo
id insertRecord(int i, String n)  //method
idi   namen          void displayInf
ormation() System.out.println(id" "name)//met
hod      public static void main(String args)
  Student s1new Student()     Student s2new S
tudent()        s1.insertRecord(1,Hriday")
s2.insertRecord(2,Anil")        s1.displayInform
ation()     s2.displayInformation()

46
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47
Data Type Default Value Default size
boolean false 1 bit
char '\u0000' 2 byte
byte 0 1 byte
short 0 2 byte
int 0 4 byte
long 0L 8 byte
float 0.0f 4 byte
double 0.0d 8 byte
lowest unicode value\u0000
highest unicode value\uFFFF
48

Operators
Assignment Operator
Arithmetic Operators - / --
Relational Operators gt lt gt lt !
Logical Operators !
Compound Assignment Operators - /
Conditional Operator ?

Control Structures
Conditional
Simple if
ifelse
if-else-if ladder
Nested if
switch case
Looping
while loop
for loop
do while loop
for each/enhanced for loop

50
if Statement

if(condition)
//code to be executed

51
if else Statement

if(condition)
//code if condition is true
else
//code if condition is false

52
if-else-if ladder Statement

if(condition1)
//code to be executed if condition1 is true
else if(condition2)
//code to be executed if condition2 is true
else if(condition3)
//code to be executed if condition3 is true
...
else
//code to be executed if all the conditions are
false

53
Nested if
if(Boolean_expression 1) // Executes when
the Boolean expression 1 is true
if(Boolean_expression 2) //
Executes when the Boolean expression 2 is true

54
Core Java
55
switch Statement
Expression

switch(expression)
case value1
//code to be executed
break //optional
case value2
//code to be executed
break //optional
......
default
//code to be executed if all cases are not ma
tched

case 1?
Case Block 1
Yes
No
case 2?
Yes
Case Block 2
No
case 3?
Case Block 3
Yes
No
default Block
56

switch Statement
Expression can be byte, short, char, and int.
From JDK7 onwards, it also works with enumerated
types ( Enums in java), the String class
and Wrapper classes.
Duplicate case values are not allowed.
switch(ch)
case 1 .......
break
case 1 ......
break

The value for a case must be the same data type
as the variable in the switch.
switch(ch)
case 1 ......
break
case 2 ......
break
The value for a case must be a constant or a
literal.Variables are not allowed.

The break statement is used inside the switch to
terminate a statement sequence.
The break statement is optional. If omitted,
execution will continue on into the next case.
The default statement is optional.

59
while Loop

while(condition)
//code to be executed

60
do-while Loop

do
//code to be executed
while(condition)

61
for Loop

for(initializationconditionincr/decr)
//code to be executed

break Statement
Used to break loop or switch statement.

continue Statement
Used to continue loop.
Skips the remaining code at specified condition.

64
Polymorphism
Compile-time Polymorphism
Run-time Polymorphism
Method Overriding
Method Overloading
65

Method Overloading
When a class have multiple methods by same name
but different parameters.
Advantage
Increases the readability of the program.
Different ways to overload the method
By changing number of arguments
By changing the data type of arguments
Method Overloading is not possible by changing
the return type or access specifier of a method.

Method Overloading by changing the no. of
arguments
class Calculation
void sum(int a,int b)
System.out.println(ab)
void sum(int a,int b,int c)
System.out.println(abc)
public static void main(String args)
Calculation objnew Calculation()
obj.sum(40,10,10)
obj.sum(50,20)

Method Overloading by changing data type of
argument
class Calculation
void sum(int a,int b)
System.out.println(sum of int (ab))
void sum(double a,double b)
System.out.println((sum of double (ab))
public static void main(String args)
Calculation objnew Calculation()
obj.sum(19.5,10.5)
obj.sum(12,20)

Overloading Main method
class Test
public static void main(int a)
System.out.println(a)
public static void main(String args)
System.out.println("main() method invoked")
main(10)

Method Overloading and Type Promotion

If we have overloaded methods with one
byte/short/int/long parameter then always method
with int is called.
To invoke method with long parameter L should be
append to the value while calling
Method with byte/short is called only if we pass
the variable of that type or we typecast the
value.
If we have overloaded method with one
int/float/double and we pass long value then
method with int parameter is called.
If we have overloaded method with one
float/double and we pass long value then method
with float parameter is called.

Ambiguity in Method Overloading
class Test
void sum(int a,long b)System.out.println(ab)
void sum(long a,int b)System.out.println(ab)
public static void main(String args)
Test objnew Test()
obj.sum(20,20)// ambiguity
OutputCompile Time Error

Constructor
Special type of method that is used to initialize
the object.
Invoked at the time of object creation.
Can be overloaded.
Rules for creating java constructor
Constructor name must be same as its class name
Constructor must have no explicit return type
Types of java constructors
Default constructor (no-arg constructor)
Parameterized constructor

If there is no constructor in a class, compiler
automatically creates a default constructor.

class Student
int id
String name
void display()System.out.println(id" "name)
public static void main(String args)
Student s1new Student()
Student s2new Student()
s1.display()
s2.display()

Parameterized constructor
Used to provide different values to the distinct
objects.
Example
class Student
int id
String name
Student(int i,String n)
id i
name n
void display()System.out.println(id"
"name)
public static void main(String args)
Student s1 new Student(1,abc")
Student s2 new Student(2,xyz")
s1.display()
s2.display()

Java Copy Constructor
There is no copy constructor in java. But, we can
copy the values of one object to another like
copy constructor in C.
There are many ways to copy the values of one
object into another in java.
They are
By constructor
By assigning the values of one object into
another
By clone() method of Object class

In this example, we are going to copy the values
of one object into another using java
constructor.
class Student6
int id
String name
Student6(int i,String n)
id i
name n
Student6(Student6 s)
id s.id
name s.name
void display()System.out.println(id"
"name)
public static void main(String args)
Student6 s1 new Student6(111,"Karan")
Student6 s2 new Student6(s1)
s1.display()

Copying values without constructor
class Student7
int id
String name
Student7(int i,String n)
id i
name n
Student7()
void display()System.out.println(id"
"name)
public static void main(String args)
Student7 s1 new Student7(111,"Karan")
Student7 s2 new Student7()
s2.ids1.id
s2.names1.name
s1.display()
s2.display()

static keyword
static is a non-access modifier.
Used for memory management mainly.
The static can be
variable (also known as class variable)
method (also known as class method)
block
nested class

static variable
Used to refer the common property of all objects
e.g. company name of employees,college name of
students etc.
Gets memory only once in class area at the time
of class loading.
Local variables cannot be static.
Advantage
It makes your program memory efficient (i.e it
saves memory).

//Program for static variable
class Student
int id
String name
static String college SVC"
Student(int i,String n)
idi
name n
void display ()
System.out.println(id" "name" "college)
public static void main(String args)
Student s1 new Student(1,Hriday")
Student s2 new Student(2,Anil")

Class Area
collegeSVC
id2name Anil
s2
id1name Hriday
s1
Stack
Heap
82

static method
Method defined with the static keyword.
Belongs to the class rather than object of a
class.
Can be invoked without the need for creating an
instance of a class.
Can access static data members and can change the
value of it.
Cannot use non static data member or call
non-static method directly.
this and super cannot be used in static context.

static block
Used to initialize the static data member.
Is executed before main method at the time of
classloading.
Example
class A
static
System.out.println("static block invoked")
public static void main(String args)
System.out.println(main invoked")

Can we execute a program without main() method?
Yes, one of the way is static block but in
previous version of JDK not in JDK 1.7.
class A3
static
System.out.println("static block is invoked")
System.exit(0)

this keyword in java
A reference variable that refers to the current
object.
Usage of this keyword
this keyword can be used to refer current class
instance variable.
this() can be used to invoke current class
constructor.
this can be used to invoke current class method
(implicitly)
this can be used to return the current class
instance from the method.
this can be passed as an argument in the method
call.
this can be passed as argument in the constructor
call.

this() to invoke current class constructor
Used for constructor chaining.
Used to reuse the constructor.
Call to this() must be the first statement in
constructor.
Syntax
this() // call default constructor
this(value1,value2,.....) //call parametrized
constructor

87
//this() to invoke current class
constructor public class Rectangle private
int x, y private int width, height
public Rectangle() this(0, 0, 0, 0)
public Rectangle(int width, int height)
this(0, 0, width, height)
public Rectangle(int x, int y, int width, int
height) this.x x this.y
y this.width width
this.height height .
88

this to invoke current class method (implicitly)
Can be used to invoke method of the current
class.
If you don't use the this keyword, compiler
automatically adds this keyword while invoking
the method.

this keyword used to refer current class instance
variable
class Student
int id
String name
Student(int id,String name)
this.id id
this.name name
void display()System.out.println(id"
"name)
public static void main(String args)
Student s1 new Student(1,Hriday")
Student s2 new Student(2,"Anil")
s1.display()
s2.display()

this() constructor call (constructor chaining)
class Student
int id
String name
Student ()System.out.println("default
constructor invoked")
Student(int id,String name)
this ()//it is used to invoke current
class constructor.
this.id id
this.name name
void display()System.out.println(id"
"name)
public static void main(String args)
Student s1 new Student(1,Hriday")
Student s2 new Student(2,"Anil")
s1.display()
s2.display()

91
this to pass as an argument in the method
class Test     int a     int
b           Test()              a
40         b 60                // Method
that receives 'this' keyword as
parameter     void display(Test
obj)              System.out.println("a " a
" b " b)             // Method that
pases current class instance     void
get()              display(this)          pub
lic static void main(String args)
Test object new Test()         object.get()
   Output- a40 b60
92
Core Java
93
this to pass as argument in the constructor call
class B     A obj     B(A obj)
       this.objobj          void display()
    System.out.println(obj.data)
//10
class A     int data10      A()
B bnew B(this)       b.display()          p
ublic static void main(String args)
A anew A()
94

this keyword in java
A reference variable that refers to the current
object.
Usage of this keyword
this keyword can be used to refer current class
instance variable.
this() can be used to invoke current class
constructor.
this can be used to invoke current class method
(implicitly)
this can be used to return the current class
instance from the method.
this can be passed as an argument in the method
call.
this can be passed as argument in the constructor
call.
this keyword cannot be used in static context.

Inheritance
A mechanism in which one object acquires all the
properties and behaviors of parent object.
Represents the IS-A relationship, also known
as parent-child relationship
extends keyword indicates that you are making a
new class that derives from an existing class.
The meaning of "extends" is to increase the
functionality.
A class which is inherited is called parent or
super class and the new class is called child or
subclass.
Inheritance in java is used
For Method Overriding (so runtime polymorphism
can be achieved).
For Code Reusability.
Syntax of Java Inheritance
class Subclass-name extends Superclass-name
//methods and fields

96
Types of inheritance in java
Multiple inheritance is not supported in java
through class
97
Object class

Object class is the superclass of all the classes
in java by default.
Every class in Java is directly or indirectly
derived from the Object class.
Object class is present in java.lang package.

class A ..
class A extends Object ..
98
Object class
class A extends Object .. class B extends
A .
class A . class B extends A .
99
Commonly used Methods of Object class
Method Description
public boolean equals(Object obj) compares the given object to this object.
protected Object clone() throws CloneNotSupportedException creates and returns the exact copy (clone) of this object.
public String toString() returns the string representation of this object.
public final void notify() wakes up single thread, waiting on this object's monitor.
public final void notifyAll() wakes up all the threads, waiting on this object's monitor.
public final void wait(long timeout)throws InterruptedException causes the current thread to wait for the specified milliseconds, until another thread notifies (invokes notify() or notifyAll() method).
protected void finalize()throws Throwable is invoked by the garbage collector before object is being garbage collected.
100
Single Inheritance
101
Multilevel Inheritance
102
Hierarchical Inheritance
103
Multiple Inheritance

A feature where a class can inherit properties of
more than one parent class.
Java doesnt allow multiple inheritance by using
classes ,for simplicity and to avoid the
ambiguity caused by it.
It creates problem during various operations like
casting, constructor chaining etc.

104
Multiple Inheritance and Ambiguity
class A    void show()
System.out.println("Hello")
   class B    void show()
System.out.println("Welcome")
class C extends A,B //suppose if it was
allowed     public static void main(String ar
gs)       C objnew C()
obj.show() //ambiguity
105
Multiple Inheritance and Diamond Problem
106
Multiple inheritance in Java by interface

If a class implements multiple interfaces, or an
interface extends multiple interfaces.

107
Polymorphism
Compile-time Polymorphism
Run-time Polymorphism
Method Overriding
Method Overloading
108

Method Overriding
If subclass (child class) has the same method as
declared in the parent class, it is known as
method overriding in java.
Usage
Used to provide specific implementation of a
method that is already provided by its super
class.
Used for runtime polymorphism

Example
class Vehicle   void run()System.out.println("Ve
hicle is running")      class Bike extends Vehi
cle   void run()System.out.println("Bike is runn
ing )      public static void main(String args
)   Bike obj  new Bike()   obj.run()
109
At compile-time
class Shape void draw()
System.out.println("No Shape") class
Rectangle extends Shape void draw()
System.out.println("Drawing rectangle")
class Circle extends Shape void
draw() System.out.println("Drawing
circle") class Triangle extends
Shape void draw()
System.out.println("Drawing circle")
public class Test public static void
main(String args) Shape s
snew Shape() s.draw() s new
Circle() s.draw() snew
Rectangle() s.draw() snew
Triangle() s.draw()
110
At run-time
class Shape void draw()
System.out.println("No Shape") class
Circle extends Shape void draw()
System.out.println("Drawing circle")
class Rectangle extends Shape void
draw() System.out.println("Drawing
rectangle") class Triangle extends
Shape void draw()
System.out.println("Drawing circle")
public class Test public static void
main(String args) Shape s
snew Shape() s.draw() s new
Circle() s.draw() snew
Rectangle() s.draw() snew
Triangle() s.draw()
111

Rules for Method Overriding
Method must have same name as in the parent class
Method must have same parameter as in the parent
class.
Method must have same return type(or sub-type).
Must be IS-A relationship (inheritance).
private method cannot be overridden.
final method cannot be overridden
static method cannot be overridden because
static method is bound with class whereas
instance method is bound with object.
We can not override constructor as parent and
child class can never have constructor with same
name.
Access Modifier of the overriding method (method
of subclass) cannot be more restrictive than the
overridden method of parent class.
Binding of overridden methods happen at runtime
which is known as dynamic binding.

112

class Test2
public static void main(String args)
SBI snew SBI()
ICICI inew ICICI()
AXIS anew AXIS()
System.out.println("SBI Rate of Interest "s.getR
ateOfInterest())
System.out.println("ICICI Rate of Interest "i.ge
tRateOfInterest())
System.out.println("AXIS Rate of Interest "a.get
RateOfInterest())

//Method overriding
class Bank
int getRateOfInterest()return 0
class SBI extends Bank
int getRateOfInterest()return 8
class ICICI extends Bank
int getRateOfInterest()return 7
class AXIS extends Bank
int getRateOfInterest()return 9

113

ExceptionHandling with MethodOverriding in Java
If the superclass method does not declare an
exception, subclass overridden method cannot
declare the checked exception.

class Child extends Parent
void msg()throws IOException
System.out.println(child")
   public static void main(Strin
g args)
Parent pnew Child()
p.msg()
class Parent   void msg()
System.out.println("parent")

OutputCompile Time Error
114

If the superclass method does not declare an
exception, subclass overridden method cannot
declare the checked exception but can declare
unchecked exception.

class Parent      void msg()
System.out.println("parent")

class Child extends Parent
void msg()throws ArithmeticException
        System.out.println("child")
       public static void main(String args)
      Parent pnew Child()
p.msg()
Outputchild
115

If the superclass method declares an exception,
subclass overridden method can declare same,
subclass exception or no exception but cannot
declare parent exception.

class Child extends Parent
void msg()throws Exception
System.out.println("child")
      public static void main(String
args)
Parent pnew Child()
try
p.msg()
catch(Exception e)

class Parent      void msg()throws Arithme
ticException
System.out.println("parent")

OutputCompile Time Error
116
Example in case subclass overridden method
declares subclass exception
class Parent void msg()throws
Exception
System.out.println("parent")
Class Child extends Parent void
msg()throws ArithmeticException
System.out.println("child")
public static void main(String args)
Parent pnew Child()
try
p.msg()
catch(Exception e)
Outputchild
117

Covariant Return Type
Specifies that the return type may vary in the
same direction as the subclass.
Since Java5, it is possible to override method by
changing the return type if subclass overrides
any method, but it changes its return type to
subclass type.

class ShapeFactory public Shape newShape()
class CircleFactory extends
ShapeFactory public Circle newShape()

class Shape class Circle extends
Shape
118

super keyword
A reference variable that is used to refer
immediate parent class object.
super keyword cannot be used in static context.
Usage
super is used to refer immediate parent class
instance variable.
super() is used to invoke immediate parent class
constructor,.
super is used to invoke immediate parent class
method.

119
super used to refer immediate parent class
instance variable class Vehicle     int speed7
0         class Bike extends Vehicle     int sp
eed100            void display()      System.ou
t.println(super.speed)// Vehicle speed
System.out.println(speed)// Bike
speed            public static void main(String a
rgs)      Bike bnew Bike()      b.display()

120
super() to invoke parent class
constructor class Vehicle
Vehicle()System.out.println("Vehicle created")
class Bike extends Vehicle Bike()
super()//invokes parent class constructor
System.out.println("Bike created")
public static void main(String args)
Bike bnew Bike()
121
super() to invoke parent class constructor

If used, super() must be the first statement
inside the constructor, hence either super() or
this() can be used inside the constructor.
super() is added in each class constructor
automatically by compiler if there is no super()
or this().

122
super used to invoke parent class method class
Vehicle void show()System.out.println("Vehi
cle Runing") class Bike extends
Vehicle void show()System.out.println("Bike
Runing ") void display()
super.show() show() public static
void main(String args) Bike bnew
Bike() b.display()
123

final Keyword
Used to restrict the user.
final can be
Variable?If you make any variable as final, you
cannot change the value of final variable(It will
be constant).
Method-gtIf you make any methode as final, you
cannot override it.
Class-gtIf you make any class as final, you cannot
extend it.

124

//final method
class Bike
final void run()System.out.println("running")
class Honda extends Bike
void run()System.out.println("running safely w
ith 100kmph")
public static void main(String args)
Honda honda new Honda()
honda.run()
OutputCompile Time Error

//final variable
class Bike9
final int speedlimit90//final variable
void run()
speedlimit400
public static void main(String args)
Bike9 objnew Bike9()
obj.run()
//end of class
OutputCompile Time Error

125
//final class final class Bike      class Honda1
extends Bike     void run()System.out.println("
running safely with 100kmph")          public st
atic void main(String args)     Honda1 honda n
ew Honda()     honda.run()           OutputCo
mpile Time Error
126
final Keyword

final method is inherited but you cannot override
it.
A final variable that is not initialized at the
time of declaration is known as blank final
variable.
We can initialize blank final variable only in
constructor. For example
class Test
final int i//blank final variable
Test()
i80
......
If a method is declared private and final in
superclass then we can have method with the same
signature in subclass also because private is not
inherited.
Constructor cannot be declared as final because
constructor is never inherited.

127

static blank final variable
static final variable that is not initialized at
the time of declaration .
Can be initialized only in static block.
Example
class Test
static final int data//static blank final varia
ble
static data50
public static void main(String args)
System.out.println(Test.data)

128
Polymorphism in Java Polymorphism in java is a
concept by which we can perform a single action
by different ways. Polymorphism is derived from 2
greek words poly and morphs. The word "poly"
means many and "morphs" means forms. So
polymorphism means many forms. There are two
types of polymorphism in java compile time
polymorphism and runtime polymorphism. We can
perform polymorphism in java by method
overloading and method overriding. If you
overload static method in java, it is the example
of compile time polymorphism. Here, we will focus
on runtime polymorphism in java. Runtime
Polymorphism in Java Runtime polymorphism or
Dynamic Method Dispatch is a process in which a
call to an overridden method is resolved at
runtime rather than compile-time. In this
process, an overridden method is called through
the reference variable of a superclass. The
determination of the method to be called is based
on the object being referred to by the reference
variable. Let's first understand the upcasting
before Runtime Polymorphism. Upcasting When
reference variable of Parent class refers to the
object of Child class, it is known as upcasting.
For example
129

class A
class B extends A
A anew B()//upcasting
Real example of Java Runtime Polymorphism

130

class Bank
int getRateOfInterest()return 0
class SBI extends Bank
int getRateOfInterest()return 8
class ICICI extends Bank
int getRateOfInterest()return 7
class AXIS extends Bank
int getRateOfInterest()return 9

class Test3
public static void main(String args)
Bank b1new SBI()
Bank b2new ICICI()
Bank b3new AXIS()
System.out.println("SBI Rate of Interest "b1.get
RateOfInterest())
System.out.println("ICICI Rate of Interest "b2.g
etRateOfInterest())
System.out.println("AXIS Rate of Interest "b3.ge
tRateOfInterest())

131

Java Runtime Polymorphism with data member
Method is overridden not the data members, so
runtime polymorphism can't be achieved by data
members.
class Bike
int speedlimit90
class Honda3 extends Bike
int speedlimit150
public static void main(String args)
Bike objnew Honda3()
System.out.println(obj.speedlimit)//90
Output90

132

Abstract class in Java
A class that is declared with abstract keyword,
is known as abstract class in java. It can have
abstract and non-abstract methods (method with
body).
Abstraction in Java
Abstraction is a process of hiding the
implementation details and showing only
functionality to the user.
Another way, it shows only important things to
the user and hides the internal details for
example sending sms, you just type the text and
send the message. You don't know the internal
processing about the message delivery.
Abstraction lets you focus on what the object
does instead of how it does it.
Ways to achieve Abstraction
Abstract class (0 to 100)
Interface (100)

133

Abstract class in Java
A class that is declared as abstract is known as
abstract class. It needs to be extended and its
method implemented. It cannot be instantiated.
Example abstract class
abstract class A
abstract method
A method that is declared as abstract and does
not have implementation is known as abstract
method.
Example abstract method
abstract void printStatus()//no body and
abstract

134

Example of abstract class that has abstract
method
In this example, Bike the abstract class that
contains only one abstract method run. It
implementation is provided by the Honda class.
abstract class Bike
abstract void run()
class Honda4 extends Bike
void run()System.out.println("running
safely..")
public static void main(String args)
Bike obj new Honda4()
obj.run()
OUTPUT-running safely..

135

Understanding the real scenario of abstract class
In this example, Shape is the abstract class, its
implementation is provided by the Rectangle and
Circle classes. Mostly, we don't know about the
implementation class (i.e. hidden to the end
user) and object of the implementation class is
provided by the factory method.
A factory method is the method that returns the
instance of the class. We will learn about the
factory method later.
In this example, if you create the instance of
Rectangle class, draw() method of Rectangle class
will be invoked.
abstract class Shape
abstract void draw()
class Rectangle extends Shape
void draw()System.out.println("drawing
rectangle")
class Circle1 extends Shape
void draw()System.out.println("drawing
circle")
//In real scenario, method is called by
programmer or user
class TestAbstraction1
public static void main(String args)

136

An abstract class can have data member, abstract
method, method body, constructor and even main()
method.
Rule If there is any abstract method in a class,
that class must be abstract.
If you are extending any abstract class that have
abstract method, you must either provide the
implementation of the method or make this class
abstract.

137

Interface in Java
An interface in java is a blueprint of a class.
It has static constants and abstract methods
only.
The interface in java is a mechanism to achieve
fully abstraction. There can be only abstract
methods in the java interface not method body. It
is used to achieve fully abstraction and multiple
inheritance in Java.
Java Interface also represents IS-A relationship.
It cannot be instantiated just like abstract
class.
The java compiler adds public and abstract
keywords before the interface method and public,
static and final keywords before data members.

138
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139
Understanding relationship between classes
and interfaces
140
Simple example of Java interface In this
example, Printable interface have only one
method, its implementation is provided in the A
class. interface printable void print()
class A6 implements printable public void
print()System.out.println("Hello") public
static void main(String args) A6 obj new
A6() obj.print()
141

Multiple inheritance in Java by interface

142

interface Printable
void print()
interface Showable
void show()
class A7 implements Printable,Showable
public void print()System.out.println("Hello")
public void show()System.out.println("Welcome")
public static void main(String args)
A7 obj new A7()
obj.print()
obj.show()

143

Multiple inheritance is not supported through
class in java but it is possible by interface
interface Printable
void print()
interface Showable
void print()
class TestTnterface1 implements Printable,Showable
public void print()System.out.println("Hello")
public static void main(String args)
TestTnterface1 obj new TestTnterface1()
obj.print()

144

Interface inheritance
A class implements interface but one interface
extends another interface .
interface Printable
void print()
interface Showable extends Printable
void show()
class Testinterface2 implements Showable
public void print()System.out.println("Hello")
public void show()System.out.println("Welcome")
public static void main(String args)
Testinterface2 obj new Testinterface2()
obj.print()
obj.show()

145

Marker or tagged interface?
An interface that have no member is known as
marker or tagged interface. For example
Serializable, Cloneable, Remote etc. They are
used to provide some essential information to the
JVM so that JVM may perform some useful
operation.
//How Serializable interface is written?
public interface Serializable
Nested Interface in Java
Note An interface can have another interface
i.e. known as nested interface. We will learn it
in detail in the nested classes chapter. For
example
interface printable
void print()
interface MessagePrintable
void msg()

146
Abstract class Interface
1) Abstract class can have abstract and non-abstract methods. Interface can have only abstract methods.
2) Abstract class doesn't support multiple inheritance. Interface supports multiple inheritance.
3) Abstract class can have final, non-final, static and non-static variables. Interface has only static and final variables.
) Abstract class can have static methods, main method and constructor. Interface can't have static methods, main method or constructor.
5) Abstract class can provide the implementation of interface. Interface can't provide the implementation of abstract class.
6) The abstract keyword is used to declare abstract class. The interface keyword is used to declare interface.
7) Examplepublic abstract class Shapepublic abstract void draw() Examplepublic interface Drawablevoid draw()
4
147
Core Java
148

Java Package
Group of similar types of classes, interfaces and
sub-packages.
Types
built-in package (such as java, lang, io, util,
sql etc.)
user-defined package.
Advantages
Used to categorize the classes and interfaces so
that they can be easily maintained.
Provides access protection.
Removes naming collision.

149
Naming Conventions
Package names are written in all lower case to
avoid conflict with the names of classes or
interfaces. The prefix of a unique package name
is always written in all-lowercase ASCII letters
and should be one of the top-level domain names,
currently com, edu, gov, mil, net, org, or one of
the English two-letter codes identifying
countries as specified in ISO Standard 3166,
1981. Subsequent components of the package name
vary according to an organization's own internal
naming conventions. Such conventions might
specify that certain directory name components be
division, department, project, machine, or login
names.
150
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151

Ways to access the package from outside the
package
import packageName.
import packageName.classname
fully qualified name.
If you import a package, subpackages will not be
imported.

152
Core Java
153

Access Modifiers
Specifies accessibility (scope) of a data member,
method, constructor or class.
Types
private
package-private (no explicit modifier)
protected
public
There are many non-access modifiers such as
static, abstract, synchronized, native, volatile,
transient etc.

154

private access modifier
Accessible only within class.
Role of Private Constructor
If you make any class constructor private, you
cannot create the instance of that class from
outside the class. For example
class A
private A()//private constructor
void msg()System.out.println("Hello java")
public class Simple
public static void main(String args)
A objnew A()//Compile Time Error
Note A class cannot be private or protected
except nested class.

155
Access Modifier within class within package outside package by subclass only outside package
private Y N N N
package-private (no explicit modifier) Y Y N N
protected Y Y Y N
public Y Y Y Y
Note A class cannot be private or protected
except nested class.
156
The following table shows where the members of
the A class are visible for each of the access
modifiers that can be applied to them.
Visibiity
Modifier A B C D
public Y Y Y Y
protected Y Y Y N
no modifier Y Y N N
private Y N N N
157

Java Array
Collection of similar type of elements.
Contiguous memory location.
Fixed size.
Index based, first element of the array is stored
at 0 index.

158

Types of Array
Single Dimensional Array
Multidimensional Array
Single Dimensional Array in java
Syntax
dataType arr (or)
dataType arr (or)
dataType arr
Instantiation
arrayRefVarnew datatypesize
Declaration, Instantiation and Initialization
int a33,3,4,5//declaration, instantiation an
d initialization

159
Core Java
160
Diamond problem

An ambiguity that can arise as a consequence of
allowing multiple inheritance.
Since Java 8 default methods have been permitted
inside interfaces, which may cause ambiguity in
some cases.

161
Diamond StructureCase1
interface A default void m()
System.out.println("m() from interface A ")
interface B extends A interface C
extends A public class D implements B,C
public static void main(String args)
new D().m()
162
Diamond StructureCase2
interface A default void m()
System.out.println("m() from interface A ")
interface B extends A _at_Override
default void m()
System.out.println("m() from interface B ")
interface C e

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