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Core Java

By – Asst. Prof. Shardul Gavande

What is Java?

Java is a purely object oriented, platform

independent, open source programming

language.

Developed by James Gosling at Sun

Microsystems in the year 1991 earlier

known as “Oak”

In the year 1995 it was officially renamed

to “Java”

Features of Java

Compiled and Interpreted

Platform independent and Portable

Object Oriented

Robust and Secure

Distributed

Familiar, Simple and Small

Multithreaded and Interactive

High Performance

Dynamic and Extensible

How Java Works?

Java Virtual Machine (JVM)

Code Structure in Java

Anatomy of Main Class

Classes in Java

A class is a blueprint for an object. It tells

the virtual machine how to make an object

of that particular type. For example, you

might use the Button class to make dozens

of different buttons, and each button might

have its own colour, size, shape, label and so

on.

Classes define the structure of an object.

It defines two things of an object :

1) What the object knows (Variables)

2) What is object does (Methods)

Objects in Java

How to create Objects ?

Steps to create Objects

How and Where objects are

created?

Objects are created based on it’s classes.

JVM created objects during the execution

of our java program.

Objects of classes are created in “Heap

Memory”

Heap Memory is a runtime memory which

is allocated during the execution of our java

program.

Java Data types

Data types specify the different sizes and

values that can be stored in the variable.

There are two types of data types in Java:

Primitive data types: The primitive data

types include boolean, char, byte, short, int,

long, float and double.

Non-primitive data types: The non-

primitive data types include Classes,

Interfaces, and Arrays.

Type Casting in java

Assigning a value of one type to a variable

of another type is known as Type Casting.

Automatic Type casting take place when,

1)the two types are compatible

2) the target type is larger than the source

type

Type Casting in java

When you are assigning a larger type value to a

variable of smaller type, then you need to perform

explicit type casting.

Arrays in Java

Java array is an object which contains

elements of a similar data type. It is a data

structure where we store similar elements.

We can store only a fixed set of elements

in a Java array.

Array in java is index-based, the first

element of the array is stored at the 0

index.

Variables in java

A variable is a container which holds the

value while the java program is executed. A

variable is assigned with a data type.

int data=50; //Here data is variable

Types of Variable

Local Variable

A variable declared inside the body of the

method is called local variable. You can use

this variable only within that method and

the other methods in the class aren't even

aware that the variable exists

Eg :

public void show()

{

int a;

}

Instance Variable A variable declared inside the body of the method is

called local variable. You can use this variable only within

that method and the other methods in the class aren't

even aware that the variable exists

public class Vehicle

{

◦ int speed; // instance variable

◦ int color; // instance variable

◦ public void drive()

◦ {

◦ }

}

Static Variable

When a variable is declared as static, then a single

copy of variable is created and shared among all

objects at class level. Static variables are,

essentially, global variables. All instances of the

class share the same static variable.

Example of static variable

To access static variable

Rules for declaring variables in java

A variable name can consist of Capital letters A-Z,

lowercase letters a-z, digits 0-9 and two special

characters such as underscore and dollar sign.

The first character must be a letter.

Blank spaces cannot be used in variable names.

Java keywords cannot be used as variable names.

Variable names are case-sensitive.

Java Tokens

A token is the smallest element of a

program that is meaningful to the

compiler. They are :

Reserved Keywords

Identifiers

Literals

Operators

Separators

Keywords

Identifiers

In programming languages, identifiers are used

for identification purpose. In Java, an

identifier can be a class name, method name,

variable name or a label.

public class Test

{

public static void main(String[] args)

{

int a = 20;

}

}

Literals

Any constant value which can be assigned

to the variable is called as literal/constant.

Eg: int x = 100; // Here x is a literal

There are :

Integer Literal

Floating point literal

Character literals

String literals

Boolean literals

Operators Operator in java is a symbol that is used to perform

operations. For example: +, -, *, / etc.

Types of operators :

1) Arithmetic

2) Relational

3) Logical

4) Assignment

5) Increment and Decrement

6) Conditional

7) Bitwise

8) Special

Arithmetic Operator

They are used to perform simple arithmetic

operations on primitive data types.

* : Multiplication

/ : Division

% : Modulo

+ : Addition

– : Subtraction

Relational Operator

These operators are used to check for

relations like equality, greater than, less than.

They return boolean result after the

comparison and are extensively used in

looping statements as well as conditional if

else statements.

Relational Operator ==, Equal to : returns true of left hand side is equal to

right hand side.

!=, Not Equal to : returns true of left hand side is not

equal to right hand side.

<, less than : returns true of left hand side is less than

right hand side.

<=, less than or equal to : returns true of left hand side

is less than or equal to right hand side.

>, Greater than : returns true of left hand side is greater

than right hand side.

>=, Greater than or equal to: returns true of left hand

side is greater than or equal to right hand side.

Logical Operator

These operators are used to perform “logical AND” and

“logical OR” operation, i.e. the function similar to AND

gate and OR gate in digital electronics.

&&, Logical AND : returns true when both conditions

are true.

||, Logical OR : returns true if at least one condition is

true.

Assignment Operator

Assignment operator is used to assign a value to any

variable. It has a right to left associativity, i.e value given on

right hand side of operator is assigned to the variable on

the left and therefore right hand side value must be

declared before using it or should be a constant.

Eg: variable = value;

Assignment Operator

+=, for adding left operand with right operand and then

assigning it to variable on the left.

-=, for subtracting left operand with right operand and

then assigning it to variable on the left.

*=, for multiplying left operand with right operand and

then assigning it to variable on the left.

/=, for dividing left operand with right operand and then

assigning it to variable on the left.

%=, for assigning modulo of left operand with right

operand and then assigning it to variable on the left.

Increment && Decrement Operator

Increment Operator : It is used to increment a value by 1.

There are two varieties of increment operator:

Post-Increment (i++) : Value is first used for computing

the result and then incremented.

Pre-Increment (++i) :Value is incremented first and

then result is computed.

Decrement Operator : It is used for decrementing the

value by 1. There are two varieties of decrement operator.

Post-decrement (i--) :Value is first used for computing

the result and then decremented.

Pre-decrement (--i) :Value is decremented first and

then result is computed.

Increment && Decrement Operator

public class Test {

public static void main(String[] args)

{

int a = 10;

int b = ++a;

System.out.println(b);

}

}

Conditional Operator

The character pair ? : is a ternary operator

available in Java. This operator is used to

construct conditional expression of the

form

exp1 ? exp2 : exp3

Example :

a= 10; b= 15;

x= (a >b) ? a : b

Bitwise Operator

These operators are used to perform manipulation of individual bits of a number. They can be used with any of the integer types.

&, Bitwise AND operator: returns bit by bit AND of input values.

|, Bitwise OR operator: returns bit by bit OR of input values.

^, Bitwise XOR operator: returns bit by bit XOR of input values.

~, Bitwise Complement Operator: This is a unary operator which returns the one’s compliment representation of the input value, i.e. with all bits inversed.

Special Operator

Java supports some special operators such as

instance of operator and member selection

operator (.)

The instance of is an object reference operator

and returns true if the object on the left hand

side is an instance of the class given on the right

hand side. The operator allows us to determine

whether the object belongs to a particular class

or not.

Eg : employee.age //reference to variable age

Employee.salary() //reference to method salary

Separators

If statement in java

The Java if statement tests the condition. It

executes the if block if condition is true.

Syntax :

if(condition){

//code to be executed

}

If statement in java

public class IfExample {

public static void main(String[] args) {

//defining an 'age' variable

int age=20;

//checking the age

if(age>18){

System.out.print("Age is greater than 18);

}

}

}

If- else statement in java

The Java if-else statement also tests the

condition. It executes the if block if

condition is true otherwise else block is

executed.

if(condition){

//code if condition is true

}else{

//code if condition is false

}

If- else statement in java

If- else-if ladder in java The if-else-if ladder statement executes one

condition from multiple statements.

if(condition1){

//code to be executed if condition1 is true

}else if(condition2){


}

else if(condition3){


}

...

else{

//code to be executed if all the conditions are false

}

If- else-if ladder in java

Nested if in java

The nested if statement represents the if

block within another if block. Here, the inner

if block condition executes only when

outer if block condition is true.

if(condition){


if(condition){


}

}

Nested if in java

While loop in java

The Java while loop is used to iterate a part

of the program several times. If the number

of iteration is not fixed, it is recommended

to use while loop.

while(condition){


}

While loop in java

Do While loop in java

The Java do-while loop is used to iterate a part of

the program several times. If the number of

iteration is not fixed and you must have to

execute the loop at least once, it is recommended

to use do-while loop.

do{


}while(condition);

Do While loop in java

For loop in java

The Java for loop is used to iterate a part of

the program several times. If the number of

iteration is fixed, it is recommended to use

for loop.

for(initialization;condition;incr/decr){

//statement or code to be executed

}

For loop in java

Switch statement in java

The Java switch statement executes one statement from multiple conditions.

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 matched;

}

Switch statement in java

break statement in java

When a break statement is encountered

inside a loop, the loop is immediately

terminated and the program control

resumes at the next statement following

the loop.

jump-statement;

break;

break statement in java

continue statement in java

The continue statement is used in loop

control structure when you need to jump

to the next iteration of the loop

immediately. It can be used with for loop

or while loop.

jump-statement;

continue;

continue statement in java

Constructor in Java

Constructors are used to allocate memory

to the object in heap memory.

It is a member method

It has same name as class name

It does not return anything

It gets automatically called

Types of Constructor

Default Constructor

No- argument Constructor

Parameterized Constructor

Default Constructor

No- argument Constructor


A constructor that has parameters is

known as parameterized constructor. If we

want to initialize fields of the class with

your own values, then use a parameterized

constructor.

Inheritance in java

Inheritance in Java is a mechanism in which

one object acquires all the properties and

behaviors of a parent object. It is an important

part of OOPs

class Subclass-name extends Superclass-name

{

//methods and fields

}

The 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.

Inheritance in java

Types of inheritance in java

Super keyword in java

The super keyword in Java is a reference

variable which is used to refer immediate

parent class object.

Whenever you create the instance of

subclass, an instance of parent class is

created implicitly which is referred by

super reference variable.

Usage Super keyword in java

It is used to call immediate parent class

instance variables


methods


constructor

Super keyword in java

“this” keyword in java

String Manipulation in Java

Strings, which are widely used in Java

programming, are a sequence of characters.

Syntax : String stringname;

String greeting = "Hello world!";

String Array :

Syntax : String arrayname[] = new String[3];

char charArray[] = new char[4]

charArray[0]=’J’

charArray[1]=’A’

charArray[2]=’V’

charArray[3]=’A’

String Methods

s2= s1.toLowerCase – Converts the string s1 to

all lowercase

s2=s1.toUpperCase – Converts the string s1 to

all uppercase

s2=s1.replace(‘x’, ‘y’) – Replaces all appearances

of x with y

s2=s1.trim() – Remove white spaces at the

beginning and end of the string s1

s1.equals(s2) – Returns true if s1 is equal to s2

s1.length()- Gives the length of s1

s1.charAt(n) – Gives the nth character of

s1

s1.compareTo(s2) – Returns negative if s1

< s2, positive if s1>s2 and zero if s1 is

equal to s2

s1.concat(s2) - Concatenates s1 and s2

s1.substring(n)- Gives substring starting

from nth character

s1.substring(n,m)- Gives substring

starting from nth character up to mth

String.valueOf(variable) – Converts the

parameter value to string representation

S1.equalsIgnoreCase(s2) – Returns true if s1 =

s2,ignoring the rest of the unequal cases

S1.toString() – Create a string representation of

the object s1

S1.indexOf(x) – Gives the position of the first

occurrence of ‘x’ in the string s1

S1.indexOf(‘x’,n) – Gives the position of x that

occurs after nth position in the string s1

String Tokenizers

The java.util.StringTokenizer class

allows an application to break a string

into tokens.

import java.util.StringTokenizer;

public class Main

{

public static void main(String[] args) {

StringTokenizer st1 = new StringTokenizer("Hi! I am good. How about you?");

for (int i = 1; st1.hasMoreTokens(); i++)

{

System.out.println("Token "+i+":“ +st1.nextToken());

}

}

}

String Buffer

Java StringBuffer class is used to create

mutable (modifiable) string. The

StringBuffer class in java is same as String

class except it is mutable i.e. it can be

changed.

We can use StringBuffer to append,

reverse, replace, concatenate and

manipulate Strings or sequence of

characters.

StringBuffer append() method

class StringBufferExample{

public static void main(String args[]){

StringBuffer sb=new StringBuffer("Hello”);

sb.append("Java");//now original string is change

d

System.out.println(sb);//prints Hello Java

}

}

StringBuffer insert() method

class StringBufferExample2{


StringBuffer sb=new StringBuffer("Hello ");

sb.insert(1,"Java");//now original string is chang

ed

System.out.println(sb);//prints HJavaello

}

}

StringBuffer replace() method



StringBuffer sb=new StringBuffer("Hello");

sb.replace(1,2,"Java");

System.out.println(sb);//prints HJallo

}

}

StringBuffer delete() method




sb.delete(1,3);

System.out.println(sb);//prints Hlo

}

}

StringBuffer reverse() method




sb.reverse();

System.out.println(sb);//prints olleH

}

}

StringBuffer capacity() method

The capacity() method of StringBuffer class

returns the current capacity of the buffer. The

default capacity of the buffer is 16. If the

number of character increases from its current

capacity, it increases the capacity by

(oldcapacity*2)+2. For example if your current

capacity is 16, it will be (16*2)+2=34.



StringBuffer sb=new StringBuffer();

System.out.println(sb.capacity());//default 16

sb.append("Hello");

System.out.println(sb.capacity());//

sb.append("java is my favourite language");

System.out.println(sb.capacity());//now (16*2)+

2=34 i.e (oldcapacity*2)+2

}

}

Polymorphism

Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.

Types of polymorphism

1) Static Polymorphism

2) Dynamic Polymorphism

Method Overloading (Static Poly.)

A function with same name same class but different

arguments. Also known as compile time polymorphism

Method Overriding (Dynamic Poly.)

A function with same name same parameters but

different class.

Abstraction in Java

A class which is declared with the abstract

keyword is known as an abstract class in

Java.

It can have abstract and non-abstract

methods.

An abstract class cannot have objects.

Non - Abstract Methods

A method which is defined is a non-

abstract method.

Example:

public void call()

{

System.out.println(“Calling”);

}

Abstract Methods

A method which is declared is an abstract method.

Example:

abstract class Mobile

{

public void call()

{System.out.println(“Calling”);

}

abstract void dance(); // abstract method

}

Packages

Package in java is a mechanism to encapsulate a group of

classes, sub packages and interfaces.

Built-in Packages These packages consist of a large number of classes

which are a part of Java API. Some of the commonly

used built-in packages are:

1) java.lang: Contains language support classes(e.g

classed which defines primitive data types, math

operations). This package is automatically imported.

2) java.io: Contains classed for supporting input /

output operations.

3) java.util: Contains utility classes which implement

data structures like Linked List, Dictionary and support

; for Date / Time operations.

4) java.applet: Contains classes for

creating Applets.

5) java.awt: Contain classes for

implementing the components for

graphical user interfaces (like button ,

;menus etc).

6) java.net: Contain classes for

supporting networking operations.

User Defined Packages

Encapsulation

Encapsulation in Java is a mechanism of wrapping

the data (variables) and code acting on the data

(methods) together as a single unit. In

encapsulation, the variables of a class will be

hidden from other classes, and can be accessed

only through the methods of their current class.

Therefore, it is also known as data hiding.

Public member

If we declare a method as public, then we

can access that member from anywhere

but corresponding class should be public.

How to access public class from

another class?

First we need to import the package.

In our case it was

javaapplication1.newpackage -> Only

then we can extend NewClass.java file and

use the variable abc defined as public in

JavaApplication1.java file.

Default member

If a member is declared as default, then we

can access that member only within

current package & we can’t access from

outside of package.

Default access also known as package level

access.

Here the default member is rollno

accessible only in same package subclass

and non-subclass.

Protected member

If a member is declared as protected, then we can access

that member within current package anywhere, but

outside the package only in child class and not in main

class.

Within current package, we can access protected

members either by parent reference or by child

reference.

Outside package, we can access protected members only

by giving child reference.

Here we have protected member shown in main class as

int caseno which is accessible from different subclasses

within the package as well as outside the package

Private member

If a member is declared as private, then we can

access that member only within the current class.

Interfaces

Since multiple inheritance is not possible in java we have the concept of interfaces.

A Java class can implement multiple Java Interfaces.

Syntax:

interface <interface-name>

{

//methods

} All methods in an interface are implicitly public and abstract.

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