Java Tutorial

SESSION - I

JAVA BASICS

April 22, 2023

Contents• JAVA Overview

• OOPS Concepts in Java

• Class Fundamentals

• Classes

• Principles of OOP

• Simple Class and Method

• Methods

• Public/private

• Using objects

• Primitive Types and Variables

• Initialisation

• Declarations

• Assignment

• Basic Mathematical Operators

• Statements & Blocks

• Flow of Control

• If – The Conditional Statement

• Relational Operators

• If… else

• Nested if … else

• else if

• The Switch Statement

• The for loop

• while loops

• Continue

• Constructors

April 22, 2023

Java – Overview

• Java is:

– platform independent programming language

– similar to C++ in syntax

• Java has some interesting features:

– automatic garbage collection,

– simplifies pointers; no directly accessible pointer to memory,

– multi-threading!

April 22, 2023

Windows LinuxSolaris

JVM(win) JVM(Sol) JVM(Lin)

Java Prog Java Prog Java Prog

XXX

JVM(XXX)

Java Prog

Platform Dependent JVM and Platform Independent Program

April 22, 2023

How it works…!

Source Code

Create/Modify Source Code

Compile Source Code i.e. javac Welcome.java

Bytecode

Run Byteode i.e. java Welcome

Result

If compilation errors

If runtime errors or incorrect result

•Java Compiler converts Java Source code to intermediate instruction set called ‘byte’ code.

•‘byte’ code is instruction set to a virtual machines called JVM – Java Virtual Machine

•JVM will convert byte instruction to underlying machine instruction during run time i.e. when program is running.

•Thus byte code remains platform independent and JVM is platform specific.

April 22, 2023

Object-Oriented Programming

• Java supports OOD

– Encapsulation

– Polymorphism

– Inheritance

• Java programs contain nothing but definitions and instantiations of

classes

– Everything is encapsulated in a class!

April 22, 2023

Principles of OOP• Encapsulation

– Objects hide their functions (methods) and data (instance variables)

• Inheritance

– Each subclass inherits all variables of its superclass

• Polymorphism

– Interface same despite different data types

car

auto-maticmanual

Super class

Subclasses

draw() draw()

April 22, 2023

Classes

• OOP - object oriented programming code built from objects are

called classes

• Each class definition is coded in a separate .java file

• Name of the object must match the class/object name

April 22, 2023

Simple Class and Method

• A class typically has two parts ‘data’ and ‘action’.

• Data – is represented by variables within the class.

• Action – represented by methods (functions in C). Methods manipulate data.

• Thus class typically encapsulates data and methods.

April 22, 2023

Using objects

• Here, code in one class creates an instance of another class and does

something with it …

Fruit plum=new Fruit();

int cals;

cals = plum.total_calories();

• Dot operator allows you to access (public) data/methods inside Fruit

class

April 22, 2023

Methods

• A method is a named sequence of code that can be invoked by other

Java code.

• A method takes some parameters, performs some computations and

then optionally returns a value (or object).

• Methods can be used as part of an expression statement.

public float convertCelsius(float tempC)

{

return( ((tempC * 9.0f) / 5.0f) + 32.0 );

}

April 22, 2023

Simple class

Class Fruit{

int grams;

int cals_per_gram;

int total_calories() {

return(grams*cals_per_gram);

}

}

April 22, 2023

Primitive Types and Variables

• Boolean, char, byte, short, int, long, float, double etc.

• These basic (or primitive) types are the only types that are not objects.

This means that you don’t use the new operator to create a primitive

variable.

• Declaring primitive variables:

float a;

int b, index = 2;

double c = 1.2;

boolean valueOk = false;

April 22, 2023

Initialisation

• If no value is assigned prior to use, then the compiler will give an

error

• Java sets primitive variables to zero or false in the case of a boolean

variable

• All object references are initially set to null

• An array of anything is an object

– Set to null on declaration

– Elements to zero false or null on creation

April 22, 2023

Declarations

int a = 1.2; // compiler error

boolean b = 1; // compiler error

double c = 5 / 4; // no error!

float d = 5.8f; // correct

double e = 5.0 / 4.0; // correct

• 1.2f is a float value accurate to 7 decimal places.

• 1.2 is a double value accurate to 15 decimal places.

April 22, 2023

Assignment

• All Java assignments are right associative

int a = 1, b = 2, c = 5

a = b = c

System.out.print(“a= “ + a + “b= “ + b + “c= “ + c)

• What is the value of a, b & c

• Done right to left: a = (b = c);

April 22, 2023

Basic Mathematical Operators

• * / % + - are the mathematical operators

• * / % have a higher precedence than + or -

double myVal = a + b % d – c * d / b;

• Is the same as:

double myVal = (a + (b % d)) – ((c * d) / b);

April 22, 2023

Relational Operators

== Equal (careful)

!= Not equal

>= Greater than or equal

<= Less than or equal

> Greater than

< Less than

April 22, 2023

Statements & Blocks

• A simple statement is a command terminated by a semi-colon:

name = “Tina”;

• A block is a compound statement enclosed in curly brackets:

{

name1 = “Tina”; name2 = “Mehta”;

}

• Blocks may contain other blocks

April 22, 2023

Flow of Control

• Java executes one statement after the other in the order they are

written

• Many Java statements are flow control statements:

Alternation: if, if else, switch

Looping: for, while, do while

Escapes: break, continue, return

April 22, 2023

If – The Conditional Statement

• The if statement evaluates an expression and if that evaluation is true

then the specified action is taken

if ( x < 10 ) x = 10;

• If the value of x is less than 10, make x equal to 10

• It could have been written:

if ( x < 10 )

x = 10;

• Or, alternatively:

if ( x < 10 ) { x = 10; }

April 22, 2023

If… else

• The if … else statement evaluates an expression and performs one

action if that evaluation is true or a different action if it is false.

if (x != oldx) {

System.out.print(“x was changed”);

}

else {

System.out.print(“x is unchanged”);

}

April 22, 2023

Nested if … else

if ( myVal > 100 ) {

if ( remainderOn == true) {

myVal = mVal % 100;

}

else {

myVal = myVal / 100.0;

}

}

else

{

System.out.print(“myVal is in range”);

}

April 22, 2023

else if

• Useful for choosing between alternatives:

if ( n == 1 ) {

// execute code block #1

}

else if ( j == 2 ) {

// execute code block #2

}

else {

// if all previous tests have failed, execute code block #3

}

April 22, 2023

The Switch Statement

switch ( n ) {

case 1:

// execute code block #1

break;

case 2:

// execute code block #2

break;

default:

// if all previous tests fail then //execute code block #4

break;

}

April 22, 2023

The for loop

• Loop n times

for ( i = 0; i < n; n++ ) {

// this code body will execute n times

// i from 0 to n-1

}• Nested for:

for ( j = 0; j < 10; j++ ) {

for ( i = 0; i < 20; i++ ){

// this code body will execute 200 times

}

}

April 22, 2023

while loop

int response=10;

while (response < 1) {

System.out.println(response);

response++;

}

April 22, 2023

do {… } while loops

int response=10;

do {

System.out.println(response);

response++;

}while (response < 1);

April 22, 2023

Break

• A break statement causes an exit from the innermost containing

while, do, for or switch statement.

for ( int i = 0; i < maxID, i++ ) {

if ( userID[i] == targetID ) {

index = i;

break;

}

} // program jumps here after break

April 22, 2023

Continue

• Can only be used with while, do or for.

• The continue statement causes the innermost loop to start the next

iteration immediately

for ( int i = 0; i < maxID; i++ ) {

if ( userID[i] != -1 ) continue;

System.out.print( “UserID ” + i + “ :” + userID);

}

April 22, 2023

Constructors• A special method that is invoked while object is being created.• It MUST have same name as class name and does not have return type• It can be parameterized• When no constructor is defined java compiler provides a default

constructor

• invokes a constructor method with which you can set the initial data of

an object

• You may choose several different type of constructor with different

argument lists

April 22, 2023

public class book1

{

String title;

String author;

double price ;

int percentDiscount;

public book1(String tit, String auth,double prc,int perDisc)

{

title = tit;

author = auth;

price = prc;

percentDiscount = perDisc;

}

double getDiscountedPrice()

{

return price - (price*percentDiscount/100);

}

public static void main(String args[])

{

book1 b1 = new book1("Java 2","Herbert",435,14);

book1 b2 = new book1("Ajax Project in Java Technology",

"Smith",300,30);

System.out.println(b1.getDiscountedPrice());

System.out.println(b2.getDiscountedPrice());

}

}

April 22, 2023

Packages• In java application there can not be two classes with same

name.

• When application involves 100s of classes and 10s of third party library being used, it is very difficult to come out with unique meaningful name

• This can be achieved in java using ‘package’s.

• A package is name space within which each class has unique name

• A package can also be used as access control – it is possible to define a class or member of a class which is visible only within a specified package.

April 22, 2023

Packages Syntaxpackage xpublic class a {}

package x.ypublic class a {}

Class a belongs to package x

Class a belongs to package x.y. This is fine as both class belong to different package

package com.bredge.samplepublic class b {}

Class b belongs to package com.bredge.sample

April 22, 2023

Access Specifiers• There are three access Specifiers namely ‘public’, ‘private’ and

protected and four access levels(including default).

• Members of a class can be public, private or protected.

• That basically means member variables and methods can either be public, private or protected.

• Public members are accessible within and outside class.

• Private members are accessible only within class.

• The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.

• When neither public nor private or protected mentioned, default access will be considered.

• Class can be public or default only.

April 22, 2023

Access Specifiers

• Default access specifier, i.e. when there is no public, private or protected is mentioned, has package level visibility.

• Default class members are visible to all other classes which belong to same package

private protected public defaultSame class Y Y Y YSame package sub-class N Y Y YSame package non-subclass N Y Y YDifferent Package subclass N Y Y NDifferent Package non-subclass N N Y N

Thank You