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Intro to JAVA. By: Riyaz Malbari. History of JAVA. Came into existence at Sun Microsystems, Inc. in 1991. Was initially called “ Oak ” but was renamed JAVA. Originally it was not developed for the Internet but as a software to be embedded in various consumer electronic devices.

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intro to java

Intro to JAVA

By:

Riyaz Malbari

04/07/04

history of java
History of JAVA
  • Came into existence at Sun Microsystems, Inc. in 1991.
  • Was initially called “Oak” but was renamed JAVA.
  • Originally it was not developed for the Internet but as a software to be embedded in various consumer electronic devices.
  • Derives much of its character from C/C++.

04/07/04

types of programs
Applications

a program that runs on the computer, under its operating system.

it is like a normal program created using C/C++.

Applets

a tiny JAVA program that can be transmitted over the Internet.

it is an intelligent program.

Types of Programs

04/07/04

bytecode
Bytecode
  • output of a JAVA compiler is Bytecode.
  • executed by the JAVA run-time system, which is called the JAVA Virtual Machine (JVM).
  • creates truly portable programs.
  • assures security while downloading programs over the Internet.

04/07/04

object oriented programming
Object-Oriented Programming

The core of JAVA

  • Encapsulation
  • Inheritance
  • Polymorphism

04/07/04

a simple program
A simple program
  • /* A program to display the message Hello World.

Call this file “hello.java”. */

class hello {

// The program begins with a call to main().

public static void main (String args [ ]) {

System.out.println (“Hello World.”) ;

}

}

  • save as text file hello.java
  • at command line type>javac hello.java
  • execute program by typing>java hello

04/07/04

data types
Data Types

Type Size Range

byte 8 bits -128 to 127

short 16 bits -32768 to 32767

int 32 bits -2147483648 to 2147483647

long 64 bits -9223372036854775808 to 9223372036854775807

float 32 bits 1.7e-308 to 1.7e308

double 64 bits 3.4e-038 to 3.4e038

char 16 bits 0 to 65536

boolean --------- true or false

04/07/04

variables
Variables
  • basic unit of storage
  • a combination of an identifier, a type and an optional initializer
  • Syntax: type identifier [=value][, identifier[=value],…];
  • eg. int a,b,c;

int d=4, e, f=7;

byte w=12;

double pi=3.14159;

char x=‘x’;

  • Dynamic initialization:

class DyIn {

public static void main (String args[ ]) {

double a=3.0, b=4.0 ;

double c=Math.sqrt (a*a+b*b) ; // c is dynamically initialized

System.out.println (“Hypotenuse is “ + c);

}

}

04/07/04

variable scope
Variable scope
  • scope is defined by a block { }
  • variables that are defined within the scope are not visible to the code that is defined outside the scope.
  • scopes can be nested
  • objects declared in the outer scope will be visible to code within the inner scope
  • eg. class Scope {

public static void main (String args[ ]) {

int x ; // known to all code within main

x=10;

if (x= =10) {

int y=20; // known only to this block

System.out.println (“x and y:” + x + ““ + y +) ;

x=y*2;

}

y = 100; // y is unknown here and so there is a compile-time error

}

}

  • variables declared in the inner and outer scope cannot have the same name
  • eg. class scopeerr {

public static void main (String args[ ]) {

int a=10;

{

int a=20; // Compile-time error as variable ‘a’ is already defined

}

}

}

04/07/04

arrays
Arrays
  • collection of variables of the same data type addressed by a common name
  • an array element is accessed by its index
  • can have one or more dimensions
  • one-dimensional array:

declaration: type var_name[ ];

eg. int month[];

allocation: var_name=new type[size];

eg. month = new int[12];

both can be done together: int month[ ] = new int[12];

assignment: month[0] = 31;

month[1] = 28;

  • values can also be assigned during declaration:

int month[ ] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

  • Multi-dimensional arrays: int twoD[ ][ ] = new int[2][3];

04/07/04

class
Class
  • fundamental concept in JAVA
  • defines a data type which can be used to create objects of that type
  • class is a template for an object and object is an instance of a class
  • declared by the class keyword
  • Syntax: class classname

type var;

type methodname (parameter list) {

// body of method

}

Instance

variables

Methods

Class Members

04/07/04

example
Example…
  • class Box {

double width;

double height; // This class declares an object of type Box.

double depth;

}

class BoxDemo {

public static void main (String args[ ]) {

Box mybox = new Box(); // An object called mybox is created

double vol;

// Assign values to mybox’s instance variables.

mybox .width = 10;

mybox .height = 20;

mybox .depth = 15;

vol = mybox .width * mybox .height * mybox .depth;

System.out.println (“The volume is “ + vol) ;

}

}

  • Save the file as BoxDemo.java
  • Compilation creates two .class files, one for Box and the other for BoxDemo
  • Execute the BoxDemo.class file

04/07/04

slide14
changing the instance variables of one object have no effect on that of another
  • class Box {

double width;

double height;

double depth;

}

class BoxDemo {

public static void main (String args[ ]) {

Box mybox1 = new Box();

Box mybox2 = new Box();

double vol;

mybox1 .width = 10;

mybox1 .height = 20;

mybox 1.depth = 15;

mybox2 .width = 3;

mybox2 .height = 6;

mybox2 .depth = 9;

vol = mybox1 .width * mybox1 .height * mybox1 .depth;

System.out.println (“The volume is “ + vol) ;

vol = mybox2 .width * mybox2 .height * mybox2 .depth;

System.out.println (“The volume is “ + vol) ;

}

}

04/07/04

slide15
adding a method to the box class…
  • class Box {

double width;

double height;

double depth;

void volume() {

System.out.println (“Volume is ” + (width * height * depth)) ;

}

}

class BoxDemo {

public static void main (String args[ ]) {

Box mybox = new Box();

double vol;

mybox .width = 10;

mybox .height = 20;

mybox .depth = 15;

mybox .volume();

}

}

04/07/04

slide16
Returning a value…
  • class Box {

double width;

double height;

double depth;

double volume() {

return width * height * depth ;

}

}

class BoxDemo {

public static void main (String args[ ]) {

Box mybox = new Box();

double vol;

mybox .width = 10;

mybox .height = 20;

mybox .depth = 15;

vol = mybox .volume();

System.out.println (“Volume is “ + vol) ;

}

}

04/07/04

constructors
Constructors
  • initializes the object immediately upon creation
  • has the same name as that of the class
  • class Box {

double width;

double height;

double depth;

Box() { // A constructor for the Box class

width = 10;

height = 20;

depth = 15;

}

double volume() {

return width * height * depth ;

}

}

class BoxDemo {

public static void main (String args[ ]) {

Box mybox = new Box();

double vol;

vol = mybox .volume();

System.out.println (“Volume is “ + vol) ;

}

}

04/07/04

method overloading
Method Overloading
  • implementation of polymorphism
  • two or more methods within the same class have the same name but different parameter declarations
  • overloaded methods must differ in the type and/or number of their parameters
  • class OverloadDemo {

void test() {

System.out.println (“No parameters”) ;

}

void test(int a) { // Overload test for one integer parameter

System.out.println (“a: ” + a) ;

}

void test(int a, int b) { // Overload test for two integer parameters

System.out.println (“a and b: ” + a + ““ + b) ;

}

}

class Overload {

public static void main (String args[ ]) {

OverloadDemo ob = new OverloadDemo() ;

ob.test();

ob.test(10);

ob.test(10, 20);

}

}

04/07/04

inheritance
Inheritance
  • defines a general class that defines traits common to a set of related items
  • other specific classes add things that are unique to it
  • a class that is inherited is called a superclass while the one that does the inheriting is called the subclass
  • the extends19keyword is used to inherit a class
  • multilevel hierarchies can be created

04/07/04

slide20

class A { // Creating a superclass A

int i, j;

void showij() {

System.out.println (“i and j: “ + i +” “ + j) ;

}

}

class B extends A { // Creating a subclass B by extending class A

int k;

void showk() {

System.out.println (“k: “ + k) ;

}

void sum() {

System.out.println (“i + j + k: “ + (i +j + k)) ;

}

}

class Inheritance {

public static void main (String args[]) {

A superOb = new A();

B subob = new B();

superOb.i = 10;

superOb.j = 20;

superOb.showij();

subOb.i = 3;

subOb.j = 6;

subOb.k = 9;

subOb.showij();

subOb.showk();

subOb.sum();

}

}

04/07/04

packages
Packages
  • partitions the class name space
  • classes defined in a package are not accessible by code outside that package
  • syntax: package package_name;
  • the import statement is used to bring certain packages or the entire package into visibility
  • package Demo;

class Balance {

String name;

double bal;

Balance(String n, double b) {

name = n;

bal = b;

}

void show() {

System.out.println(name + “: $” + bal) ;

}

}

class AccBal {

public static void main(String args[ ]) {

Balance current[] = new Balance[3] ;

current[0] = new Balance(“Bush”, 0.99);

current[1] = new Balance(“Donald”, 9.99);

current[2] = new Balance(“Rice”, 19.99);

for (int i=0; i<3; i++) current[i].show() ;

}

}

04/07/04

access specifiers
Access Specifiers
  • public No restrictions. Accessible from anywhere.
  • private Only accessible from within the class.
  • package Accessible from package members.
  • protected Also accessible from subclasses and package members.

04/07/04

java for security
JAVA for Security

Basic concepts:

  • Authentication
  • Authorization
  • Confidentiality
  • Integrity

JAVA tools:

  • JAAS (JAVA Authentication And Authorization Service)
  • JSSE (JAVA Secure Socket Extensions)
  • GSS-API (JAVA Generic Security Service Application Programming Interface)

04/07/04

thank you
Thank you!

04/07/04