Intro to JAVA

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Intro to JAVA

• By
• Riyaz Malbari

2
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.

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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.

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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.

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JAVA Virtual Machine
6
Object-Oriented Programming

• The core of JAVA
• Encapsulation
• Inheritance
• Polymorphism

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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 typegtjavac hello.java
• execute program by typinggtjava hello

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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

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Variables

• basic unit of storage
• a combination of an identifier, a type and an
  optional initializer
• Syntax type identifier value,
  identifiervalue,
• eg. int a,b,c
• int d4, e, f7
• byte w12
• double pi3.14159
• char xx
• Dynamic initialization
• class DyIn
• public static void main (String args
  )
• double a3.0, b4.0
  
• double cMath.sqrt
  (aabb) // c is dynamically initialized
• System.out.println (Hypotenuse is
  c)

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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
• x10
• if (x 10)
• int y20 //
  known only to this block
• 
  System.out.println (x and y x y )
• xy2
• 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 )

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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_namenew typesize
• eg. month new int12
• both can be done together int month
  new int12
• assignment month0 31
• 
  month1 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
  int23

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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

Methods
Instance variables
Class Members
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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 myboxs 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)

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• 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

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• 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

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• 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()

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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

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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)

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Inheritance
20
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()

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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 )
  

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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.

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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)

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JAVA Security Mechanisms

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Thank you!