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Intro to JAVA

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

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  1. Intro to JAVA By: Riyaz Malbari 04/07/04

  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++. 04/07/04

  3. 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

  4. 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

  5. JAVA Virtual Machine 04/07/04

  6. Object-Oriented Programming The core of JAVA • Encapsulation • Inheritance • Polymorphism 04/07/04

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  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(); } } 04/07/04

  21. 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

  22. 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

  23. 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

  24. JAVA Security Mechanisms 04/07/04

  25. Thank you! 04/07/04

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