Introduction tojavaandxml lc-slides01-fp2005-ver 1.0

Introduction to Java and XML
Day 1

ER/CORP/CRS/LA10/003
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Copyright © 2005, Infosys
Technologies Ltd
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Course Objective
To introduce Java Architecture & appreciate basic syntax in Java Language
To apply Object Oriented Concepts using Java
To illustrate how to make use of standard Java Class Library and create
reusable classes.
To introduce Exception Handling in Java
To learn what is required to package and deploy a Java application
To understand XML and XML parsing

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Session Plan
Day 1
– Review of Object Oriented Concepts
– Java architecture
– The basic constructs in Java

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Class and Object
What is a Class?
– A class is a blueprint or prototype that defines the variables and the
methods (functions) common to all objects of a certain kind.
What is an Object?
– An object is a representative or specimen of a class. Software objects are
often used to model real-world objects you find in everyday life.

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Features of OOP
Abstraction:
– The process of extracting the essential information and hiding the irrelevant details
Encapsulation:
– The process of binding code and data together in the form of a capsule
Inheritance:
– The feature by which one class acquires the properties and functionalities of another
class
Polymorphism:
– The feature that allows the same interface to be used for a general set of actions

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Introduction to Java
A language developed by Sun Microsystems
A general-purpose language
High-level language
Developed initially for consumer devices
Popular platform to develop enterprise applications
– Finds use in internet based applications

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Features of Java
Object-oriented
Simpler language
– Compared to earlier OO languages like C++, it is simple
– Designed considering the pitfalls of earlier languages
Robust
Architecture Neutral / Portable
– Example: Java code compiled on Windows can be run on Unix without recompilation
Secure
– Built -in security features like absence of pointers and confinement of the java program within its
runtime environment
Support for Multithreading at language level
Designed to handle Distributed applications

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Features of Java- (Contd…)
Please find more explanation of the previous slide in the notes page
(This slide is intentionally left blank)

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Platform independence
Java is a language that is platform independent.
A platform is the hardware and software environment in which a program
runs
Once compiled, code will run on any platform without recompiling or any
kind of modification.
– “Write Once Run Anywhere”
This is made possible by making use of a Java Virtual Machine commonly
known as JVM

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Java Virtual Machine (JVM) (1 of 2)
The source code of Java will be stored in a text file with extension .java
The Java compiler compiles a .java file into byte code
The byte code will be in a file with extension .class
The .class file that is generated is the machine code of this processor.
– Byte code is a binary language
The byte code is interpreted by the JVM
JVM can be considered as a processor purely implemented with software.
The interface that the JVM has to the .class file remains the same irrespective of the
underlying platform.
– This makes platform independence possible

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Java Virtual Machine (JVM) (2 of 2)
The JVM interprets the .class file to the machine language of the underlying
platform.
The underlying platform processes the commands given by the JVM

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Source File (HelloWorld.java)
Java Architecture:
Compiler (javac)
Machine Code or Byte code
(HelloWorld.class)
JVM
Class Loader
Byte Code Verifier
Interpreter
JIT Code
Generator
Runtime
Operating System
Hardware

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Java Architecture- (Contd…)
(This slide is intentionally left blank)

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Installing and using Java
Before we begin, something on installation
– Java 2 SDK (v1.4 or higher)
– Can be downloaded freely from http://java.sun.com
– Also available in the intranet
Setting Environment variables for Java
Environment Variable:
– A variable that describes the operating environment of the process
– Common environment variables describe the home directory, command search path
etc.

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Environment variables used by JVM
JAVA_HOME: Java Installation directory
– This environment variable is used to derive all other env. variables used by JVM
– In Windows: set JAVA_HOME=C:jdk1.4.3
– In UNIX: export JAVA_HOME=/var/usr/java
CLASSPATH
– In Windows: set PATH=%PATH%;%JAVA_HOME%libtools.jar;.
– In UNIX: set PATH=$PATH:$JAVA_HOME/lib/tools.jar:.
PATH
– Path variable is used by OS to locate executable files
– In Windows: set PATH=%PATH%;%JAVA_HOME%bin
– In UNIX: set PATH=$PATH:$JAVA_HOME/bin
This approach helps in managing multiple versions of Java – Changing JAVA_HOME will
reflect on CLASSPATH and PATH as well
Set these environment variables on the command prompt and type ‘javac’
– Displays all the options of using ‘javac’

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Source File Layout - Hello World
We will have the source code first
Type this into any text editor
public class HelloWorldApp {
public static void main(String[]args){
System.out.println(“Hello World!”);
}
}
Save this as HelloWorldApp.java
Important :
– Take care!! cAsE of file name matters

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To Compile
Open a command prompt
Set Environment variables (explained earlier)
Go to the directory in which you have saved your program.
Type javac HelloWorldApp.java
– If it says bad command or file name then check the path setting
– If it does not say anything, and you get the prompt, then the compilation was
successful.

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To execute
Type in the command prompt
java HelloWorldApp
The result

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Best Practices
One .java file must contain only one class declaration
The name of the file must always be same as the name of the class
Stand alone Java program must have a public static void main defined
– it is the starting point of the program.
– Not all classes require public static void main
Code should be adequately commented
Must follow indentation and coding standards

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Primitive Data Types in Java
Integer data types
byte (1 byte)
short (2 bytes)
int (4 bytes)
long (8 bytes)
Floating Type
float (4 bytes)
double (8 bytes)
Textual
char (2 bytes)
Logical
boolean (1 byte) (true/false)
Notes:
All numeric data types are signed
The size of data types remain the
same on all platforms (standardized)
char data type in Java is 2 bytes
because it uses UNICODE character
set. By virtue of it, Java supports
internationalization
UNICODE is a character set which
covers all known scripts and
language in the world

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Reference Types in Java (1 of 3)
Objects, Arrays are accessed using reference variables in Java
A reference variable is similar to a pointer (stores memory address of an
object)
Java does not support the explicit use of addresses like other languages
Java does not allow pointer manipulation or pointer arithmetic

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Reference Types in Java (2 of 3)
A reference type cannot be cast to primitive type
A reference type can be assigned ‘null’ to show that it is not referring to any
object
– ‘null’ is a keyword in Java

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Comment entry in Java
A single line comment in Java will start with //
//////// This is a single line comment in Java
A multi line comment starts with a /* and ends with a */
/* This is a multi line
comment
in Java */

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Variables in Java
Using primitive data types is similar to other languages
int count;
int max=100;
In Java variables can be declared anywhere in the program
for (int count=0; count < max; count++) {
int z = count * 10;
}
In Java, if a local variable is used without initializing it, the compiler will show an error
BEST PRACTICE: Declare a variable in program only when
required. Do not declare variables upfront like in C.

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Typecasting of primitive data types –(Contd…)

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Operators and Assignments –(Contd…)

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Operators and Assignments –(Contd…)

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Constituents of a Class
The main method may or may not be present depending on whether the class is a starter class
public class Student {public class Student {public class Student {public class Student {
private intprivate intprivate intprivate int rollNorollNorollNorollNo;;;;
private String name;private String name;private String name;private String name;
Student(){Student(){Student(){Student(){
//initialize data members//initialize data members//initialize data members//initialize data members
}}}}
Student(StringStudent(StringStudent(StringStudent(String nameParamnameParamnameParamnameParam){){){){
name =name =name =name = nameParamnameParamnameParamnameParam;;;;
}}}}
public intpublic intpublic intpublic int getrollNogetrollNogetrollNogetrollNo (){(){(){(){
returnreturnreturnreturn rollNorollNorollNorollNo;;;;
}}}}
}}}}
Data Members
(State)
Constructor
Method (Behavior)

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Access Modifiers – private and public
Data members are always kept private
– It is accessible only within the class
The methods which expose the behavior of the object are kept public
– However, we can have helper methods which are private
Key features of object oriented programs
– encapsulation (binding of code and data together)
– State (data) is hidden and Behavior (methods) is exposed to external world
Access modifiers (public, private etc) will be covered in more details in the later
slides

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Creating Objects in Java
The new operator creates the object and returns a reference to it
Memory allocation of objects happens in the heap area
Reference returned can be stored in reference variables
E.g. :
Student obj1;
obj1 = new Student();
Or
Student obj2 = new Student(“Jack”);
obj1 is reference
variable
new keyword creates
an object and returns
a reference to it

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Constructors (1 of 2)
A constructor is a special method that is used to initialize a newly created
object
Called just after the memory is allocated for the object
Can be used to initialize the objects to required or default values at the time of
object creation
It is not mandatory for the coder to write a constructor for the class
When writing a constructor, remember that:
– it has the same name as the class
– it does not return a value not even void
– It may or may not have parameters (arguments)

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Constructors (2 of 2)
If no user defined constructor is provided for a class, compiler initializes
member variables to its default values. Examples:
– numeric data types are set to 0
– char data types are set to null character(‘0’)
– reference variables are set to null

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Lifetime of objects (1 of 2)
Student obj1 = new student();
Student obj2 = new student();
The two Student objects are now living on the heap
-- References: 2
-- Objects: 2
Student obj3 = obj2;
-- References: 3
-- Objects: 2
2
1
heap
obj1
obj2
2
1
heap
obj1
obj2
obj3

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Lifetime of objects (2 of 2)
obj3 = obj1;
-- References: 3
-- Objects: 2
obj2 = null;
-- Active References: 2
-- null references: 1
-- Reachable Objects: 1
-- Abandoned objects: 1 2
1
heap
obj1
obj2
ob3
2
1
obj1
obj2
ob3
heap
Null reference
This object can be
garbage collected
(Can be Removed
from memory)

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Garbage Collection
In C language, it is the programmer’s responsibility to de-allocate memory
allocated dynamically
– free() function is used to de-allocate
In Java, JVM will automatically do the memory de-allocation
– Called “Garbage collection”
– However programmer has to ensure that reference to the object is released
• If a reference variable is declared within a function, the reference is invalidated soon as the
function call ends
• Other way of explicitly releasing the reference is to set the reference variable to null
• Setting a reference variable to null means that it is not referring to any object
An object which is not referred by any reference variable is removed from
memory by the garbage collector
Primitive types are not objects. They cannot be assigned null

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Variables and their scope (1 of 3)
Instance variables (Also known as member variables)
– Declared inside a class
– Outside any method or constructor
– Belong to the object
– Stored in the heap area along with the object to which they belong
– Lifetime depends on the lifetime of the object
Local variables (Also known as stack variables)
– Declared inside a method
– Method parameters are also local variables
– Stored in the program stack along with method calls and live until the call ends

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Strings in Java
Unlike C, String is a system defined class in Java
Declaring “Hello World” in code will create and object of type string with data “Hello
World” and returns a reference to it.
String is defined in the Java API under the package ‘java.lang’
– Packages are covered in detail later.
– The fully qualified name of String class in Java is ‘java.lang.String’
Unlike C, the string is of fixed length and memory for the string is managed totally by the
String class
– Prevents buffer overruns
– NULL terminator not used in strings
Unlike C, string is not a simple array of characters
– internally it is stored as an array, but not exposed to programmer

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Control Structures in Java
Similar to the “C” Programming Language
Conditionals
– if-else, else if statements
– switch case statements
Loops
– for
– while
– do-while

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Coding standards and Best practices for naming
classes and variables
Class name should begin with uppercase and camel casing
– Eg. Student, ArrayList
Name should tell what the variable, method or class does
No short form of words
Variable name should start with lower case and to follow camel casing
– Eg. int numberOfStudents;
Method names should begin with lowercase and follow camel casing
– Eg. void displayUserChoice()

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Commenting code in Java (1 of 3)
File header
– Description of the file
/* This java file contains a class with a method to compute the/* This java file contains a class with a method to compute the/* This java file contains a class with a method to compute the/* This java file contains a class with a method to compute the
* Sum of two numbers. This method is invoked from another class* Sum of two numbers. This method is invoked from another class* Sum of two numbers. This method is invoked from another class* Sum of two numbers. This method is invoked from another class
* by passing the necessary values as parameters* by passing the necessary values as parameters* by passing the necessary values as parameters* by passing the necessary values as parameters
*/*/*/*/

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Class header
– description of the class
– Date
– @ author
– @ version
– Note: Do not type class name in header
/**
* This class contains a method to print sum of two numbers.
* Date: 12-Jan-2005
* @author E&R Dept, Infosys Technologies Limited
* @version 1.0
*/

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Method header
– description of the method
– @param
– @return
– Note: Do not type method name in header
/**
* Computes the sum of the two integer variables passed
* as parameters
* @param number1 The First number
* @param number2 The Second number
* @return the sum of the two numbers passed as arguments
*/

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Arrays in Java (1 of 7)
An array is a data structure which defines an ordered collection of a fixed
number of homogeneous data elements
The size of an array is fixed and cannot increase to accommodate more
elements
Arrays in Java are objects and can be of primitive data types or reference
variable type
All elements in the array must be of the same data type
20 10045 7050
An array holding 5 int elements
An array holding 4 rabbit objects

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Declaring Array Variables
<elementType>[] <arrayName>;
or
<elementType> <arrayName>[];
where <elementType> can be any primitive data type or reference type
Example:
int intArray[];
Pizza[] mediumPizza, largePizza;

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Constructing an Array
<arrayName> = new <elementType>[<noOfElements>];
Example:
int intArray[];
Pizza mediumPizza[], largePizza[];
intArray = new int[10];
mediumPizza = new Pizza[5];
largePizza = new Pizza[2];
Declaration and Construction combined
int intArray[] = new int[10];
Pizza mediumPizza[] = new Pizza[5];

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Declaring and Initializing an Array
<elementType>[] <arayName> = {<arrayInitializerCode>};
Example:
int intArray[] = {1, 2, 3, 4};
char charArray[] = {‘a’, ‘b’, ‘c’};
Pizza pizzaArray[] = {new Pizza(), new Pizza()};

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Unlike C, Java checks the boundary of an array while accessing an element in
it
Java will not allow the programmer to exceed its boundary
If x is a reference to an array, x.length will give you the length of the array
So setting up a for loop as follows is very common in Java
for(int i = 0; i < x.length; ++i){for(int i = 0; i < x.length; ++i){for(int i = 0; i < x.length; ++i){for(int i = 0; i < x.length; ++i){
x[i] = 5;x[i] = 5;x[i] = 5;x[i] = 5;
}}}}

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this keyword(1 of 2)
class Counter{
private int data;
Counter(int var1){
data=var1;
}
public void increment(){
++data;
}
public int getData(){
return data;
}
}
class CounterTest{
public static void main(String args[]){
Counter a = new Counter(10);
a.increment();
System.out.println(a.getData());
System.out.println(a.getData());
Counter b = new Counter(25);
b.increment();
System.out.println(b.getData());
}
}

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this reference (2 of 2)
In the Counter example, when we call a.increment(), the increment() method
will say ++data
Whose data is it?
The methods of a class will automatically have a reference called this
The ‘this’ reference will always refer to the object that called the method

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static (1 of 4)
static keyword can be used in 3 scenarios:
– For class variables
– For methods
– For a block of code

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static (2 of 4)
static variable
– It is a variable which belongs to the class
– A single copy to be shared by all instances of the class
– For using static variables, creation of instance is not necessary
– Accessed using <class-name>.<variable-name> unlike instance variables which are accessed
as <object-name>.<variable-name>
static method
– It is a class method
– Accessed using class name.method name
– For using static methods, creation of instance is not necessary
– A static method can only access other static data and methods. It cannot access non-static
members

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static (3 of 4)
Class Duck {Class Duck {Class Duck {Class Duck {
private int size;private int size;private int size;private int size;
private static intprivate static intprivate static intprivate static int duckCountduckCountduckCountduckCount;;;;
public Duck(){public Duck(){public Duck(){public Duck(){
duckCountduckCountduckCountduckCount++;++;++;++;
}}}}
public voidpublic voidpublic voidpublic void setSizesetSizesetSizesetSize (int s){(int s){(int s){(int s){
size = s;size = s;size = s;size = s;
}}}}
public intpublic intpublic intpublic int getSizegetSizegetSizegetSize (int s){(int s){(int s){(int s){
return size;return size;return size;return size;
}}}}
public static voidpublic static voidpublic static voidpublic static void main(Stringmain(Stringmain(Stringmain(String argsargsargsargs[]){[]){[]){[]){
System.out..println(System.out..println(System.out..println(System.out..println(““““SizeSizeSizeSize of the duck is;of the duck is;of the duck is;of the duck is;”””” + size);+ size);+ size);+ size);
}}}}
}}}}
The static duckCount variable is
initialised to 0, ONLY when the
class is first loaded, NOT each time
a new instance is made
Each time the constructor is invoked ie
an object gets created, the static
variable duckCount will be incremented
thus keeping a count of the total no of
Duck objects creataed
Which duck? Whose size? A static
method cannot access anything
non-static
Compilation
error

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static (4 of 4)
static block
– The static block is a block of statement inside a Java class that will be executed
when a class is first loaded and initialized
• A class is loaded typically after the JVM starts
• In some cases a class is loaded when the program requires it
– A static block helps to initialize the static data members just like constructors help to
initialize instance members
class Test{class Test{class Test{class Test{
static {static {static {static {
//Code goes here//Code goes here//Code goes here//Code goes here
}}}}
}}}}

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Referring to Java documentation
Java provides a rich set of library classes
Demo on how to refer to library classes

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Summary:
– Review of Object Oriented Concepts
– Java architecture
– The basic constructs in Java

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