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Introduction to JAVA
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Introduction
 History
 Java is related to C++, which is a direct descendent of C.
Much of the character of Java is inherited from these two
languages.
 From C, Java derives its syntax. Many of Java’s objectoriented features were determined by C++.
 Java was developed by James Gosling, Patrick Naughton,
Chris Warth, Ed Frank, and Mike Sheridan at Sun
Microsystems, Inc.in 1991. It took 18 months to develop the
first working version. This language was initially called “Oak”
but was renamed “Java” in 1995.
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Comparison between OOP and
pure OOP
 Unlike
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C++, java is pure object oriented programming
language.
Pure OOP organizes a program around its data (i.e. objects)
and set of well defined interfaces (i.e. methods) to that data.
Pure OOP means all of the data types in the language are
objects and all of the operations on those objects are
methods.
You can’t code anything in java without declaring classes and
objects.
Everything in java is an object. All the program codes and
data resides within classes and objects.
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Advantages of java
 Simple:
 If you already understand the basic concepts of object-
oriented programming, moving to Java will require very
little effort because Java inherits the C/C++ syntax and
many of the object-oriented features of C++.
 Java was designed to be easy to use and is therefore easy to
write, compile, debug, and learn than other programming
languages.
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 Secure:
 When you use a Java-compatible Web browser, you can safely
download Java applets without fear of viral infection.
 Java achieves this protection by providing a “firewall”
between a networked application and your computer.
 Java system not only verify all memory access but also ensure
that no viruses are communicated with an applet.
 The absence of pointers in java ensures that programs cannot
gain access to memory locations without proper
authorization.
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 Portable and Platform Independent:
 One of the most significant advantages of Java is its ability to
move easily from one computer system to another.
 Today many types of computers and operating systems are in
used throughout the world and many are connected to the
Internet.
 We can download a java applet from a remote computer onto
our local system via internet and execute it locally.
 You can run java program on any type of operating system
because of its portability.
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 Object- Oriented:
 One of the most key features of java is Object Oriented.
 Java is object-oriented because programming in Java is
centered on creating objects, manipulating objects, and
making objects work together. This allows you to create
modular programs and reusable code.
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 Robust:
 Robust means reliable.
 It provides many safeguards to ensure reliable code.
 It has strict compile time and runtime checking for data
types.
 In C/C++, the programmer must manually allocate and free
all dynamic memory. This sometimes leads to problems,
because programmers will either forget to free memory that
has been previously allocated or, worse, try to free some
memory that another part of their code is still using.
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 Java virtually eliminates these problems by managing
memory allocation and deallocation.
 Deallocation is completely automatic, because Java provides
garbage collection for unused objects.
 Java also incorporates the concept of exception handling
which captures serious errors and eliminates any risk of
crashing the system.
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 Multithreaded:
 Java supports multithreaded programming, which allows you
to write programs that do many things simultaneously.
 Architecture-Neutral:
 Nowadays Operating system upgrades, processor upgrades,
and changes in core system resources can all combine that
make a program failure and doesn’t work properly.
 Once you written a java program, you can run this program
anywhere, anytime forever.
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 Interpreted and High Performance:
 The output of a Java compiler is not executable code. Rather,
it is bytecode.
 The Java bytecode was carefully designed so that it would be
easy to translate directly into native machine code for very
high performance by using a just-in-time compiler.
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 Compiled and Interpreted:
Usually, a computer language is either compiled or
interpreted. Java combines both these approaches. Thus
making java a two stage system.
 First, java compiler translates source code into bytecode.
Bytecodes are not machine instructions and therefore, in the
second stage, java interpreter generates machine code that
can be directly executed by the machine that is running the
java program. We can thus say that java is both a compiled
and an interpreted language.
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 Distributed:
 Distributed computing involves several computers on a
network working together. Java is designed to make
distributed computing easy with the networking capability
that is inherently integrated into it.
 Writing network programs in Java is like sending and
receiving data to and from a file. For example, three
programs running on three different systems, communicating
with each other to perform a joint task.
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 Dynamic:
 Java programs carry with them substantial amounts of run-
time type information that is used to verify and resolve
accesses to objects at run time. This makes it possible to
dynamically link code in a safe manner.
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Features of java
 Encapsulation
Encapsulation is the mechanism that binds together code and
the data it manipulates, and keeps both safe from outside
interference and misuse.
Or
The wrapping up of data and methods into single unit is
known as encapsulation.
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 Inheritance
Inheritance is the process by which one object acquires the
properties of another object. This is important because it
supports the concept of hierarchical classification.
 For example, the bird sparrow is a part of the class flying
bird, which is again a part of the class bird.
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 Polymorphism
 Polymorphism is a feature that allows one interface to be
used for a general class of actions.
 The concept of polymorphism is often expressed by the
phrase “one interface, multiple methods.” This means that it is
possible to design a generic interface to a group of related
activities.
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What is byte code?
 The output of a Java compiler is not executable code. it is
bytecode.
 Bytecode is a highly optimized set of instructions to be executed
by the Java run-time system, which is called the Java Virtual
Machine (JVM). JVM is an interpreter for Bytecode.
 Bytecodes are a set of instructions that is not specific to any one
processor.
 When we compile a program written in C or in most other
languages, the compiler translates our program into machine
codes. Those instructions are specific to the processor your
computer is running.
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 So, for example, if you compile your code on a Pentium
system, the resulting program will run only on other
Pentium systems. If you want to use the same program on
another system, you have to go back to your original source,
get a compiler for that system, and recompile your code.
 Things are different when you write code in Java. The Java
development environment has two parts: a Java compiler and
a Java interpreter. The Java compiler takes your Java program
and instead of generating machine codes from your source
files, it generates bytecodes.
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Traditional compiled programs
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 To run a java program, you run a program called a bytecode
interpreter, which in turn executes your Java program.
 Having your Java programs in bytecode form means that
instead of being specific to any one system, your programs
can be run on any platform and any operating or window
system as long as the Java interpreter is available.
 Only the JVM needs to be implemented for each platform.
once the run-time package exists for a given system, any Java
program can run on it.
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Java programs
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Applications of java
 To develop web browser
 To enhance capabilities of web browser.
 To develop web server
 To enhance capabilities of web server
 To develop protocols
 Java encryption/decryption algorithm implementation
enables security on the internet.
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Basic structure of java program
suggested
optional
optional
optional
optional
Essential
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 Documentation Section :
 The documentation section comprises a set of comment
lines. In addition to two styles of comments, java also uses a
third style of comment /**…..*/ known as documentation
comment.
 Package Statement :
 The first statement allowed in a java file is a package
statement. This statement declares a package name and
informs the computer that the classes defined here belong to
this package.The package statement is optional.
Ex: package student
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 Import Statements:
 The next thing after package statement is import statements.
This is similar to the #include statement in C.
Ex: import student test;
 This statement instructs the interpreter to load the test class
contained in the package student. Using import statements,
we can have access to classes that are part of other named
packages.
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 Interface Statements:
 An interface is like a class but includes a group of method
declarations. This is also an optional section and is used only
when we wish to implement the multiple inheritance feature
in the program.
 Class Definitions:
 A java program may contain multiple class definitions.
Classes are the primary and essential elements of a java
program.
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First simple program
Example.java
class Example
{
public static void main (String args [])
{
System.out.println ("Hello World.");
}
}
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Compiling the program
 To compile the Example program, execute the compiler,
javac, specifying the name of the source file on the
command line, as shown here:
C:\>javac Example.java
 The javac compiler creates a file called Example.class that
contains the bytecode version of the program. The Java
bytecode is the intermediate representation of your program
that contains instructions the Java interpreter will execute.
Thus, the output of javac is not code that can be directly
executed.
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Run the program
 To actually run the program, you must use the Java
interpreter, called java. To do so, pass the class name
Example as a command-line argument, as shown here:
C:\>java Example
 When the program is run, the following output is displayed:
Hello World.
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 When Java source code is compiled, each individual class is
put into its own output file named after the class and using
the .class extension.
 When you execute the Java interpreter as just shown, you are
actually specifying the name of the class that you want the
interpreter to execute.
 It will automatically search for a file by that name that has
the .class extension. If it finds the file, it will execute the
code contained in the specified class.
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Description
 class Example
 This line uses the keyword class to declare that a new class is
being defined. Example is an identifier that is the name of
the class.
 public static void main (String args [])
 This line begins the main ( ) method. This is the line at
which the program will begin executing. All Java applications
begin execution by calling main ( ).
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 The public keyword is an access specifier, which allows
the programmer to control the visibility of class members. In
this case, main ( ) must be declared as public, since it must
be called by code outside of its class when the program is
started.
 The keyword static allows main ( ) to be called without
having to instantiate a particular instance of the class. This is
necessary since main ( ) is called by the Java interpreter
before any objects are made.
 The keyword void simply tells the compiler that main ( )
does not return a value.
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 In main ( ), there is only one parameter, String args [ ],
which declares a parameter named args, which is an array of
instances of the class String. In this case, args receives any
command-line arguments present when the program is
executed.
 System.out.println ("HelloWorld.");
 This line outputs the string “hello World.” followed by a new
line on the screen. Output is actually accomplished by the
built-in println ( ) method.In this case,println( ) displays
the string which is passed to it.
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 System is a predefined class that provides access to the
system, and out is the output stream that is connected to the
console.
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Java input output operators
 I/O streams
 A stream in java is a path along which data flows.
 It has a source and destination.
 Input stream reads the data from source and pass the data to
program.
 Output stream writes the data from program to destination.
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Java stream classes
Java streams
Byte Stream
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Character Stream
I/P Stream
O/P Stream
File I/P Stream
Data I/P Stream
Object I/P Stream
Buffered I/P
Stream
File O/P Stream
Data O/P Stream
Object O/P Stream
Buffered O/P Stream
Reader
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Writer
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 The most basic java I/O is writing to and reading from
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console.
Java contains the built-in system, input stream and print
stream classes to perform these operations.
To read from the keyboard, the statement
System.in.read();
It will read a sequence of characters until the end of stream,
i.e.the enter key has been detected or an exception has been
thrown.
To write data to the screen,
Sytem.out.println(“hi”);
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Reading a data from keyboard
import java.io.*;
public class Echo
{
public static void main(String args[]) throws IOException
{
InputStreamReader reader;
BufferedReader bufReader;
reader=new InputStreamReader(System.in);
bufReader=new BufferedReader(reader);
String s;
while(null!=(s=bufReader.readLine()))
System.out.println(s);
}
}
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Output:
Hello world
Hello world
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