Overview of Java

Home	»	Java	»	Overview of Java

Contents

• Simple Java Program
• An Application with Two Classes
• Java Tokens
• Implements a Java Program
• Java Virtual Machine
• Command Line Arguments

Simple Java Program

A Simple Example of java Program
//A Simple Example of java Program class abc { public static void main(String arg[]) { System.out.print("A Simple Example of java Program"); } }

We know java is Purely OOP Language So that Every Program of Java Must be Written into the Classes For this Class is Keyword and abc is name of Class.

Comment

The first line is a single line comment which will be ignored during compilation. Comments make you understand what your program does. Use /*.............*/ for multiple line comments.

Class Declaration

In Java, everything must be declared in a class, which is declared by the keyword Class. Public is a keyword which makes this class available and accessible to any one.(Simple) one is a Java identifier that specifies the name of the class to be defined.

Opening Brace

In Java, every class definition begins with an opening brece "{" and ends with a closing brace "}". Pair of "{}" braces define the scope of any method or class.

The Main Line
The next line of code is shown here:
Public static void main(String arg[]

Public Here Public is a Keyword which declares main method is public or unprotected so that it will be Accessible to all classes. Static Static is used for Describing that this is the method which belongs to this class not to other means this is the main method of Class only Main Method Must be Declared as Static so that Interpreter Will Execute this Method First ,before Creating the Objects of the Class. Void Void means The Main Function will never Return a Value or it will Simply Prints the text on the Screen.

The Output Line
The next line of code is shown here:

System.out.print("A Simple Example of java Program");

This Line is Similar to printf in C and Cout in C++ For Displaying the Results on the Screen at the Time of Execution in this System is name of Package and Out i.e Output is name of class and print is the name of Method that belongs to the output class For Displaying the Results on the Screen.

An Application with Two Classes

The one example discussed above contain only one class that contains main() method. Multiple classes are required in a real-life application. Let's consider a program in Java with two classes.

//Program with two classes Class Box { double width; double height; double length; } //This class declares an object of type Box. class BoxDemo { public static void main(String arg[]) { Box mybox = new Box(); double vol; //assign values to mybox's instance variables mybox.width=10; mybox.height = 15; mybox.length = 20; //compute volume of the Box vol = mybox.width * mybox.height * mybox.length; System.out.print("Volume is =" +vol); } }

The above program contains two classes Box and BoxDemo. The Box class declares three variables. The class BoxDemo contains the main method that initiates the execution.

The main method create a Box object by the statement.

Box mybox = new Box();

After this statement executes, mybox will be an instance of Box. Thus, it will have "physical" reality. Every Box object will contain its own copy of instance variables width, height and length. The dot (.) operator is used to access these variables.

Java Tokens

A java Program is made up of Classes and Methods and in the Methods are the Container of the various Statements And a Statement is made up of Variables, Constants, operators etc Tokens are the Smallest unit of Program There are Five Types of Tokens.

1. Reserve Word or Keywords
2. Identifier
3. Literals
4. Operators
5. Separators

Keywords

A Keyword is that which have a special meaning those are already been explained to the java language like int, float, class, public etc these are the reserve keywords Always Remember that we can t give a name to a variable as the name of a keyword Java Provides us the 60 Keywords All the Keywords are to be written into the lower case because java is a Case Sensitive means Upper case and Lower case Letters are Different in java Some Keywords are inherit by the java language from c and C++ it adds only 27 keywords like System, Interface, abstract, final etc.

Identifiers
The Identifiers are those which are used for giving a name to a variable ,class and method ,packages ,interfaces etc There Are Some Rules against for using the Identifiers.
1) Name of Identifier not be a Keyword
2) Must be Start from Alphabet not from digit
3) Uppercase and Lowercase are Different
4) Can be any length

Literals Literals are Sequence of Characters like Digits ,alphabets ,letters those are used for Representing the Value of the Variable Like Integer Literals, String Literals, Float Literals.

Operator Operators are the Special Symbols those have specific functions associated with them for Performing the operations it needs some Operands
Operands are those on which operations are performed by the Operators
Like 2 +3
In this + is the Operator and 2 and 3 Operands.

Separators
These are Special Symbols used to Indicate the group of code that is either be divided or arrange into the Block The Separators includes Parentheses, Open Curly braces Comma, Semicolon, or either it will be period or dot Operator.

Implements a Java Program

The steps involved in the implementation of a java program are as follows:

1. Creating the program
2. Compling the program
3. Running the program

Remember to install the Java Development Kit (JDK) properly before creating the program in Java.

Creating the program

A Program can be created using any text editor. Let us consider an example program.

First of all we will install JDK using any java version . for example now we will use jdk1.4 kit. after install java now you will open jdk1.4 using following commands:

//A Simple test program import java.io.*; class First { public static void main(String arg[]) { System.out.print("Welcome to Java World!"); System.out.print("Hello"); System.out.print("Let us begin learning Java"); } }

The above program must be saved with the file name "First.java" under the "c:/jdk1.4/bin/ dir. This file is called the Source file. Note that file name must be the class name of the class containing the main method along with the file extension "java".

Compiling the Program

To compile the program, type the following instruction at the command prompt:

javac First.java

If the program is compiled successfully, the javac compiler creates a file called First.class contains the bytecode of the program.

Running the Program

Java Interpreter is used to run a stand-alone program. To run the program type the following instruction at the command prompt.

Download Complete Program

-

Java Virtual Machine

We Know that Compiler translates the Human Code into the Byte Codes which is not machine Language and for Converting Bytes Code into Machine Language Interpreter is used JVM is used in interpreting Code When We Interpret the Code or when we Translate the Byte Code into the Machine Code then Java virtual Machine is used JVM is the built if software which converts bytes code into machine language Process of JVM :-

When We Compile Source Program using java Compiler known as Javac Then after Compilation it will create Virtual Machine Code or Byte Code Then When we Interpret the Code of the java then Virtual Machine will Sends These Byte Codes to interpreter then the Interpreter will Generate the Machine Code.

Command Line Arguments

There is Situation . when output of a program is depend on input of another program So for providing the input , java provides us Command line Arguments These are the parameters those are provided at Run Time or when w Interpret java program for Execution But Always Remember these are Always String by default java also provide us Length Method which is built in Method for calculating the Length of Arguments those are supplied at the Time of Execution .

<< Previous Topic	Next Topic >>