Here is our first java source code

first.java
 

class first { public static void main(String args[]) { System.out.println("hello world"); } }

 

For the most computer languages, the name of the file that holds the source code to a program is arbitrary. However, this is not the case with Java. The first thing that you must learn about Java is that the name you give to a source code file is very important. For this example, the name of the source file sould be first.java. let’s see why.

In Java, a source file is officially called a compilation unit. It is a text file that contains one or more class definations. The Java compiler requires that source file use the .java filename extension. Notice that the file extension is four character long.

Compiling the program

To compile the first program, execute the compiler, javac, specifying the name of the source file on the command line, as shown here:
 
 

C:\>javac first.java
 
 

The javac compiler creates a file called first.class that contains the bytecode version of the program. As discussed earlier, 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.

To actually run the program, you must use the Java Interpreter, called java. To do so, pass the class name first as a command-line argument, as shown here:
 
 

c:\>java first
 
 

When the program is run the following output is displayed:
 
 

Hello World
 

source code		Compiler		byte-code file		Interpreter		output screen
first.java	è	javac	è	first.class	è	java	è	Hello World

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. This is why it is a good idea to give your Java source files the same name as the class they contain-the name of the source file will match the name of the .class file. 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.
 
 

Explanation of program
 
 

class first {

This line uses the keyword class to declare that a new class is defined. First is an identifier that is the name of the class. The entire class defination, including all of its members, will be between the opening curly brace ( { ) and the closing curly brace ( } ).
 
 

public static void main(String args[ ])
 
 

All Java applications begin execution by calling main(). ( This is like C/C++.)

The public keyword is an access specifier, which allows the programmer to control the visibility of class members. When a class member is preceded by public, then that member may be accessed by code outside the class in which it is declared.

static allows main() to be called without having to instantiate a particular instance of the class. This is necessary since main() is called by Java interpreter before any objects are made.

The keyword void simply tells the compiler that main() does not return a value.

Any information that you need to pass to a method is received by variables specified within the set of parentheses that follow the name of the method. These variables are called parameters. If there is no parameter required for a given method, you still need to include the empty parentheses. In main(), there is only one parameter. String args[] declares a parameter named args, which is an array of instances of the class String.

System.out.println("Hello world");

This line outputs the string "Hello world" followed by a new line on the screen. Output is accomplished by the build-in println() method. println() displays the string which is passed to it. The same can be used for other datatypes too. System is a predefined class that provides access to the system, and out is the output stream that is connected to the console. And all statement in Java end with a semicolon.
 
 

second.java
 

/* This is the first Java program written  using jdk 1.1.1 compiler */ class second { public static void main(String args[]) { System.out.println("hello world"); // it prints hello world to screen } }

 

Compile the above program and run it. You will not find any difference between first and second Java program. In Java the comments can be given in two ways. If it is multiline comment /* */ are to be used. If it is single line comment then // can be used.
 
 

third.java
 

class third { public static void main(String args[]) { int i=20; System.out.println("i ="+i ); // it prints the value of i. } }

 

The above program creates int type variable with the name i and assigns the value 20 to it at the time of declaration. Afterwards the value is concatenated with the string and given to println() method to direct it to screen. No data type conversion is required while concatenating the numeric value to character string.