02_the hello world application

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2. The Hello World Application

Java

Summer 2008 Instructor: Dr. Masoud Yaghini


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The Hello World Application

OutlineIntroduction

Creating Your First ApplicationA Closer Look at the "Hello World!" ApplicationReferences


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Introduction


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The "Hello World!" ApplicationYour first application, HelloWorldApp, willsimply display the greeting "Hello World!".We can use: An Integrated Development Environments (IDE) or

A simple text editor.


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"Hello World!" for Microsoft WindowsTo write your first program, you'll need: The Java SE Development Kit 6 (JDK 6) A text editor

You can download the Windows version ofJDK6 from:http://java.sun.com/javase/6/download.jsp

We use a text editor to create and edit sourcefiles. We will can use Notepad , a simple editor

included with the Windows platforms.


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Update the PATH variableSet the PATH variable if you want to be able toconveniently run the JDK executables(javac.exe, java.exe, javadoc.exe, etc.) fromany directory without having to type the fullpath of the command.If you don't set the PATH variable, you need tospecify the full path to the executable everytime you run it, such as: C:> "\Program Files\Java\jdk1.6.0_\bin\javac"

MyProgram.java


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Update the PATH variableTo set the PATH permanently, add the full pathof the jdk1.6.0_\bin directory to thePATH variable.Set the PATH as follows on MicrosoftWindows: Click Start > Control Panel > System on Windows

XP or Start > Settings > Control Panel > Systemon Windows 2000.

Click Advanced > Environment Variables . Add the location of bin folder of JDK installation for

PATH in User Variables and System Variables.A typical value for PATH is: C:\Program

Files\Java\jdk1.6.0_\bin


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Update the PATH variablePATH environment variable is a series ofdirectories separated by semi-colons (;) and isnot case sensitive.Microsoft Windows looks for programs in thePATH directories in order, from left to right.Add the path to the right end of the PATH inthe User Variables.

The new path takes effect in each newcommand window you open after setting thePATH variable.


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Creating Your First Application


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Creating Your First ApplicationTo create this program, you will: Create a source file .

A source file contains code, written in the Javaprogramming language.

Compile the source file into a .class file .

The Java programming language compiler (javac) takesyour source file and translates its text into instructions thatthe Java virtual machine can understand.The instructions contained within this file are known as

bytecodes . Run the program .

The Java application launcher tool (java) uses the Javavirtual machine to run your application.


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Create a Source FileStart your editor, and in a new document, typein the following code:

/** * The HelloWorldApp class implements an application that * simply prints "Hello World!" to standard output.

*/ class HelloWorldApp {

public static void main(String[] args) { System.out.println("Hello World!"); // Display the string.

} }


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Create a Source FileType all code, commands, and file namesexactly as shown.Save the code in a file with the nameHelloWorldApp.java .


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Compile the Source File into a .class FileBring up a shell, or "command," window.You can do this from the Start menu by choosingCommand Prompt (Windows XP), or by choosing Run .. . and then entering cmd .


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Compile the Source File into a .class FileTo compile your source file, change your currentdirectory to the directory where your file is located.If your source directory is java on the D drive, type thefollowing command at the prompt and press Enter:

D:

cd C:\java


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Compile the Source File into a .class FileIf you enter dir at the prompt, you should seeyour source file:


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Compile the Source File into a .class FileAt the prompt, type the following command and press Enter:

javac HelloWorldApp.java

The compiler has generated a bytecode file,HelloWorldApp.class .

At the prompt, type dir to see the new file that was generated


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Run the ProgramIn the same directory, enter the followingcommand at the prompt:

java HelloWorldApp


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javac & java are Case SensitiveBoth the compiler (javac) and launcher (java)are case-sensitive.In other words, HelloWorldApp is notequivalent to helloworldapp.


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A Closer Look at the

"Hello World!" Application


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A Closer Look at the "Hello World!" Application

Here again is its code:



public static void main(String[] args) { System.out.println("Hello World!"); //Display the string.

} }


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The "Hello World!" application consists of threeprimary components: Source code comments, The HelloWorldApp class definition, and The main method.

The following explanation will provide you witha basic understanding of the code


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The following bold text defines the comments:



public static void main(String[] args) { System.out.println("Hello World!"); //Display the string.

} }


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Source code commentsComments are ignored by the compiler but are usefulto other programmers.

The Java programming language supports three kindsof comments: /* text */

The compiler ignores everything from /* to */.

/** documentation */ This indicates a documentation comment (doc comment, forshort).The compiler ignores this kind of comment, just like it ignorescomments that use /* and */.The javadoc tool uses doc comments when preparingautomatically generated documentation.

// text

The compiler ignores everything from // to the end of the line.


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HelloWorldApp class definitionHelloWorldApp class definition:

class name { ...}

The keyword class begins the class definitionfor a class named name, and the code for eachclass appears between the opening and losingcurly braces marked in bold above.

We will classes in detail later.For now it is enough to know that everyapplication begins with a class definition.


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The main MethodThe following bold text begins the definitionof the main method:

/** * The HelloWorldApp class implements an application that

* simply prints "Hello World!" to standard output.*/ class HelloWorldApp {

public static void main(String[] args) {

System.out.println("Hello World!"); //Display the string. }

}


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The main MethodIn the Java programming language, everyapplication must contain a main method whosesignature is:public static void main(String[] args)

It's the entry point for your application and willsubsequently call up all the other methodsrequired by your program.

The modifiers public and static can be writtenin either order ( public static or static public ) butthe convention is to use public static


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The main MethodThe main method accepts a single argument:an array of elements of type String.

public static void main( String[] args )

You can name the argument anything youwant, but most programmers choose "args" or"argv."This array is the mechanism through which theruntime system passes information to yourapplication.


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The main MethodEach string in the array is called a command-line argument.Command-line arguments let users affect theoperation of the application without recompilingit.The "Hello World!" application ignores itscommand-line arguments, but you should beaware of the fact that such arguments do exist.


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The main MethodFinally, the following line uses the Systemclass from the API to print the "Hello World!"message to standard output:

System.out.println("Hello World!");


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References


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ReferencesS. Zakhour, S. Hommel, J. Royal, I.Rabinovitch, T. Risser, M. Hoeber, The JavaTutorial: A Short Course on the Basics , 4thEdition, Prentice Hall, 2006.


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02_the hello world application

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