JAVA First Day

About the Java Technology
Java technology is both a programming language and a platform.

The Java Programming Language

The Java programming language is a high-level language that can be characterized by
all of the following buzzwords:

Simple Architecture neutral
Object oriented Portable
Distributed High performance
Multithreaded Robust
Dynamic Secure

Each of the preceding buzzwords is explained in The Java Language Environment , a
white paper written by James Gosling and Henry McGilton.

In the Java programming language, all source code is first written in plain text files
ending with the .java extension. Those source files are then compiled into .class files
by thejavac compiler. A .class file does not contain code that is native to your
processor; it instead contains bytecodes — the machine language of the Java Virtual
Machine1 (Java VM). The java launcher tool then runs your application with an
instance of the Java Virtual Machine.

An overview of the software development process.

Because the Java VM is available on many different operating systems, the
same .class files are capable of running on Microsoft Windows, the Solaris™
Operating System (Solaris OS), Linux, or Mac OS. Some virtual machines, such as
the Java HotSpot virtual machine, perform additional steps at runtime to give your
application a performance boost. This include various tasks such as finding
performance bottlenecks and recompiling (to native code) frequently used sections of
code.

Through the Java VM, the same application is capable of
running on multiple platforms.

The Java Platform

A platform is the hardware or software environment in which a program runs. We've
already mentioned some of the most popular platforms like Microsoft Windows,
Linux, Solaris OS, and Mac OS. Most platforms can be described as a combination of
the operating system and underlying hardware. The Java platform differs from most
other platforms in that it's a software-only platform that runs on top of other
hardware-based platforms.

The Java platform has two components:

The Java Virtual Machine
The Java Application Programming Interface (API)

You've already been introduced to the Java Virtual Machine; it's the base for the Java
platform and is ported onto various hardware-based platforms.

The API is a large collection of ready-made software components that provide many
useful capabilities. It is grouped into libraries of related classes and interfaces; these
libraries are known as packages.

The API and Java Virtual Machine insulate the program
from the underlying hardware.

As a platform-independent environment, the Java platform can be a bit slower than
native code. However, advances in compiler and virtual machine technologies are
bringing performance close to that of native code without threatening portability.

The terms"Java Virtual Machine" and "JVM" mean a Virtual Machine for the Java
platform.

What Can Java Technology Do?
The general-purpose, high-level Java programming language is a powerful software
platform. Every full implementation of the Java platform gives you the following
features:

Development Tools: The development tools provide everything you'll need for
compiling, running, monitoring, debugging, and documenting your
applications. As a new developer, the main tools you'll be using are
the javac compiler, the java launcher, and the javadoc documentation tool.
Application Programming Interface (API): The API provides the core
functionality of the Java programming language. It offers a wide array of useful
classes ready for use in your own applications. It spans everything from basic
objects, to networking and security, to XML generation and database access,
and more. The core API is very large; to get an overview of what it contains,
consult the Java Platform Standard Edition 7 Documentation.
Deployment Technologies: The JDK software provides standard mechanisms
such as the Java Web Start software and Java Plug-In software for deploying
your applications to end users.
User Interface Toolkits: The Swing and Java 2D toolkits make it possible to
create sophisticated Graphical User Interfaces (GUIs).
Integration Libraries: Integration libraries such as the Java IDL API, JDBC™
API, Java Naming and Directory Interface™ (JNDI) API, Java RMI, and Java

Remote Method Invocation over Internet Inter-ORB Protocol Technology (Java
RMI-IIOP Technology) enable database access and manipulation of remote
objects.

How Will Java Technology Change My Life?
We can't promise you fame, fortune, or even a job if you learn the Java programming
language. Still, it is likely to make your programs better and requires less effort than
other languages. We believe that Java technology will help you do the following:

Get started quickly: Although the Java programming language is a
powerful object-oriented language, it's easy to learn, especially for
programmers already familiar with C or C++.
Write less code: Comparisons of program metrics (class counts, method
counts, and so on) suggest that a program written in the Java programming
language can be four times smaller than the same program written in C++.
Write better code: The Java programming language encourages good
coding practices, and automatic garbage collection helps you avoid memory
leaks. Its object orientation, its JavaBeans™ component architecture, and its
wide-ranging, easily extendible API let you reuse existing, tested code and
introduce fewer bugs.
Develop programs more quickly: The Java programming language is
simpler than C++, and as such, your development time could be up to twice
as fast when writing in it. Your programs will also require fewer lines of
code.
Avoid platform dependencies: You can keep your program portable by
avoiding the use of libraries written in other languages.
Write once, run anywhere: Because applications written in the Java
programming language are compiled into machine-independent bytecodes,
they run consistently on any Java platform.
Distribute software more easily: With Java Web Start software, users will
be able to launch your applications with a single click of the mouse. An
automatic version check at startup ensures that users are always up to date
with the latest version of your software. If an update is available, the Java
Web Start software will automatically update their installation.

"Hello World!" for the NetBeans IDE
It's time to write your first application! These detailed instructions are for users of the
NetBeans IDE. The NetBeans IDE runs on the Java platform, which means that you

can use it with any operating system for which there is a JDK available. These
operating systems include Microsoft Windows, Solaris OS, Linux, and Mac OS X.

A Checklist
Creating Your First Application
o Create an IDE Project
o Add JDK 6 to the Platform List (if necessary)
o Add Code to the Generated Source File
o Compile the Source File
o Run the Program
Continuing the Tutorial with the NetBeans IDE

A Checklist

To write your first program, you'll need:

1. The Java SE Development Kit (JDK 6 has been selected in this example)
o For Microsoft Windows, Solaris OS, and Linux: Java SE Downloads
Index page
o For Mac OS X: developer.apple.com
2. The NetBeans IDE
o For all platforms: NetBeans IDE Downloads Index page


Your first application, HelloWorldApp, will simply display the greeting "Hello World!"
To create this program, you will:

Create an IDE project

When you create an IDE project, you create an environment in which to build
and run your applications. Using IDE projects eliminates configuration issues
normally associated with developing on the command line. You can build or
run your application by choosing a single menu item within the IDE.

Add code to the generated source file

A source file contains code, written in the Java programming language, that
you and other programmers can understand. As part of creating an IDE project,
a skeleton source file will be automatically generated. You will then modify the
source file to add the "Hello World!" message.

Compile the source file into a .class file

The IDE invokes the Java programming language compiler (javac), which
takes your source file and translates its text into instructions that the Java
virtual machine can understand. The instructions contained within this file are
known as bytecodes.

Run the program

The IDE invokes the Java application launcher tool (java), which uses the Java
virtual machine to run your application.
Create an IDE Project

To create an IDE project:

1. Launch the NetBeans IDE.
o On Microsoft Windows systems, you can use the NetBeans IDE item in
the Start menu.
o On Solaris OS and Linux systems, you execute the IDE launcher script
by navigating to the IDE's bin directory and typing ./netbeans.
o On Mac OS X systems, click the NetBeans IDE application icon.
2. In the NetBeans IDE, choose File | New Project.

NetBeans IDE with the File | New Project menu item
selected.

3. In the New Project wizard, expand the Java category and select Java
Application as shown in the following figure:

NetBeans IDE, New Project wizard, Choose Project
page.

4. In the Name and Location page of the wizard, do the following (as shown in the
figure below):
o In the Project Name field, type Hello World App.
o In the Create Main Class field, type helloworldapp.HelloWorldApp.
o Leave the Set as Main Project checkbox selected.

NetBeans IDE, New Project wizard, Name and
Location page.

5. Click Finish.

The project is created and opened in the IDE. You should see the following
components:

The Projects window, which contains a tree view of the components of the
project, including source files, libraries that your code depends on, and so on.
The Source Editor window with a file called HelloWorldApp open.
The Navigator window, which you can use to quickly navigate between
elements within the selected class.

NetBeans IDE with the HelloWorldApp project open.

Add JDK 6 to the Platform List (if necessary)

It may be necessary to add JDK 6 to the IDE's list of available platforms. To do this,
choose Tools | Java Platforms as shown in the following figure:

Selecting the Java Platform Manager from the Tools Menu

If you don't see JDK 6 (which might appear as 1.6 or 1.6.0) in the list of installed
platforms, click "Add Platform", navigate to your JDK 6 install directory, and click
"Finish". You should now see this newly added platform:

The Java Platform Manager

To set this JDK as the default for all projects, you can run the IDE with the --
jdkhome switch on the command line, or by entering the path to the JDK in
thenetbeans_j2sdkhome property of
your INSTALLATION_DIRECTORY/etc/netbeans.conf file.

To specify this JDK for the current project only, select Hello World App in the
Projects pane, choose File | Project Properties (Hello World App), click on Libraries,
then select JDK 6 under the Java Platform pulldown menu. You should see a screen
similar to the following:

The IDE is now configured for JDK 6.

Add Code to the Generated Source File

When you created this project, you left the Create Main Class checkbox selected in
the New Project wizard. The IDE has therefore created a skeleton class for you. You
can add the "Hello World!" message to the skeleton code by replacing the line:
// TODO code application logic here

with the line:

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

Optionally, you can replace these four lines of generated code:
/**
*
* @author
*/

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

These four lines are a code comment and do not affect how the program runs. Later
sections of this tutorial explain the use and format of code comments.

Be Careful When You Type

Note: Type all code, commands, and file names exactly as shown. Both the compiler
(javac) and launcher (java) are case-sensitive, so you must capitalize consistently.

HelloWorldApp is not the same as helloworldapp.

Save your changes by choosing File | Save.

The file should look something like the following:
/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/

package helloworldapp;

/**
*/
public class HelloWorldApp {

/**
* @param args the command line arguments
*/
public static void main(String[] args) {
}

}

Compile the Source File into a .class File

To compile your source file, choose Run | Build Main Project from the IDE's main
menu.

The Output window opens and displays output similar to what you see in the
following figure:

Output window showing results of building the HelloWorld
project.

If the build output concludes with the statement BUILD SUCCESSFUL, congratulations!
You have successfully compiled your program!

If the build output concludes with the statement BUILD FAILED, you probably have a
syntax error in your code. Errors are reported in the Output window as hyper-linked
text. You double-click such a hyper-link to navigate to the source of an error. You can
then fix the error and once again choose Run | Build Main Project.

When you build the project, the bytecode file HelloWorldApp.class is generated. You
can see where the new file is generated by opening the Files window and expanding
the Hello World App/build/classes/helloworldapp node as shown in the following
figure.

Files window, showing the generated .class file.

Now that you have built the project, you can run your program.
Run the Program

From the IDE's menu bar, choose Run | Run Main Project.

The next figure shows what you should now see.

The program prints "Hello World!" to the Output window
(along with other output from the build script).

Congratulations! Your program works!

Continuing the Tutorial with the NetBeans IDE

The next few pages of the tutorial will explain the code in this simple application.
After that, the lessons go deeper into core language features and provide many more
examples. Although the rest of the tutorial does not give specific instructions about
using the NetBeans IDE, you can easily use the IDE to write and run the sample code.
The following are some tips on using the IDE and explanations of some IDE behavior
that you are likely to see:

Once you have created a project in the IDE, you can add files to the project
using the New File wizard. Choose File | New File, and then select a template
in the wizard, such as the Empty Java File template.
You can compile and run an individual file (as opposed to a whole project)
using the IDE's Compile File (F9) and Run File (Shift-F6) commands. If you
use the Run Main Project command, the IDE will run the file that the IDE
associates as the main class of the main project. Therefore, if you create an
additional class in your HelloWorldApp project and then try to run that file
with the Run Main Project command, the IDE will run the HelloWorldApp file
instead.
You might want to create separate IDE projects for sample applications that
include more than one source file.
As you are typing in the IDE, a code completion box might periodically appear.
You can either ignore the code completion box and keep typing, or you can
select one of the suggested expressions. If you would prefer not to have the
code completion box automatically appear, you can turn off the feature. Choose
Tools | Options | Editor, click the Code Completion tab and clear the Auto
Popup Completion Window checkbox.
If you want to rename the node for a source file in the Projects window, choose
Refactor from IDE's main menu. The IDE prompts you with the Rename dialog
box to lead you through the options of renaming the class and the updating of
code that refers to that class. Make the changes and click Refactor to apply the
changes. This sequence of clicks might seem unnecessary if you have just a
single class in your project, but it is very useful when your changes affect other
parts of your code in larger projects.
For a more thorough guide to the features of the NetBeans IDE, see
the NetBeans Documentation page.

"Hello World!" for Microsoft Windows

It's time to write your first application! The following instructions are for users of
Windows XP Professional, Windows XP Home, Windows Server 2003, Windows
2000 Professional, and Windows Vista. Instructions for other platforms are in "Hello
World!" for Solaris OS and Linux and "Hello World!" for the NetBeans IDE.

If you encounter problems with the instructions on this page, consult the Common
Problems (and Their Solutions).

A Checklist
o Create a Source File
o Compile the Source File into a .class File
o Run the Program

A Checklist

To write your first program, you'll need:

1. The Java SE Development Kit 6 (JDK 6)

You can download the Windows version now. (Make sure you download
the JDK, not the JRE.) Consult the installation instructions.

2. A text editor

In this example, we'll use Notepad, a simple editor included with the Windows
platforms. You can easily adapt these instructions if you use a different text
editor.

These two items are all you'll need to write your first application.


Your first application, HelloWorldApp, will simply display the greeting "Hello world!".
To create this program, you will:

Create a source file

A source file contains code, written in the Java programming language, that
you and other programmers can understand. You can use any text editor to
create and edit source files.

Compile the source file into a .class file

The Java programming language compiler (javac) takes your source file and
translates its text into instructions that the 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 Java virtual machine to run
your application.
Create a Source File

To create a source file, you have two options:

You can save the file HelloWorldApp.java on your computer and avoid a lot of
typing. Then, you can go straight to Compile the Source File into a .class File.
Or, you can use the following (longer) instructions.

First, start your editor. You can launch the Notepad editor from the Start menu by
selecting Programs > Accessories > Notepad. In a new document, type in the
following code:
/**
*/
class HelloWorldApp {
public static void main(String[] args) {
}
}

Be Careful When You Type

Note: Type all code, commands, and file names exactly as shown. Both the compiler
(javac) and launcher (java) are case-sensitive, so you must capitalize consistently.

HelloWorldApp is not the same as helloworldapp.

Save the code in a file with the name HelloWorldApp.java. To do this in Notepad, first
choose the File > Save As menu item. Then, in the Save As dialog box:

1. Using the Save in combo box, specify the folder (directory) where you'll save
your file. In this example, the directory is java on the C drive.
2. In the File name text field, type "HelloWorldApp.java", including the quotation
marks.
3. From the Save as type combo box, choose Text Documents (*.txt).
4. In the Encoding combo box, leave the encoding as ANSI.

When you're finished, the dialog box should look like this.

The Save As dialog just before you click Save.

Now click Save, and exit Notepad.
Compile the Source File into a .class File

Bring up a shell, or "command," window. You can do this from the Start menu by
choosing Command Prompt (Windows XP), or by choosing Run... and then
entering cmd. The shell window should look similar to the following figure.

A shell window.

The prompt shows your current directory. When you bring up the prompt, your
current directory is usually your home directory for Windows XP (as shown in the
preceding figure.

To compile your source file, change your current directory to the directory where your
file is located. For example, if your source directory is java on the C drive, type the
following command at the prompt and press Enter:
cd C:java

Now the prompt should change to C:java>.

Note: To change to a directory on a different drive, you must type an extra
command: the name of the drive. For example, to change to the java directory on
the Ddrive, you must enter D:, as shown in the following figure.

Changing directory on an alternate drive.

If you enter dir at the prompt, you should see your source file, as the following
figure shows.

Directory listing showing the .java source file.

Now you are ready to compile. At 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, as shown in the following figure.

Directory listing, showing the generated .class file

Now that you have a .class file, you can run your program.

If you encounter problems with the instructions in this step, consult the Common
Run the Program

In the same directory, enter the following command at the prompt:
java HelloWorldApp

The next figure shows what you should now see:

The program prints "Hello World!" to the screen.

Congratulations! Your program works!

If you encounter problems with the instructions in this step, consult the Common

JAVA First Day

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