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The computer age is here to stay.
Households and businesses all over the world use computers in
one way or another because computers help individuals and
businesses perform a wide range of tasks with speed, accuracy,
and efficiency. Computers can perform all kinds of tasks ranging
from running an animated 3D graphics application with background
sound to calculating the number of vacation days you have coming
to handling the payroll for a Fortune 500 company.
When you want a computer to perform
tasks, you write a program. A program is a sequence of
instructions that define tasks for the computer to execute. This
lesson explains how to write, compile, and run a simple program
written in the JavaTM language
(Java program) that tells your computer to print a one-line
string of text on the console.
But before you can write and compile
programs, you need to understand what the Java platform is, and
set your computer up to run the programs.
A Word About the Java Platform
The Java platform consists of the Java
application programming interfaces (APIs) and the Java1
virtual machine (JVM).
Java APIs are libraries of compiled code that you can use in
your programs. They let you add ready-made and customizable
functionality to save you programming time.
The simple program in this lesson uses a
Java API to print a line of text to the console. The console
printing capability is provided in the API ready for you to use;
you supply the text to be printed.
Java programs are run (or interpreted)
by another program called the Java VM. If you are familiar with
Visual Basic or another interpreted language, this concept is
probably familiar to you. Rather than running directly on the
native operating system, the program is interpreted by the Java
VM for the native operating system. This means that any computer
system with the Java VM installed can run Java programs
regardless of the computer system on which the applications were
originally developed.
For example, a Java program developed on
a Personal Computer (PC) with the Windows NT operating system
should run equally well without modification on a Sun Ultra
workstation with the Solaris operating system, and vice versa.
Setting Up Your Computer
Before you can write and run the simple
Java program in this lesson, you need to install the Java
platform on your computer system.
The Java platform is available free of
charge from the java.sun.com
web site. You can choose between the JavaŽ 2 Platform software
for Windows 95/98/NT or for Solaris. The download page contains
the information you need to install and configure the Java
platform for writing and running Java programs.
Note: Make sure you
have the Java platform installed and configured for your system
before you try to write and run the simple program presented
next.
Writing a Program
The easiest way to write a simple program
is with a text editor. So, using the text editor of your choice,
create a text file with the following text, and be sure to name
the text file ExampleProgram.java. Java programs
are case sensitive, so if you type the code in yourself, pay
particular attention to the capitalization.
//A Very Simple Example
class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
}
Here is the ExampleProgram.java
source code file if you do not want to type the program text in
yourself.
Compiling the Program
A program has to be converted to a form the
Java VM can understand so any computer with a Java VM can
interpret and run the program. Compiling a Java program means
taking the programmer-readable text in your program file (also
called source code) and converting it to bytecodes, which are
platform-independent instructions for the Java VM.
The Java compiler is invoked at the
command line on Unix and DOS shell operating systems as follows:
javac ExampleProgram.java
Note: Part of the
configuration process for setting up the Java platform is
setting the class path. The class path can be set using either
the -classpath option with the javac
compiler command and java interpreter command, or
by setting the CLASSPATH environment variable. You
need to set the class path to point to the directory where the ExampleProgram
class is so the compiler and interpreter commands can find it.
See Java
2 SDK Tools for more information.
Interpreting and Running the Program
Once your program successfully compiles
into Java bytecodes, you can interpret and run applications on
any Java VM, or interpret and run applets in any Web browser
with a Java VM built in such as Netscape or Internet Explorer.
Interpreting and running a Java program means invoking the Java
VM byte code interpreter, which converts the Java byte codes to
platform-dependent machine codes so your computer can understand
and run the program.
The Java interpreter is invoked at the
command line on Unix and DOS shell operating systems as follows:
java ExampleProgram
At the command line, you should see:
I'm a Simple Program
Here is how the entire sequence looks in a
terminal window:
Common Compiler and Interpreter
Problems
If you have trouble compiling or running
the simple example in this lesson, refer to the Common
Compiler and Interpreter Problems lesson in The
Java Tutorial for troubleshooting help.
Code Comments
Code comments are placed in source files to
describe what is happening in the code to someone who might be
reading the file, to comment-out lines of code to isolate the
source of a problem for debugging purposes, or to generate API
documentation. To these ends, the Java language supports three
kinds of comments: double slashes, C-style, and doc comments.
Double Slashes
Double slashes (//) are used
in the C++ programming language, and tell the compiler to treat
everything from the slashes to the end of the line as text.
//A Very Simple Example
class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
}
C-Style Comments
Instead of double slashes, you can use
C-style comments (/* */) to enclose one or more
lines of code to be treated as text.
/* These are
C-style comments
*/
class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
}
Doc Comments
To generate documentation for your program,
use the doc comments (/** */) to enclose lines of
text for the javadoc tool to find. The javadoc
tool locates the doc comments embedded in source files and uses
those comments to generate API documentation.
/** This class displays a text string at
* the console.
*/
class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
}
With one simple class, there is no reason
to generate API documentation. API documentation makes sense
when you have an application made up of a number of complex
classes that need documentation. The tool generates HTML
files (Web pages) that describe the class structures and contain
the text enclosed by doc comments. The javadoc
Home Page has more information on the javadoc
command and its output.
API Documentation
The Java platform installation includes API
Documentation, which describes the APIs available for you to use
in your programs. The files are stored in a doc
directory beneath the directory where you installed the
platform. For example, if the platform is installed in /usr/local/java/jdk1.2,
the API Documentation is in /usr/local/java/jdk1.2/doc/api.
More Information
See Java
2 SDK Tools for more information on setting the class path
and using the javac, and java
commands.
See Common
Compiler and Interpreter Problems lesson in The
Java Tutorial for troubleshooting help.
The javadoc
Home Page has more information on the javadoc
command and its output.
You can also view the API Documentation
for the Java 2 Platform on the java.sun.com
site.
_______
1 As used on this web site, the
terms "Java virtual machine" or "JVM" mean a
virtual machine for the Java platform.
<--BEGIN READER SURVEY-->
Reprinted with permission from
the Java
Developer Connection(SM)
Copyright Sun
Microsystems Inc.
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