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1.1 Hello World
As long ago as 1978, Brian
Kernighan and Dennis Ritchie wrote, in their classic book
The C Programming Language, that
"the first program to write is the same for all
languages." They were referring, of course, to the
"Hello World" program. The Java
implementation of Hello World is shown in Example 1-1.
Example 1-1. Hello.java
package je3.basics; // A unique class name prefix
public class Hello { // Everything in Java is a class
public static void main(String[ ] args)
{ // All programs must have main( )
System.out.println("Hello World!"); // Say hello!
} // This marks the end of main( )
} // Marks the end of the class
The first line of this program is the package
declaration. It specifies the name of the
package of which this program is part. The
program''s name (as we''ll see in the
second line) is Hello. The package name is
je3.basics. We can combine these two names to
produce a fully qualified name, je3.basics.Hello.
Using packages provides a unique namespace for every Java program. By
placing this Hello program in a package,
I''ve helped to ensure that no naming conflict will
arise if someone else defines a program that is also named
Hello. Each chapter of this book has its own
package that begins with the prefix je3 (for
JavaExamples3). In this case, since this is the basics chapter, the
package name is je3.basics.[1]
[1] In the
second edition of this book, I was more rigorous about preventing
naming conflicts: I created the package name by reversing my Internet
domain name, a procedure that should guarantee that there will be no
naming conflicts. The resulting package name for this chapter was
com.davidflanagan.examples.basics. For this
edition, I''ve settled on a less rigorous but much
easier to type alternative.
The value of "Hello
World" is that it is a template that you can expand
on in your later experiments with Java. The second and third lines of
Example 1-1 are a required part of the template.
Every programevery piece of Java code, reallyyou write
is a class. The second line of the example says that
we''re writing a class named
Hello. It also says the class is
public, which means it can be used by anyone.Every
standalone Java program requires a main( ) method.
This is where the Java interpreter begins running a Java program. The
third line of the example declares this main( )
method. It says that the method is public, that it
has no return value (i.e., its return value is
void), and that it is passed an array of strings
as its argument. The name of the array is args.
The line also says that main( ) is a
static method. (In this chapter, we work
exclusively with static methods. In Chapter 2, when we start working with objects,
you''ll learn what a static method
is, and you''ll see that nonstatic
methods are actually the norm.)In any case, you might as well go ahead and memorize this line:
public static void main(String[ ] args)
Every standalone Java program you ever write contains a line that
looks exactly like this one. (Actually, you can name the array of
strings anything you want, but it is usually called
args.)The fifth and sixth lines of Example 1-1 simply mark the end of the main(
) method and of the Hello class. Like
most modern programming languages, Java is a block-structured
language. This means that such things as classes and methods have
bodies that comprise a "block" of
code. In Java, the beginning of a block is marked by a
{, and the end is marked by a matching
}. Method blocks are always defined within class
blocks, and as we''ll see in later examples, method
blocks can contain such things as if statements
and for loops that form subblocks within the
method. Furthermore, these sorts of statement blocks can be nested
arbitrarily deep within each other.The first three
lines and the last two lines of Example 1-1 are part
of the basic framework of a Java application. It is the fourth line
of the example that is of primary interest to us. This is the line
that prints the words "Hello
World!" The System.out.println( )
method sends a line of output to "standard
output," which is usually the screen. This method is
used throughout this chapter and in many other chapters in this book.
It isn''t until Chapter 3,
however, that you''ll really understand what it is
doing. If you are curious before then, look up the
java.lang.System and
java.io.PrintStream classes in Java in a
Nutshell (or some other Java reference manual).One final point to note about this program is the use of comments.
Example 1-1 uses C++-style comments that begin with
// and continue until the end of the line. Thus,
anything between the // characters and the end of
a line is ignored by the Java compiler. You''ll find
that the examples in this book are thoroughly commented. The code and
the comments are worth studying because the comments often draw your
attention to points that are not mentioned in the main text of the
book.
1.1.1 Running Hello World
The first step in running our program is to type it in.[2] Using a text editor, type in the Hello
program as shown in Example 1-1. For now, however,
omit the package declaration on the first line.
Save the program in a file named Hello.java.
[2] Although this example is included in the online example
archive, I''m suggesting that you type it in so that
you start imprinting basic Java idioms in your brain.
I''m also going to have you modify the example, in
order to explain certain aspects of running the program.
The second step is to compile the
program. If you are using the Java Software Development Kit (SDK)
from Sun, you compile code with the javac
command.[3]
cd to the directory that contains your
Hello.java file, and type this command (assuming
that javac is in your path):
[3] If you are using some other Java programming
environment, read and follow the vendor''s
instructions for compiling and running programs.
% javac Hello.java
If the Java SDK has been properly installed,
javac runs for a short while and then produces a
file named Hello.class. This file contains the
compiled version of the program. As I said earlier, everything you
write in Java is a class, as the .class
extension on this file indicates. One important rule about compiling
Java programs is that the name of the file minus the
.java extension must match the name of the class
defined in the file. Thus, if you typed in Example 1-1 and saved it in a file named
HelloWorld.java, you would not be able to
compile it. To run the program (again using the Java SDK) type:
% java Hello
This command should produce the output:
Hello World!
The java command
is the Java interpreter; it runs the Java Virtual Machine. You pass
java the name of the class that you want to run.
Note that you are specifying the class name,
Hello, not the name of the file,
Hello.class, that contains the compiled class.The previous steps have shown you how to compile and run Java
programs that don''t have package
declarations. If you omitted the package
declaration when you typed in Hello.java, these
instructions should have worked for you (if they
didn''t, check that you typed the program in
correctly). In practice, however, all nontrivial Java programs
(including the examples in this book) do have
package declarations. Using packages makes
compiling and running Java programs a bit more complicated. As I just
noted, a Java program must be saved in a file that has a name that
matches the class name. When a class is in a package, there is a
further requirement that the class be saved in a directory that
matches the name of the package.
Go ahead and reinsert the package declaration into
Hello.java:
package je3.basics;
Now make yourself a new directory (or folder) in which
you''ll do all your work with the examples from this
book. For example, on a Windows system, you might create a folder
named c:\Examples. On a Linux system, you might
use ~/Examples. Within this directory, create a
subdirectory named je3. Then create a
subdirectory of je3 named
basics. Now copy your
Hello.java program (with the package
declaration) into this directory. On a Windows system, the resulting
file might be:
c:\Examples\je3\basics\Hello.java
After you''ve created the directory structure and put
your Java program in it, the next step is to tell the Java compiler
and interpreter where to find it. The compiler and interpreter simply
need to know the base directory you''ve chosen; they
will look for the Hello.class file in
subdirectories of this base directory, based on the package name. To
tell Java where to look, you have to set the
CLASSPATH environment variable in the manner
appropriate for your operating system. If you used the suggested name
for your base directory on a Windows system
(c:\Examples), you can use a command like the
following:
C:\> set CLASSPATH=.;c:\Examples
This tells Java to look first for classes in the current directory
(.), followed by the
c:\Examples directory.
On a Unix system using the csh shell, you can
use the following command:
% setenv CLASSPATH .:/home/david/Examples
With the sh or bash shell,
the command is:
$ CLASSPATH=.:/home/david/Examples; export CLASSPATH
You may want to automate this process by setting
CLASSPATH in a startup file, such as
autoexec.bat on Windows systems or
.cshrc on Unix systems (under
csh). With your CLASSPATH set, you can now go ahead and
compile and run the Hello program. To compile,
change directories to the examples/basics
directory that holds Hello.java. Compile the
program as before:
% javac Hello.java
This creates the Hello.class file.To run the program, you invoke the Java interpreter as before, but
now you must specify the fully qualified name of the program, so that
the interpreter knows exactly which program you want to run:
% java je3.basics.Hello
Because you''ve set the CLASSPATH,
you can run the Java interpreter from any directory on your system,
and it will always find the correct program. If you get tired of
typing such long class names, you may want to write yourself a batch
file or shell script that automates the process for you.
Note that all Java programs are compiled and run in this way, so we
won''t go through these individual steps again. Of
course, one step you don''t have to repeat is typing
in all the examples. You can download the example source code from
http://www.davidflanagan.com/javaexamples3.
