Writing Java Programs - Beginning Java 8 Fundamentals - page 45

■ the value for CLASSPATH is a list of directories, Jar files, and Zip files that are separated by a path-separator.
the path-separator is operating system dependent. on a Windows operating system, the separator is a semicolon.
on a UniX-like operating systems, for example, Linux, solaris, and mac os X, the separator is a colon (:). on Windows
operating system, a CLASSPATH entry looks like C:\;C:\jbook;C:\jp;c:\myapps.jar . on UniX-like operating systems,
it looks like /usr/jp/classes:/usr/myapps.jar .

Tip

You can also set CLASSPATH by typing the following command on command prompt. However, such CLASSPATH
setting is valid only for the current command prompt session.

C:\>SET CLASSPATH=C:\;C:\jbook;C:\javaprograms

After you have set the CLASSPATH , you can use the following command to run the Welcome class:

C:\>java com.jdojo.intro.Welcome

As I have already discussed, the JVM will convert the class name to a class file, com\jdojo\intro\Welcome.class .
The JVM reads the CLASSPATH value, which it will use to locate the class file. It reads the first entry from the CLASSPATH
value, which is C:\ , and concatenates the converted class name string to it. The JVM may use an extra backslash if
needed to create a valid file name. In your case, the resulting string will be

C:\com\jdojo\intro\Welcome.class

The JVM checks if a file with the above path exists on the machine. In this case, this check will fail, because you
do not have a C:\com\jdojo\intro\Welcome.class file on your machine. If the JVM fails to locate a class file using
an entry in the CLASSPATH , it repeats the class file search using next entry from the CLASSPATH . This process continues
until the JVM locates the class file or it has exhausted all entries in the CLASSPATH . In your case, the JVM will find the
class file in the third attempt. It will find the C:\javaprograms\com\jdojo\intro\Welcome.class file. This is not the
end of the story. Since you passed the class name com.jdojo.intro.Welcome to the JVM, the JVM makes sure that the
C:\javaprograms\com\jdojo\intro\Welcome.class was generated by a Java compiler for a class named Welcome ,
whose package declaration was com.jdojo.intro . In your case, this criterion is fulfilled.
What does the JVM do next? Now, it looks for a method declaration in the Welcome class whose declaration must
look as follows:

public static void main (String[] args)

If the JVM does not find the main method declaration, or the main method is not declared static final, it prints an
error message and aborts the execution. This is the reason why you declared the main method in your Welcome class.
In your case, the JVM will find the correct declaration for the method main. It executes the body of the main method,
which prints the following message on the console:

Welcome to the Java world.

There is another way of setting the CLASSPATH . You can set the CLASSPATH when you run your Java class. You can
use -cp or -classpath option with the java command to set the CLASSPATH while running the class. For example, you
can run the Welcome class as follows. Note that the command is entered on the command line in one line, like.

C:\>java -cp C:\;C:\jbook;C:\javaprograms com.jdojo.intro.Welcome