Organizing Java files and other practicalities - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 147

as the compiler ( javac.exe ) and JVM ( java.exe ), that you use to develop
and run Java programs. The jre (Java Runtime Environment) directories contain
the files needed to run Java programs but not the development tools such as the
compiler. The jre would be distributed with a program intended for users who do
not need the development tools. The include directory contains files for linking
Java programs to C or C

code (see Chapter 22), and a set of demonstration
programs come in the demo directory.
Many of the Java tools such as the JVM ( java.exe )build on a core in native
code but also use Java classes for higher level tasks. The lib directories contain
the JAR files with these classes.
On most platforms, a PATH environment variable indicates the location of the
Java executables. For example, the path on a Windows platform might look like:

++

PATH=C: \ WINDOWS;C: \ WINDOWS \ COMMAND;c: \ java \ jdk1.5.0 \ bin;

Here the c: \ java \ jdk1.5.0 \ bin directory contains the javac , java ,
appletviewer , and other executables. Java programs look relative to this direc-
tory for rt.jar ,which contains most of the core runtime language classes, and
other JAR files.
The compiler and JVM by default look for user class files in the directory of the
application or applet and its subdirectories (according to the package names). To
find packages in other directories, you can enter the directories in the CLASSPATH
environment variable or use the -cp or -classpath notation shown above. For
example, suppose that the file Bessel.class is located in

c: \ programs \ mymath \ functions \ Bessel.class

Then setting the CLASSPATH environment variable with

c: \ > set CLASSPATH c: \ programs;

provides the location of Bessel.class when it is needed, as in the following
subclass definition:

public class MyNewBessel extends mymath.functions.Bessel
{...}

Similarly, when an import statement indicates to the compiler what packages
to search for a class, the CLASSPATH directs it to the proper directories, as in

import myjava.functions.*;
public class MyNewBessel extends Bessel{..

The CLASSPATH can also be set on the command line:

c: \ > javac -classpath c: \ programs MyNewBessel

for compilation and

c: \ > java -cp c: \ programs MyNewBessel