Objects and classes - Data Structures and Problem Solving Using Java - page 94

An example of how the package statement is used is shown in Figure 3.16,
where we move the BigRational class to a new package, weiss.math .

3.8.3 the CLASSPATH environment variable

Packages are searched for in locations that are named in the CLASSPATH vari-
able. What does this mean? Here are possible settings for CLASSPATH , first for a
Windows system and second for a Unix system:

The CLASSPATH vari-
able specifies files
and directories that
should be searched
to find classes.

SET CLASSPATH=.;C:\bookcode\
setenv CLASSPATH .:$HOME/bookcode/

In both cases, the CLASSPATH variable lists directories (or jar files 2 ) that
contain the package's class files. For instance, if your CLASSPATH is corrupted,
you will not be able to run even the most trivial program because the current
directory will not be found.

A class in package p must be in a directory p that will be found by
searching through the CLASSPATH list; each . in the package name repre-
sents a subdirectory. Starting with Java 1.2, the current directory (directory . )
is always scanned if CLASSPATH is not set at all, so if you are working
from a single main directory, you can simply create subdirectories in it
and not set CLASSPATH . Most likely, however, you'll want to create a sep-
arate Java subdirectory and then create package subdirectories in
there. You would then augment the CLASSPATH variable to include . and
the Java subdirectory. This was done in the previous Unix declaration
when we added $HOME/bookcode/ to the CLASSPATH . Inside the bookcode direc-
tory, you create a subdirectory named weiss , and in that subdirectory, math ,
util and nonstandard . In the math subdirectory, you place the code for the
BigRational class.
An application, written in any directory at all, can then use the BigRational
class either by its full name

A class in package
p must be in a
directory p that will
be found by
searching through
the CLASSPATH list.

weiss.math.BigRational;

or simply using BigRational , if an appropriate import directive is provided.
Moving a class from one package to another can be tedious because it
may involve revising a sequence of import directives. Many development tools
will do this automatically, as one of the options for refactoring .

2. A jar file is basically a compressed archive (like a zip file) with extra files containing Java-
specific information. The jar tool, supplied with the JDK, can be used to create and expand
jar files.