Using a package stored in arbitrary directories.

The Classpath

The Classpath:

Classpath = is a parameter (variable) that tells the Java Compiler (javac) and Java Virtual Machine (java) where to look for user-defined classes and packages.
The class path is also known as the search path

Advantage of using the class path:

Without using the class path variable, the Java program and the packages used by the Java program must reside in the same directory.
Illustrated:
Using the class path variable, we can use a package stored in a arbitrary directory.
Illustrated:

Content of the Classpath variable

Content of CLASSPATH:

Example:

Suppose we have 3 packages: pack1, pack2 and pack3 stored in the following manner:
The directory paths that contains the directory of packages are:

The search path (CLASSPATH) that we need to use to tell Java the location of the packages pack1, pack2 and pack3 is:

".:Dir1:Dir2"

Notes:

There are 3 search paths specified in the above CLASSPATH:
The different paths are separated by a colon (:)
Make a habit to always provide the current directory (.) as one of the class path....

Specifying the seach path (classpath) as a command line option

Syntax to specify a search path (classpath) as a command line option:

javac -cp "SEARCH-PATH" myProgram.java

Example:

Suppose we put the ClassA.java (that is inside package myLib) under /home/cs170000/libs:
(Use your own home directory if you want to try, and replace cs170000 with your own id)
Do the following to save ClassA.java:

Compile ClassA.java:

cd /home/cs170000/libs javac myLib/ClassA.java

The official name of this class is: myLib.ClassA

Now save the myProg.java (that uses the class ClassA inside package myLib) in any directory (I will use /home/cs170000/demo )
You now have the following situation:
To compile the myProg.java program, we must tell the Java Compiler to search in classpath /home/cs170000/libs (to find myLib.ClassA):

Executing a program that uses a class that is stored inside a package

The Java Virtual Machine also needs to know the class path where the user packages are stored
(So that it can load the compile code)

Example:

cd /home/cs170000/demo (directory containing "myProg,java") java -cp ".:/home/cs170000/libs" myProg

Output:

This is PUBLIC Method81() of ClassA in PACKAGE myLib

Example Program: (Demo above code)
- The ClassA.java Prog file: click here
- The myProg.java Prog file: click here
How to run the program:

Specifying the search path using the environment variable CLASSPATH

The Java compiler and the Java Virtual Machine can obtain the search path from the "environment variable" CLASSPATH
Caveat:

Setting the environment variable CLASSPATH:

In UNIX (ksh and bash)
In Windows, travel to the CLASSPATH variable using this sequence of menus:

Example: (running myProg using CLASSPATH setting)

1. export CLASSPATH=".:/home/cs170000/libs"; 2. cd /home/cs170000/demo (the directory containing myProg) 3. Compile: javac myProg.java 4. Run: java myProg

The default search path

Default search path used by Java:

The default value of the class path is ".", meaning that only the current directory is searched.
So, if you do not define the CLASSPATH variable, Java will use:

In the previous webpage, we have taken advantage of the fact that "." (current directory) is the default search path by:

Example:

(Put the program myProg.java inside /home/cs170000/demo - should be there already)
Move the directory /home/cs170000/libs/myLib into /home/cs170000/demo:
Resulting situation:
(This is exactly what we used before in the previous webpage !!!)
Undefine the CLASSPATH variable by executing this command:
Compile amd run:
This is the original setup used and it will only work when the packages are inside the working directory (/home/cs170000/demo) because the default search path = ".".