Setting Class path in java

            PATH and CLASSPATH

This section explains how to use the PATHand CLASSPATHenvironment variables on Microsoft Windows, Solaris, and Linux. Consult the installation instructions included with your installation of the Java Development Kit (JDK) software bundle for current information.
After installing the software, the JDK directory will have the structure shown below.

The bindirectory contains both the compiler and the launcher.

Update the PATH Environment Variable (Microsoft Windows)

You can run Java applications just fine without setting the PATHenvironment variable. Or, you can optionally set it as a convenience.
Set the PATHenvironment variable if you want to be able to conveniently run the executables (javac.exe, java.exe, javadoc.exe, and so on) from any directory without having to type the full path of the command. If you do not set the PATHvariable, you need to specify the full path to the executable every time you run it, such as:

C:\Java\jdk1.7.0\bin\javac MyClass.java

The PATHenvironment variable is a series of directories separated by semicolons (;). Microsoft Windows looks for programs in the PATHdirectories in order, from left to right. You should have only one bindirectory for the JDK in the path at a time (those following the first are ignored), so if one is already present, you can update that particular entry.
The following is an example of a PATHenvironment variable:

C:\Java\jdk1.7.0\bin;C:\Windows\System32\;C:\Windows\;C:\Windows\System32\Wbem

It is useful to set the PATHenvironment variable permanently so it will persist after rebooting. To make a permanent change to the PATHvariable, use the System icon in the Control Panel. The precise procedure varies depending on the version of Windows:
Windows XP

1. Select Start, select Control Panel. double click System, and select the Advanced tab.
2. Click Environment Variables. In the section System Variables, find the PATHenvironment variable and select it. Click Edit. If the PATHenvironment variable does not exist, click New.
3. In the Edit System Variable (or New System Variable) window, specify the value of the PATHenvironment variable. Click OK. Close all remaining windows by clicking OK.

Windows Vista:

1. From the desktop, right click the My Computer icon.
2. Choose Properties from the context menu.
3. Click the Advanced tab (Advanced system settings link in Vista).
4. Click Environment Variables. In the section System Variables, find the PATHenvironment variable and select it. Click Edit. If the PATHenvironment variable does not exist, click New.
5. In the Edit System Variable (or New System Variable) window, specify the value of the PATHenvironment variable. Click OK. Close all remaining windows by clicking OK.

Windows 7:

1. From the desktop, right click the Computer icon.
2. Choose Properties from the context menu.
3. Click the Advanced system settings link.
4. Click Environment Variables. In the section System Variables, find the PATHenvironment variable and select it. Click Edit. If the PATHenvironment variable does not exist, click New.
5. In the Edit System Variable (or New System Variable) window, specify the value of the PATHenvironment variable. Click OK. Close all remaining windows by clicking OK.

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Note: You may see a PATHenvironment variable similar to the following when editing it from the Control Panel:

%JAVA_HOME%\bin;%SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem

Variables enclosed in percentage signs (%) are existing environment variables. If one of these variables is listed in the Environment Variables window from the Control Panel (such as JAVA_HOME), then you can edit its value. If it does not appear, then it is a special environment variable that the operating system has defined. For example, SystemRootis the location of the Microsoft Windows system folder. To obtain the value of a environment variable, enter the following at a command prompt. (This example obtains the value of the SystemRootenvironment variable):

echo %SystemRoot%

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Update the PATH Variable (Solaris and Linux)

You can run the JDK just fine without setting the PATHvariable, or you can optionally set it as a convenience. However, you should set the path variable if you want to be able to run the executables (javac, java, javadoc, and so on) from any directory without having to type the full path of the command. If you do not set the PATHvariable, you need to specify the full path to the executable every time you run it, such as:

% /usr/local/jdk1.7.0/bin/javac MyClass.java

To find out if the path is properly set, execute:

% java -version

This will print the version of the javatool, if it can find it. If the version is old or you get the error java: Command not found, then the path is not properly set.
To set the path permanently, set the path in your startup file.
For C shell (csh), edit the startup file (~/.cshrc):

set path=(/usr/local/jdk1.7.0/bin )

For bash, edit the startup file (~/.bashrc):

PATH=/usr/local/jdk1.7.0/bin:
export PATH

For ksh, the startup file is named by the environment variable, ENV. To set the path:

PATH=/usr/local/jdk1.7.0/bin:
export PATH

For sh, edit the profile file (~/.profile):

PATH=/usr/local/jdk1.7.0/bin:
export PATH

Then load the startup file and verify that the path is set by repeating the javacommand:
For C shell (csh):

% source ~/.cshrc
% java -version

For ksh, bash, or sh:

% . /.profile
% java -version

Checking the CLASSPATH variable (All platforms)

The CLASSPATHvariable is one way to tell applications, including the JDK tools, where to look for user classes. (Classes that are part of the JRE, JDK platform, and extensions should be defined through other means, such as the bootstrap class path or the extensions directory.)
The preferred way to specify the class path is by using the -cpcommand line switch. This allows the CLASSPATHto be set individually for each application without affecting other applications. Setting the CLASSPATHcan be tricky and should be performed with care.
The default value of the class path is ".", meaning that only the current directory is searched. Specifying either the CLASSPATH variable or the -cpcommand line switch overrides this value.
To check whether CLASSPATHis set on Microsoft Windows NT/2000/XP, execute the following:

C:> echo %CLASSPATH%

On Solaris or Linux, execute the following:

% echo $CLASSPATH

If CLASSPATHis not set you will get a CLASSPATH: Undefined variable error (Solaris or Linux) or simply %CLASSPATH% (Microsoft Windows NT/2000/XP).
To modify the CLASSPATH, use the same procedure you used for the PATHvariable.
Class path wildcards allow you to include an entire directory of .jarfiles in the class path without explicitly naming them individually. For more information, including an explanation of class path wildcards, and a detailed description on how to clean up the CLASSPATHenvironment variable, see the Setting the Class Path technical note.

Step 0 - Install the predefined jdk .

which is described in the installation procedure

Step 1 - Start the Control Panel

Click the Windows Start button and then Control Panel in the box which appears.

Step 2 - Click on Performance and Maintenance

Step 3 - Click on System

Step 4- Click on the Advanced tab

Step 5- Click on Environment Variables

Step 6 - Create a new user environment variable

The dialog box below has an area in the top half for user defined environment variables. The bottom half is for maintaining and viewing the values of Windows system environment variables. Click on the upper New to define a new variable, namely the classpath.

Step 7 - Create classpath
 

Step 8 - Add another path to the value of classpath

Step 9 - Add one more path to the value of classpath