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    <p>The classpath is the place(s) where the java compiler (command: javac) and the JVM (command:java) look in order to find classes which your application reference. What does it mean for an application to reference another class ? In simple words it means to use that class somewhere in its code: </p> <p>Example:</p> <pre><code>public class MyClass{ private AnotherClass referenceToAnotherClass; ..... } </code></pre> <p>When you try to compile this (javac) the compiler will need the AnotherClass class. The same when you try to run your application: the JVM will need the AnotherClass class. In order to to find this class the javac and the JVM look in a particular (set of) place(s). Those places are specified by the classpath which on linux is a colon separated list of directories (directories where the javac/JVM should look in order to locate the AnotherClass when they need it). </p> <p>So in order to compile your class and then to run it, you should make sure that the classpath contains the directory containing the AnotherClass class. Then you invoke it like this: </p> <pre><code>javac -classpath "dir1;dir2;path/to/AnotherClass;...;dirN" MyClass.java //to compile it java -classpath "dir1;dir2;path/to/AnotherClass;...;dirN" MyClass //to run it </code></pre> <p>Usually classes come in the form of "bundles" called jar files/libraries. In this case you have to make sure that the jar containing the AnotherClass class is on your classpaht:</p> <pre><code>javac -classpath "dir1;dir2;path/to/jar/containing/AnotherClass;...;dirN" MyClass.java //to compile it java -classpath ".;dir1;dir2;path/to/jar/containing/AnotherClass;...;dirN" MyClass //to run it </code></pre> <p>In the examples above you can see how to compile a class (MyClass.java) located in the working directory and then run the compiled class (Note the "." at the begining of the classpath which stands for current directory). This directory has to be added to the classpath too. Otherwise, the JVM won't be able to find it.</p> <p>If you have your class in a jar file, as you specified in the question, then you have to make sure that jar is in the classpath too , together with the rest of the needed directories. </p> <p>Example: </p> <pre><code>java -classpath ".;dir1;dir2;path/to/jar/containing/AnotherClass;path/to/MyClass/jar...;dirN" MyClass //to run it </code></pre> <p>or more general (assuming some package hierarchy):</p> <pre><code>java -classpath ".;dir1;dir2;path/to/jar/containing/AnotherClass;path/to/MyClass/jar...;dirN" package.subpackage.MyClass //to run it </code></pre> <p>In order to avoid setting the classpath everytime you want to run an application you can define an environment variable called CLASSPATH. </p> <p>In linux, in command prompt: </p> <pre><code>export CLASSPATH="dir1;dir2;path/to/jar/containing/AnotherClass;...;dirN" </code></pre> <p>or edit the ~/.bashrc and add this line somewhere at the end;</p> <p>However, the class path is subject to frequent changes so, you might want to have the classpath set to a core set of dirs, which you need frequently and then extends the classpath each time you need for that session only. Like this: </p> <pre><code>export CLASSPATH=$CLASSPATH:"new directories according to your current needs" </code></pre>
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