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copied!<p>Clearly the contents of <code>/usr/share/java/</code> don't automatically get added to the classpath - it's just a common location where APT packages put Java libraries. It's up the developer to reference them correctly.</p> <p>JARs in the <code>ext/</code> subdirectory of a Java installation do get added to the classpath automatically. However, <strong>do not</strong> put your own JARs in there. It's a terrible practice because it doesn't match how Java apps are deployed "in the real world".</p> <p>The correct way is using the <code>-cp</code> parameter explicitly <strong>when compiling AND running</strong> your app. Java doesn't compile library code into your <code>.class</code> files, a <code>.class</code> file only refers to names of other classes which are then loaded as-needed from the class path when your app runs. The <code>-cp</code> parameter takes only <code>.jar</code> files, or directories with <code>.class</code> files in them. You can also use wildcards in the value of that parameter. For more information on wrangling the class path, check the <a href="http://docs.oracle.com/javase/7/docs/technotes/tools/windows/classpath.html" rel="nofollow">tool documentation on setting the class path</a>. </p> <p>You using a build tool that sets it for you automatically, like an IDE or <a href="https://maven.apache.org/" rel="nofollow">Maven</a> or another build system with dependency management. (<a href="http://www.gradle.org/" rel="nofollow">Gradle</a> or <a href="https://ant.apache.org/ivy/" rel="nofollow">Ant+Ivy</a>.) If you're writing a Java app that uses third party libraries, I <em>very strongly</em> suggest you learn and use one of those. (Also, most IDEs can work with Maven's configuration files letting you use the same build settings in a team with people using mixed or no IDEs.) Generally if you're invoking a compiler directly you're not doing it right.</p>

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