public class JWhich {
    /**
    * Returns the absolute pathname of the class file
    * containing the specified class name, as prescribed
    * by the current classpath.
    * @param className Name of the class.
    */
    public static String which(String className) {
        // It is necessary to prepend the slash (/). Otherwise, Class.getResource will 
        // prepend the package name of the current class.
        if (!className.startsWith("/")) {
            className = "/" + className;
        }

        className = className.replace('.', '/');
        className = className + ".class";

        java.net.URL classUrl = new JWhich().getClass().getResource(className);

        return (classUrl == null) ? null : classUrl.getFile();
    }

    public static void main(String args[]) {
        if (args.length == 0) {
            System.err.println("Usage: java JWhich <classname>");
            return;
        } 

        String path = JWhich.which(args[0]);
        if (path == null) {
            System.out.printf("Class '%s' not found on classpath\n", args[0]);
        }
        else {
            System.out.printf("Class '%s' found in: %s\n", args[0], path);
        }
    }
}