/**
     * Initialises cache file member variable if not already initialised.
     */
    private void initCacheFile() {
        if (cacheFile == null) {
            if (cacheFileName == null || cacheFileName.equals(""))
            {    /* Old Code that broke under Linux
                 * cacheFileName = System.getProperty("java.io.tmpdir")
                 *       + "ExchangeRates.xml";
                 */

                /*
                 * The name of the cache file is saved as a part of the package
                 * Preferences.  This code checks to see if a name has been saved.
                 * If not a new File is created in the temp directory identified
                 * by java.util.tmpdir.  The Preferences are then updated.
                 */
                 if (prefs.get("cacheFileName", "").equals(""))
                 {
                    cacheFile = new File(System.getProperty("java.util.tmpdir",
                        "ExchangeRates.xml"));
                    prefs.put("cacheFileName", cacheFile.getName());
                 }
                else {
                    cacheFile = new File(prefs.get("cacheFileName",""));
                }

          }
        }
        cacheFileName = cacheFile.getName();
    }