protected char[] getCacheFileName(String key) 
{
                             
        if ((key == null) || (key.length() == 0)) {
            throw new IllegalArgumentException("Invalid key '" + key + "' specified to getCacheFile.");
        }
        char[] chars = key.toCharArray();
        StringBuffer sb = new StringBuffer(chars.length + 8);
        for (int i = 0; i <  chars.length; i++) {
            char c = chars[i];
            int pos = CHARS_TO_CONVERT.indexOf(c);
            if (pos  >= 0) {
                sb.append('_");
                sb.append(i);
            } else {
                sb.append(c);
            }
        }
        char[] fileChars = new char[sb.length()];
        sb.getChars(0, fileChars.length, fileChars, 0);
        return fileChars;
}

Build cache file name for the specified cache entry key.