private ConcurrentHashMap <String, CustomFile> fileIdMap =
                     new ConcurrentHashMap <String, SwitchConfigurationFile>();

public void importFile () {
    CustomFile file = new CustomFile (generateFileID(), param1, param2, param3, param4);
    fileIdMap.put (file.getID(), file);
}

private String generateFileID () {
   String generatedValue = RandomStringUtils.randomAlphanumeric(5);
   while (fileIdMap.containsKey(generatedValue)) {
       generatedValue = RandomStringUtils.randomAlphanumeric(5);
   }
   //I was thinking here to put the generated value into the Map
   //but at this moment I don't have the CustomFile instance
   //and null values are not allowed
   //maybe:
   //fileIdMap.put (generatedValue, new CustomFile());
   return generatedValue;
}

private String generateFileID () {
   CustomFile file = new CustomFile();
   String generatedValue = RandomStringUtils.randomAlphanumeric(5);
   while (fileIdMap.putIfAbsent(generatedValue, file) != null) {
       generatedValue = RandomStringUtils.randomAlphanumeric(5);
   }
   return generatedValue;
}