How to monitor file modifications and know what changes were made

I'm working on a Java project where I need to monitor files in a certain directory and be notified whenever changes are made on one of the files, this can be achieved using WatchService. Furthermore, I want to know what changes were made, for example: "characters 10 to 15 where removed", "at index 13 characters 'abcd' were added"... I'm willing to take any solution even based on c language monitiring the fileSystem. I also want to avoid the diff solution to avoid storing the same file 2 times, and for the complexity of the algorithm, it takes to much time for big files. Thank you for help. :)