For librarian login : username : 1 passowrd : 1 Using data saved in binary files, librarian can create a bill with the requests of the client which is then printed in .txt format
For Manager login: username : 1 password : 2 Manager can check stock books, supply them based on their category and/or add a new category or book, he can also check the statistics of librarians like: number of bills, books sold, money made in a day/month/year, finally the manager can also check the stats of each book meaning that it will show when a book was bought or sold based on day/month/year/total format
For administrator login: username : 1 password : 3 Administrator can check all employess, librarians and managers, editing their data and/or adding new employers, each field of the employees go through regex checkers (which are at Librarian.java), he can check the same stats of each book as manager but also the total income of the library and costs.