* Import needed libraries, com.twmacinta.util.MD5 is for hashing user password.
*/
publicclassManager {
/**
* Initialize allUsers: An Arraylist to keep all users [username, hashed password, name, lastname, privilege]
* Initialize allBooks: An Arraylist to keep all books info = [name, author, currentAddress, oldAddress, recommendedTimes], comments = [], usernameWhoCommented = [] > all in order.
* Initialize username: The username of user who logins to system.
* Initialize loginForm: LoginFrom object > to show login form.