The Phonebook class is an abstraction of a phonebook. The essence of a phonebook is that it allows you to store contact information about people you know and to retrieve $($and possibly update$)$ this information at a later time. Therefore, a phonebook must maintain a collection of its contacts information that facilitate information storage and retrieval. This collection is a Map that maps each username $($a String object$)$ to its contact information $($an Info object$).$