Java EE (Java Enterprise Edition) General Discussion

hi,
please help me in understanding the concept of Primary Key.
For each and every entity bean represents a data base row. i think it is correct. if multiple users are accessing the same data base row, there will be different primary key for each and every client. is it correct or not. can we say this as "concurrency"?. if all the clients are manipulating the bean, who manages it either application server or data base server?. why does we must implement both hashCode() and equals() methods?. Provide me the links where i can find the answers.