I have a question for yesterday's lab. 

THIS IS THE ANSWER CLASS

public class Answer {

    Integer     id;

    String      content;
    User        author;
    Date        timestamp;

    Set<Vote>   votes;

}

THIS IS THE ANSWERS TABLE

create table answers (
    id          integer primary key,
    content     text,
    author_id   integer,
    timestamp   timestamp,
    question_id integer references questions(id)
);

QUESTION IS:  Why isn't there a reference to the author_id?  Isn't it class type?  
              Shouldn't it be, author_id integer references users(id)?  

Thank you.

Thank you Professor.  Also one more question

THIS IS THE VOTE CLASS

public class Vote {

    Answer      answer;
    User        voter;
    int         value;

}

THIS IS THE VOTES TABLE

create table votes (
    answer_id   integer references answers(id),
    voter_id    integer references users(id),
    value       integer,
  primary key(answer_id, voter_id)
);
 
QUESTION IS: SHOULDN'T THERE BE ANOTHER attribute called answer_id references answers(id)?  
In the table, the answer_id references answers(id) because it was a list created in a new table.  The voter_id 
references the user, and the value is an integer.  But what about the Answer answer; in the VOTE CLASS? 
It's missing from the table?  Also, how do you know that both answer_id, and voter_id should be primary keys?

Sorry if I'm asking too many questions.