HashMap multiple keys to one element

Question

I couldn't think of a better title to the problem but my question isn't really with mapping two different keys to one value. I know how to do that. My question is, is there a way to have a specific key-key pair map to a value?

For example, I'm trying to implement an online grading system. This database has the exams keyed by their examId. "Midterm 1" maps to Midterm1 exam instance. There is also a solution manual for every exam. Then I want to add a student's answer sheet to the database and compare the student's answer to the solutions manual to calculate the score.

I'm implementing the AnswerSheet class and am stumped by this problem. A student's answer corresponds to a certain problem on a certain exam. Is there a way to have examId and questionNumber (combined) be a key that maps to the answer the student wrote down? It wouldn't be enough to just map the question number to the answer as question 1 on midterm 1 is not the same as question 1 on midterm 2. I am trying to think of a way to have this:

exam1 question1 ---------> "Student Answer for exam 1"

exam1 question5 ---------> "Student Answer for exam 1"

exam2 question1 ---------> "Student Answer for exam 2"

I hope I have adequately explained my conundrum. Hopefully someone can point me in the right direction.

@khuong291 Could you elaborate on that? I still want to preserve the mapping aspect of it for later use in retrieving the respective answer sheet and comparing answers. — move_slow_break_things
– move_slow_break_things, Commented Nov 7, 2015 at 5:57
It sounds like what you really want is a list of lists or map of lists, i.e., Map<List<Answer>>. Numbered answers don't need to be mapped, just retrieved by index. — Kevin Krumwiede
– Kevin Krumwiede, Commented Nov 7, 2015 at 6:04

chrisl08 · Accepted Answer · 2015-11-07 05:58:02Z

1

Assuming that "exam", "question" and "Student Answer" are objects mapped to the database through your ORM, and they each have a primary key, you can concatenate the 2 primary keys of "exam" and "question" into a string value, and use this value to store "Student Answer". Something like this:

//built a hash key using primary keys of objects
String hkey= "EXAM:" + exam.examId().toString() + 
             "-QST:" + question.questionId().toString();

map.put(hkey, studentAnswer);

Note that you need the prefixes "EXAM:" and "QST:" just in case you have primary key number collisions.

answered Nov 7, 2015 at 5:58

chrisl08

1,6581 gold badge16 silver badges25 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

move_slow_break_things Over a year ago

Yes. That's a good idea. Ultimately, I think what I really wanted was a way to "stamp" every answer sheet with the exam ID. The answer sheet itself would be mapping question numbers to their answers. The exam ID's only use here is to differentiate one map from the other and I wonder if there's a way to achieve that functionality. Thanks for your response though.

Ken Geis · Accepted Answer · 2015-11-07 07:54:54Z

0

Check out the class Table in Guava.

A collection that associates an ordered pair of keys, called a row key and a column key, with a single value.

If you don't want to use Guava, you can just use a List (such as ArrayList) to contain the two keys and then use that composite key to access your map. See List.equals(..) and List.hashcode() to see that they do the right thing to enable this.

answered Nov 7, 2015 at 7:54

Ken Geis

9226 silver badges17 bronze badges

Collectives™ on Stack Overflow

HashMap multiple keys to one element

2 Answers 2

1 Comment

Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Related