Where to store class-relationship information?

Question

Assume, we have classes A_1 to A_n and classes B_1 to B_n.

Each A_i has relationships (abstract spoken; a relation could be 'A_4 likes B_2') to a set of B_j's and the same the other way round.

Question: Where is a good place to store the relationship information ? Is there a good pattern for this kind of problem ?

Solution 1: Each A_i has a list field store the B_j's it has a relation with.

Solution 2: Each B_i has a list field store the A_j's it has a relation with.

Solution 1 and 2 are basically the same.

Solution 3: Store the information in a third 'information' class, e.g. as a matrix.

Answer 1

No matter which one you choose between Solution 1 and Solution 2 some information about what a class likes is only stored in another class, which seems a bit backwards.
Therefore, I would definitely go for a separate RelationKeeper class, something like this:

public enum RelationType {
    LIKES,HATES...
}
public class Relation {
   RelationType type;
   Class relatedClass; //Or in what type and form this is stored
}
public class RelationKeeper {
   public List<Relation> getRelationsFor(Class c) {
   }
   public void setRelationsFor(Class c, List<Relation> relations) {
   }
   public void addRelationFor(Class c, Relation relation) {
   }
   ...etc
}

This way you can customize and change the internal storage method to fit your needs, depending on if you want to be able to retrieve only "outgoing" relations of a class or if you also want to be able to fetch "inbound" relations.