Question

I am not sure what data structure I should use to solve it,
I have a list of categories and list of values that are associated to them as following:

Categories           Values 
Smart             **person1**,person2
Handsome          person3,person6,**person1**
Hard-Working      person7,person9

For example, person1 is Smart and Handsome because he is in both Handsome and Smart categories. (a value(person) can be in many categories) and person2 is Smart but is not Handsome.

person object has name,age and gender for example person 1 is

John 43 Male

I am wondering how I can design it, I was thinking of a two-dimention array but it is not a good idea as I would have some spare places

Cat1 v3,v6 Spare Spare
Cat2 v10 Spare Spare Spare
cat4 v6,v7,v8,v9

I suppose the best is to have a list but not sure how to find out which value is for which category or categories ( like V6 in above example that is associated to both cat1 and cat4)

 V3 v6 v7 v8 v9 v10

Answer 1

You could use something like

Map<Category, List<Person>>

Since Map should provide O(1) access, you want your key to be what you use to look up the most (Category in your case, from what I gather). So if you have

Map<Category, List<Person>> categoryLookup = new HashMap<Category, ArrayList<Person>>();

you can do

List<Person> values = myCategoryLookup.get(someCategory);

and then iterate over the values. References to the same value can be in multiple lists — that is not a problem.

For Yameo below, if you want to get categories for a person, you can either

1. have Person maintain a list of its categories
2. “reverse” the datastructure above; i.e. also maintain a Map<Person, List<Category>>

The alternative to these solutions is to set the relationships in the Category and Person classes themselves. For example, Person would have a List<Category> as one of its class properties, and similarly Category would have a List<Person> as one of its class properties. But I get the sense this is more a data-structures question. This what you would probably do if you had a database that contains the actual data and were going to use an ORM tool to map the data to Java classes.