Java: How to bind values from two different arrays

Question

I have a list of usernames and one of nodes. I need a data structure to hold information about what usernames are on a given node. I would need something like HashMap<String, ArrayList<String> but I also need to perform operation on the whole username list in an easy (not on a bunch of different arrays). Does that data structure exists or I need to hold twoArrayList and an HashMap?

Answer 1

HashMap<String, ArrayList<String> seems like a reasonable solution to me. I don't see any need for custom structures here.

For the operations with usernames, you can use .keySet() or .entrySet() (or any method of iteration).

See How to efficiently iterate over each Entry in a Map.

Answer 2

I like your original thought about using a Map. I think a Map would be a good start for this problem.

Are all the usernames guaranteed to be unique? If so, I would use a Set instead of a List to represent the usernames. Maybe you could first transfer all usernames in the list to a Set<Usernames>, then you could iterate through the node/username lists to generate your Map<Node, Set<Usernames>>.

You would to keep track of the Set<Usernames> and a Map<Node, Set<Usernames>>, but I think it would store the right data and you could do fast lookup operations on the username set.

Answer 3

If something doesn't exist in Java you can try to create on your own:

Node:

public class Node {
    List<User> users = new ArrayList<>();

    public List<User> getUsers() {
        return users;
    }

    public void setUsers(List<User> users) {
        this.users = users;
    }
}

User:

public class User {
    String username;
    String name;
    String surname;
    // ...
}

You can just use List<String> usernames in your Node, if you don't need a complex object.

Then, somewhere in your app:

public List<User> getUsersOfNode(Node node) {
    return node.getUsers();
}

Operating with this is easier than with HashMap.

Answer 4

Why don't you create an object for that, with 3 private fields (two lists and one map)?