use hashmap instead of arraylist

Question

I have two Classes. one DAO that has an Arrraylist of Users (user is instace of User class) and one method in another class that checks if there is a user with our input name or not How can I use Hashmap (usernames, User) instead of Arraylist of Users?

public class UserDAO {
private static UserDAO instance;
private static String fileName = "sources/users.aaa";
//--------------------------------------------------------
private UserDAO(){

}
//--------------------------------------------------------
public boolean storeUsers(ArrayList<User> Users){
    return IOFile.writeObject(Users, fileName);
}

//--------------------------------------------------------
public ArrayList<User> getUsers(){
    ArrayList<User> Users = (ArrayList<User>) IOFile.readObject(fileName);
    return Users;
}
//--------------------------------------------------------
public static UserDAO getInstance(){
    if(instance == null)
        instance = new UserDAO();
    return instance;
}

}

and one method in another class that checks if there is a user with our input name or not:

User user = new User(firstName, lastName, userName, password);
            ArrayList<User> users =  UserDAO.getInstance().getUsers();
            for (User user2 : users) {

                if (user.equals(user2)){
                    system.out.println ("Error!!");
                    return;
                }
            }
            users.add(user);
            UserDAO.getInstance().storeUsers(users);

If you want map one Object to another then use HashMap. But to know what it is like you may cover my tutorial on HashMap at first — Volodymyr Levytskyi
– Volodymyr Levytskyi, Commented Jul 25, 2013 at 11:10

Jeff Storey · Accepted Answer · 2013-07-24 21:34:33Z

2

In this case, since you're just trying to check if the user exists, you could use a HashSet. A set has constant time lookups. So instead of your loop, it would just be users.contains(user).

You could use a map if you were looking up by something other than the actual User object, e.g. a mapping of names to users.

In either case, if you're using collection where you're checking contains, you must implement both equals and hashCode properly.

answered Jul 24, 2013 at 21:34

Jeff Storey

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2 Comments

s_puria Over a year ago

I want to have a map with key: username and Value: User.

Jeff Storey Over a year ago

in that case you can use a hashmap

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