Create an array of values from a list of objects [duplicate]

Question

I cant find a java version of this anywhere. In js this is simple. I have a list of objects with attributes and I want to create arrays for each attribute. The user class simply initializes its own attributes. The main class initializes the list with user objects and then creates lists of usernames and passwords.

User:

public class User{

    String username, password;

    public User(){}

    public User(String myName, String myPwd){

        username = myName;
        password = myPwd;
    }
}

Main:

import java.util.stream.Collectors;

public class Main{

    public static void main(String[] args){

        List<User> users = new ArrayList<>();

        int i = 0;
        while(i<5){

            users.add(new User("Brandon" + i,"LetsGo" + i));
            i++
        }

        System.out.println("Enter your username.");
                String username = scanner.nextLine();
                System.out.println("Enter your password.");
                String password = scanner.nextLine();
        
        //how do I make these lists?
        List<String> names = users.stream().map(User::username).collect(Collectors.toList());
        List<String> pwds = users.stream().map(User::password).collect(Collectors.toList());

        if(names.contains(username)){
            if(pwds.contains(password)){
                System.out.println("Welcome back " + username);
            }else{
                System.out.println("The credentials do not match our records.");
            }
        }else{
            users.add(new User(username, password));
            System.out.println("Welcome " + username);
        }
    }
}

Always search Stack Overflow thoroughly before posting.

Basil Bourque
– Basil Bourque

2022-02-23 02:27:42 +00:00
Commented Feb 23, 2022 at 2:27 — Basil Bourque
– Basil Bourque, Commented Feb 23, 2022 at 2:27
thanks, I did search far and wide and not just on this site

MVB76
– MVB76

2022-02-23 02:30:00 +00:00
Commented Feb 23, 2022 at 2:30 — MVB76
– MVB76, Commented Feb 23, 2022 at 2:30

WJS · Accepted Answer · 2022-02-23 02:32:50Z

2

First, create the user instance with the loop.

List<User> list = new ArrayList<>();
for (int i = 0; i < 5; i++) {
  System.out.println("Enter your username.");
  String username = scanner.nextLine();
  System.out.println("Enter your password.");
  String password = scanner.nextLine();
  list.add(new User(username, password));
}

Then stream the list like your doing.

List<String> names = users.stream().map(User::username).collect(Collectors.toList());
List<String> pwds = users.stream().map(User::password).collect(Collectors.toList());

But your using a method reference for a getter so you need to include a method username() in your class definition that returns a String. Same for username. These are normally prefixed with get like getUsername.

public String getUsername() {
    return username;
}

Then you can do what you want with the lists

edited Feb 23, 2022 at 2:32

answered Feb 23, 2022 at 2:28

WJS

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1 Comment

MVB76 Over a year ago

ahhh getters of course, thanks

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