public class UserDB

{ private List<User> userList;

public UserDB() {
userList = new ArrayList<>();
}

public void addUser(User user) {
userList.add(user);
}

public void removeUser(User user) {
userList.remove(user);
}

public User getUserByUsername(String username) {
for (User user : userList) {
if (user.getUsername().equals(username)) {
return user;
}
}
return null;
}

public List<User> getAllUsers() {
return userList;
}
}