I am getting null as outcome ? Could anyone catch and rectify for the same Thanks

Question

This method I created

public class Use {
    private
    String firstname;
     String lastname;
     
     public String output() {
            return "Hii my name is " + firstname + lastname;
        }   
            public String getFirstName() {
                return firstname.toUpperCase();
            }
            public void setFirstName(String jl) {
                firstname = jl.strip() ;
                
            }
            public String getLastName() {
                return lastname.toUpperCase();
                
            }
            public void setLastName(String FN) {
                lastname = FN.strip();
            }
            
    }

And I am trying to use the above method in this code:

import java.util.ArrayList;
import java.util.List;

public class MySweetProgram {

    public static void main(String[] args) {
        String[] firstnames = {"KARRIK", "KESHAV", "Sussy"};
        String[] lastnames = {"gulati", "gulati", "smith"};
        List <User> users = new ArrayList<User>();
    
        for (int  i = 0; i < firstnames.length;     i ++) {
        User user = new User();
        user.setFirstName(firstnames[i]);
        user.setLastName(lastnames[i]);
        users.add(user);
        } 
    for (User user : users) {
        System.out.println(user.getfullname());
    }       
   }
    
}   

on running getting null null null

Answer 1

import java.util.ArrayList;
import java.util.List;

public class MySweetProgram {

public static void main(String[] args) {
    String[] firstnames = { "KARRIK", "KESHAV", "Sussy" };
    String[] lastnames = { "gulati", "gulati", "smith" };
    List<User> users = new ArrayList<User>();

    for (int i = 0; i < firstnames.length; i++) {
        User user = new User();
        user.setFirstName(firstnames[i]);
        user.setLastName(lastnames[i]);
        users.add(user);
    }
    for (User user : users) {
        System.out.println(user.getfullname());
    }
}

}
//User Class
class User {
private String firstname;
private String lastname;

public String output() {
    return "Hii my name is " + firstname + lastname;
}

public String getfullname() {
    return firstname + "_" + lastname;
}

public String getFirstName() {
    return firstname.toUpperCase();
}

public void setFirstName(String jl) {
    // firstname = jl.strip();// for java 11
    firstname = jl;

}

public String getLastName() {
    return lastname.toUpperCase();

}

public void setLastName(String FN) {
    // lastname = FN.strip();//for java 11
    lastname = FN;
}
}