I'm trying to use .sort() method with an arraylist.
My method public void sortSurname() should sort alphabetically all the objects by their surname, these objects are contained in an arraylist ArrayList<Contact> contacts.
Could someone please find the issue with this code?
Here is what I've written so far:
Contact class

package main;
public class Contact {
    private String name, surname, cellphone;
    private Genere genere;
    
    public Contact(String name, String surname, String cellphone){
        this.name=name;
        this.surname=surname;
        this.cellphone=cellphone;
    }

    public String getSurname() {
        return surname;
    }
}

Rubrica class

public class Rubrica {
    private ArrayList<Contact> contacts;
    
    public Rubrica(){
        contacts = new ArrayList<>();
    }

    public void sortSurname(){
        contacts.sort((Contact c1, Contact c2) -> {
            c1.getSurname().compareTo(c2.getSurname());
        });
    }

    public void addContact(Contact c1){
        contacts.add(c1);
    }
}

Main

package main;

public class Main {
public static void main(String[] args) {
    Contact c1 = new Contact("Paolo", "Groviera", "338");
    Contact c2 = new Contact("Paolo", "Groviera", "234");
    Contact c3 = new Contact("Lampa", "Dino", "234");
    Contact c4 = new Contact("Lampa", "Dina", "0234");
    
    Rubrica r = new Rubrica();
    r.addContact(c1);
    r.addContact(c2);
    r.addContact(c3);
    r.addContact(c4);

    r.sortSurname();
    System.out.println(r);
    }
}

Thank you for the help.