Java 使用 lambda 函数对数组列表进行排序不起作用

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;
    }
}

类标题

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);
    }
}

主要的

package main;

public class 主要的 {
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.