Doubly linked list not working, specifict implementation

Question

I need to implement a Doubly linked list, and implement a addNode method that takes 3 arguments (head, tail, value). Here is my interface:

public interface DoublyLinkedListTDA {
    //void inicializar();
    void agregar(
            DoubleNode head, 
            DoubleNode tail, 
            int value);
    
}

public class DoublyLinkedList implements DoublyLinkedListTDA {


    DoubleNode head;
    DoubleNode tail;

    public void inicializar() {

        head = null;
        tail = null;

    }

    

    public void agregar(DoubleNode head, DoubleNode tail, int value) {
        DoubleNode NuevoNodo = new DoubleNode();

        if (this.head == null) {
            head = NuevoNodo;
            tail = NuevoNodo;
            NuevoNodo.value = value;
            NuevoNodo.prev = null;
            NuevoNodo.sig = null;

        } else {
            NuevoNodo.value = value;
            tail.sig = NuevoNodo;
            NuevoNodo.prev = tail;
            tail = NuevoNodo;
            NuevoNodo.sig = null;

        }

    }


}

And here is my Node Class

public class DoubleNode {
    int value;
    DoubleNode sig;
    DoubleNode prev;
}

But this is not working properly. When testing with:

package simulacro;

public class App {
    public static void main(String[] args) {

        DoublyLinkedList list = new DoublyLinkedList();

        list.inicializar();
        DoubleNode uno = new DoubleNode();
        uno.value = 7;
        uno.prev = null;

        DoubleNode dos = new DoubleNode();
        dos.value = 4;
        dos.prev = uno;
        dos.sig = null;

        uno.sig = dos;

        list.agregar(null, dos, 8);
        System.out.println(list.tail.value);

    }

}

Values, head, and tails are not updating properly. When I print the values of head and tails usually its returns null, and sometimes it return Node "dos" value, and not new tail value "8".