我试图在双链表的末尾插入一个节点，但是当我运行add方法时，代码永远不会运行完。代码如下：

public class DoublyLinkedList<T> {
    static class Node<T> {
        T data;
        Node<T> next;
        Node<T> previous;

        Node() {
            data = null;
            next = null;
            previous = null;
        }

        Node(T value) {
            data = value;
            next = null;
            previous = null;
        }
    }

    private Node<T> head;
    private int size;

    public DoublyLinkedList() {
        head = null;
    }

    public DoublyLinkedList(T value) {
        head = new Node<T>(value);
        size ++;
    }

    public void add(T value) {
        Node<T> append = new Node<T>(value);
        append.next = null;
        if(head == null) {
            append.previous = null;
            head = append;
            size ++;
            return;
        }
        Node current = head;
        while(current.next != null) {
            current = current.next;
        }
        current.next = append;
        append.previous = current;
        size ++;
    }

我很确定current.next=append这行是问题所在，但我不确定如何修复它。我做错什么了？