Java 15. In java 7 and 8, you may encounter static problem so code in java 15

DoublyLinkedListNode n = new DoublyLinkedListNode(data);

    if (llist == null) {
        return n;
    }

    // Case when the new node needs to be inserted at the beginning
    if (data < llist.data) {
        n.next = llist;
        llist.prev = n;
        return n;
    }

    DoublyLinkedListNode current = llist;
    while (current.next != null && current.next.data < data) {
        current = current.next;
    }

    // Insert the new node
    n.next = current.next;
    if (current.next != null) {
        current.next.prev = n;
    }
    current.next = n;
    n.prev = current;

    return llist;