private static class Node { E item; Node next; Node prev; Node(Node prev, E element, Node next) { this.item = element; this.next = next; this.prev = prev; } }

final Node newNode = new Node<>(null, e, f); // 构造函数的第3个参数不就是 next 元素了。

Why is the link broken?
f points to the original first and sets the new newNode to first.
At this time, it is judged that if the original linked list is empty, then last is also first.
If it is not empty, then the prev of f points to first.

I guess the reason why you think the link is broken is because you didn't see that first's next points to f. Um, next has been passed into Node
final Node newNode = new Node<>(null, e, f);
private static class Node {

E item; Node next; Node prev; Node(Node prev, E element, Node next) { this.item = element; this.next = next; this.prev = prev; } }