Implementation of a doubly linked list in Java
removeLast() assigns last to temp, which may be null.  If the list is empty, you will call toString() on null.

Otherwise, it is a pretty well made list.

I would change the names of your functions/class.


DoublyLinkList becomes DoublyLinkedList
addFirst becomes pushFront
removeFirst becomes popFront
removeLast becomes popBack
displayList becomes display ( because we know a list is calling it )
displayNode becomes display


As general advise about naming, the words you are using to describe what is happening are good hints at what names the functions/class should be.  For example, you use Doubly Linked List in the title of this post, and you use the word "popped" as output to the remove functions.