My toString method is giving me an error even though I am not calling it

I am trying to run the reverseTest method so that I know if the reverse method is working correctly but I keep getting an error in my toString method and I don't know why this is happening. Any help would be greatly appreciated.

public String toString(DoublyLinkedList dll) {
    StringBuilder sb = new StringBuilder();
    for(E s : this) {

    return sb.toString();

public void reverse(){
    Node temp = null;
    Node current = header;

    /* swap next and prev for all nodes of 
     doubly linked list */
    while (current != null) {
        temp = current.prev;
        current.prev =; = temp;
        current = current.prev;

    /* Before changing head, check for the cases like empty 
     list and list with only one node */
    if (temp != null) {
        header = temp.prev;

public static void reverseTest() {
    String [] data = {"one", "two", "three", "four", "five", "six"};
    DoublyLinkedList2 ll = new DoublyLinkedList2();

    for(String s : data) {
    System.out.println("before reverse: " + ll);
    System.out.println("after reverse: " + ll);