Doubly Linked List

Question

When I run the following code, it didn't pass all the test. Could anyone help me? I appreciate.

import java.util.Iterator;
import java.util.ConcurrentModificationException;

interface List<T> extends Iterable<T> {
void add(T x); // simple add
T remove(int i);
T get(int i);
boolean contains(T x);
int size();
default boolean isEmpty() {
return size() == 0;
}
}

public class DoublyLinkedList<T> implements List<T> {

/**
* Node is a pair containing a data field and pointers to
* the previous and next nodes in the list.
*/

class Node {
T data;
Node next, prev;

Node(T data) {
this(data, null, null);
}

Node(T data, Node prev, Node next) {
this.data = data;
this.prev = prev;
this.next = next;
}
}

final Node head; // always points to the headnode for this list
int n; // the number of nodes in this list, initially 0

/**
* Creates the empty list.
*/
public DoublyLinkedList() {
Node head = new Node(null, null, null);
head.next = head;
head.prev = head;
this.head = head;//Save it to state
}

/**
* Inserts the value x at the end of this list.
*/

public void add(T x) {
n ++;
Node last = head.prev;
Node curr = new Node(x, last, head);
last.next = curr;
head.prev = curr;
}

public T remove(int i) {
if (i < 0 || i >= size())
throw new IndexOutOfBoundsException();

if (head == null)
return null;

Node temp = head;

if(i == 0){
head.next = temp.next.next;
return temp.data;
}

for (int j = 0; temp!=null && j < i-1; j++)
temp = temp.next;

Node next = temp.next.next;
temp.next =next;

return next.data;
}

public T get(int i) {
if (i < 0 || i >= size())
throw new IndexOutOfBoundsException();
Node node = head;
for(int j = 0; j <i; j++)