问CS61b project1b无法解决Deque中的get方法

EN

Deque stringDeque =  wordToDeque(word); 

LinkListDeque stringDeque = (LinkListDeque) wordToDeque(word); 

/** Create an interface in a new file named Deque.
 * java that contains all of the methods that appear in both ArrayDeque and LinkedListDeque.
 * @param <Item>
 */

public interface Deque<Item> {

    int size = 0;
    /** Adds an item of type T to the front of the deque. */
    void addFirst(Item item);

    /** Adds an item of type T to the back of the deque. */
    void addLast(Item item);

    /** Returns true if deque is empty, false otherwise. */
    default boolean isEmpty() {
        return size == 0;
    };

    /** Prints the items in the deque from first to last,
     * separated by a space. Once all the items have been printed, print out a new line. */
    void printDeque();

    /** Removes and returns the item at the front of the deque. If no such item exists, returns null. */
    Item removeFirst();

    /** Removes and returns the item at the back of the deque. If no such item exists, returns null. */
    Item removeLast();

    /** Gets the item at the given index, where 0 is the front, 1 is the next item, and so forth.
     * If no such item exists, returns null. Must not alter the deque! */
    Item get(int index);
}

import java.util.LinkedList;
import java.util.NoSuchElementException;

/**
 * Isn't this solution kinda... cheating? Yes.
 */
public class LinkedListDeque<Item> extends LinkedList<Item> implements Deque<Item> {

    @Override
    public void printDeque() {
        System.out.println("dummy");
    }

    public Item getRecursive(int i) {
        return get(i);
    }

    @Override
    public Item removeFirst() {
        try {
            return super.removeFirst();
        } catch (NoSuchElementException e) {
            return null;
        }
    }

    @Override
    public Item removeLast() {
        try {
            return super.removeLast();
        } catch (NoSuchElementException e) {
            return null;
        }
    }
}

import java.util.Deque;
import java.util.LinkedList;

public class Palindrome {

    /** Given a String, wordToDeque should return a Deque
     * where the characters appear in the same order as in the String.
     * @param word
     */
    public Deque<Character> wordToDeque(String word) {
        Deque<Character> stringDeque = new LinkedListDeque<>();
        for (String s : word.split("")) {
            stringDeque.addLast(s.charAt(0));
        }
        return stringDeque;
    }

    /** Return true if the given word is a palindrome, and false otherwise. */
    public boolean isPalindrome(String word) {
        if (word.length() == 0 || word.length() == 1) {
            return true;
        }
        Deque stringDeque =  wordToDeque(word);
        int index = word.length() / 2;
        for (int i = 0; i < index; i += 1) {
            if (stringDeque.get(i) != stringDeque.get(word.length() - i - 1)) { return false; }
        }
        return true;
    }
}