用java声明双链表类DoublyLinkedList，实现LList接口中的方法。

好的，以下是双链表类DoublyLinkedList的声明和实现：

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

    private Node<T> head;
    private Node<T> tail;
    private int size;

    private static class Node<T> {
        T data;
        Node<T> next;
        Node<T> prev;

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

    public DoublyLinkedList() {
        head = null;
        tail = null;
        size = 0;
    }

    @Override
    public boolean isEmpty() {
        return size == 0;
    }

    @Override
    public int size() {
        return size;
    }

    @Override
    public void addFirst(T data) {
        Node<T> newNode = new Node<>(data);
        if (isEmpty()) {
            head = tail = newNode;
        } else {
            newNode.next = head;
            head.prev = newNode;
            head = newNode;
        }
        size++;
    }

    @Override
    public void addLast(T data) {
        Node<T> newNode = new Node<>(data);
        if (isEmpty()) {
            head = tail = newNode;
        } else {
            newNode.prev = tail;
            tail.next = newNode;
            tail = newNode;
        }
        size++;
    }

    @Override
    public void add(int index, T data) {
        if (index < 0 || index > size) {
            throw new IndexOutOfBoundsException("Invalid index");
        }
        if (index == 0) {
            addFirst(data);
        } else if (index == size) {
            addLast(data);
        } else {
            Node<T> newNode = new Node<>(data);
            Node<T> current = head;
            for (int i = 0; i < index; i++) {
                current = current.next;
            }
            newNode.prev = current.prev;
            newNode.next = current;
            current.prev.next = newNode;
            current.prev = newNode;
            size++;
        }
    }

    @Override
    public T removeFirst() {
        if (isEmpty()) {
            throw new NoSuchElementException("List is empty");
        }
        T data = head.data;
        if (head == tail) {
            head = tail = null;
        } else {
            head = head.next;
            head.prev = null;
        }
        size--;
        return data;
    }

    @Override
    public T removeLast() {
        if (isEmpty()) {
            throw new NoSuchElementException("List is empty");
        }
        T data = tail.data;
        if (head == tail) {
            head = tail = null;
        } else {
            tail = tail.prev;
            tail.next = null;
        }
        size--;
        return data;
    }

    @Override
    public T remove(int index) {
        if (index < 0 || index >= size) {
            throw new IndexOutOfBoundsException("Invalid index");
        }
        if (index == 0) {
            return removeFirst();
        } else if (index == size - 1) {
            return removeLast();
        } else {
            Node<T> current = head;
            for (int i = 0; i < index; i++) {
                current = current.next;
            }
            T data = current.data;
            current.prev.next = current.next;
            current.next.prev = current.prev;
            size--;
            return data;
        }
    }

    @Override
    public T getFirst() {
        if (isEmpty()) {
            throw new NoSuchElementException("List is empty");
        }
        return head.data;
    }

    @Override
    public T getLast() {
        if (isEmpty()) {
            throw new NoSuchElementException("List is empty");
        }
        return tail.data;
    }

    @Override
    public T get(int index) {
        if (index < 0 || index >= size) {
            throw new IndexOutOfBoundsException("Invalid index");
        }
        Node<T> current = head;
        for (int i = 0; i < index; i++) {
            current = current.next;
        }
        return current.data;
    }

    @Override
    public void set(int index, T data) {
        if (index < 0 || index >= size) {
            throw new IndexOutOfBoundsException("Invalid index");
        }
        Node<T> current = head;
        for (int i = 0; i < index; i++) {
            current = current.next;
        }
        current.data = data;
    }

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder("[");
        Node<T> current = head;
        while (current != null) {
            builder.append(current.data);
            if (current.next != null) {
                builder.append(", ");
            }
            current = current.next;
        }
        builder.append("]");
        return builder.toString();
    }
}

上述代码实现了LList接口中的所有方法，同时支持双向遍历。需要注意的是，双链表中的每个节点都有一个指向前一个节点的prev指针和一个指向后一个节点的next指针。