java双向链表怎么实现

Java中的双向链表可以通过定义一个Node类来实现，该类包含一个值和两个指针，分别指向前一个节点和后一个节点。具体实现如下：

public class DoublyLinkedList {
    private Node head; // 链表头节点
    private Node tail; // 链表尾节点

    // 节点类
    private class Node {
        private int value;
        private Node prev;
        private Node next;

        public Node(int value) {
            this.value = value;
            this.prev = null;
            this.next = null;
        }
    }

    // 在链表末尾添加节点
    public void add(int value) {
        Node newNode = new Node(value);
        if (head == null) {
            head = newNode;
            tail = newNode;
        } else {
            newNode.prev = tail;
            tail.next = newNode;
            tail = newNode;
        }
    }

    // 在指定位置插入节点
    public void insert(int index, int value) {
        if (index < 0 || index > size()) {
            throw new IndexOutOfBoundsException("Invalid index");
        }

        if (index == 0) {
            Node newNode = new Node(value);
            newNode.next = head;
            head.prev = newNode;
            head = newNode;
        } else if (index == size()) {
            add(value);
        } else {
            Node cur = head;
            for (int i = 0; i < index; i++) {
                cur = cur.next;
            }

            Node newNode = new Node(value);
            newNode.prev = cur.prev;
            newNode.next = cur;
            cur.prev.next = newNode;
            cur.prev = newNode;
        }
    }

    // 删除指定位置的节点
    public void remove(int index) {
        if (index < 0 || index >= size()) {
            throw new IndexOutOfBoundsException("Invalid index");
        }

        if (index == 0) {
            head = head.next;
            head.prev = null;
        } else if (index == size() - 1) {
            tail = tail.prev;
            tail.next = null;
        } else {
            Node cur = head;
            for (int i = 0; i < index; i++) {
                cur = cur.next;
            }

            cur.prev.next = cur.next;
            cur.next.prev = cur.prev;
        }
    }

    // 获取指定位置的节点值
    public int get(int index) {
        if (index < 0 || index >= size()) {
            throw new IndexOutOfBoundsException("Invalid index");
        }

        Node cur = head;
        for (int i = 0; i < index; i++) {
            cur = cur.next;
        }

        return cur.value;
    }

    // 返回链表的大小
    public int size() {
        int count = 0;
        Node cur = head;
        while (cur != null) {
            count++;
            cur = cur.next;
        }
        return count;
    }
}

使用示例：

public class Main {
    public static void main(String[] args) {
        DoublyLinkedList list = new DoublyLinkedList();

        list.add(1);
        list.add(2);
        list.add(3);

        System.out.println("Size: " + list.size()); // 输出：Size: 3

        list.insert(1, 4);

        System.out.println("Size: " + list.size()); // 输出：Size: 4
        System.out.println("Element at index 1: " + list.get(1)); // 输出：Element at index 1: 4

        list.remove(2);

        System.out.println("Size: " + list.size()); // 输出：Size: 3
        System.out.println("Element at index 2: " + list.get(2)); // 输出：Element at index 2: 3
    }
}

以上代码实现了一个简单的双向链表，并提供了添加、插入、删除和获取节点的功能。