在JAVA中的线性表的底层结构是数组，而链表在JAVA中的底层结构不是单向链表，而是双向链表。

双向链表的实现代码：

双向链表的主要函数:

头插法，尾插法，找到指定节点，指定位置插入，查找是不是含有值为key的节点，得到链表长度，删除第一个值为key的节点，删除所有值为key的节点，清除链表中的所有节点。

头插法:

public void addToHead(int n) {
        Node cur = new Node(n);
        if(head == null) {
            this.head = cur;
            this.last = cur;
        } else {
            cur.next = this.head;
            this.head.prev = cur;
            head = cur;
        }
    }

尾插法：

public void addToLast(int n) {
    Node cur = new Node(n);
    if(last == null) {
        this.head = cur;
        this.last = cur;
    } else {
        this.last.next = cur;
        cur.prev = last;
        last = cur;
    }
}

得到链表长度：

public Node findIndex(int index) {
        if (index < 0) {
            return null;
        }
        if(index >sizeOflink()) {
            return null;
        }
        Node cur = this.head;
        while(index != 0) {
            cur = cur.next;
            index--;
        }
        return cur;
    }

指定位置插入：

public void addToIndex(int n,int index) {
        Node cur = findIndex(index);
        Node node = new Node(n);
        if(cur == null) {
            System.out.println("位置不合法");
            return;
        }
        if(cur == this.head) {
            node.next = this.head;
            this.head.prev = node;
            this.head = node;
        } else if (cur == this.last) {
            node.prev = last;
            last.next = node;
            this.last = node;
        } else {
            node.next = cur;
            node.prev = cur.prev;
            cur.prev.next = node;
            cur.prev = node;
        }
    }

查找是否含有值为key的节点：

public boolean contains(int key) {
        Node cur = this.head;
        while(cur != null) {
            if(cur.val == key) {
                return true;
            }
            cur = cur.next;
        }
        return false;
    }

得到链表长度：

public int sizeOflink() {
        int count = 0;
        Node cur = this.head;
        while(cur != null) {
            cur = cur.next;
            count++;
        }
        return count;
    }

删除第一个值为key的节点：

public void remove(int key) {
        Node cur = this.head;
        while(cur != null) {
            if(cur.val == key) {
                if(this.head == this.last) {
                    this.head = null;
                    this.last = null;
                    return;
                }
                if(cur == this.head) {
                    this.head = this.head.next;
                    this.head.prev = null;
                } else if(cur == this.last) {
                    this.last = this.last.prev;
                    this.last.next = null;
                } else {
                    cur.prev.next = cur.next;
                    cur.next.prev = cur.prev;
                }
                return;
            }
            cur = cur.next;
        }
    }

删除所有值为key的节点：

public void remove(int key) {
        Node cur = this.head;
        while(cur != null) {
            if(cur.val == key) {
                if(this.head == this.last) {
                    this.head = null;
                    this.last = null;
                    return;
                }
                if(cur == this.head) {
                    this.head = this.head.next;
                    this.head.prev = null;
                } else if(cur == this.last) {
                    this.last = this.last.prev;
                    this.last.next = null;
                } else {
                    cur.prev.next = cur.next;
                    cur.next.prev = cur.prev;
                }
            }
            cur = cur.next;
        }
    }

清除链表中所有节点：

public void clear() {
        Node cur = this.head;
        while(cur != null) {
            Node curNext = cur.next;
            cur.prev = null;
            cur.next = null;
            cur = cur.next;
        }
        this.head = null;
        this.last = null;
    }