LinkedList

LinkedList是一个双向链表实现的list，允许null值线程非安全还可以作为队列或栈来使用

其继承关系如下：

可看出LinkedList不仅实现了list接口还是先了Queue或Deque接口

主要属性

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    //元素个数
    transient int size = 0;
    //指向链表第一个节点
    transient Node<E> first;
    //指向链表最后一个节点
    transient Node<E> last;
    
    private static class Node<E> {
        E item;
        Node<E> next;
        Node<E> prev;

        Node(Node<E> prev, E element, Node<E> next) {
            this.item = element;
            this.next = next;
            this.prev = prev;
        }
    }
    ... ...
}

构造器

public LinkedList() {
}
public LinkedList(Collection<? extends E> c) {
    this();
    addAll(c);
}

添加元素

作为队列

public void addFirst(E e) {
    linkFirst(e);
}

private void linkFirst(E e) {
    final Node<E> f = first;
    final Node<E> newNode = new Node<>(null, e, f);
    first = newNode;
    if (f == null)
        last = newNode;
    else
        f.prev = newNode;
    size++;
    modCount++;
}
public boolean add(E e) {
    linkLast(e);
    return true;
}

void linkLast(E e) {
    final Node<E> l = last;
    final Node<E> newNode = new Node<>(l, e, null);
    last = newNode;
    if (l == null)//当前没有元素
        first = newNode;
    else
        l.next = newNode;//将当前节点追加到last后
    size++;
    modCount++;
}

作为list

public void add(int index, E element) {
    checkPositionIndex(index);

    if (index == size)
        linkLast(element);
    else
        linkBefore(element, node(index));
}
//查找指定索引位置的节点
Node<E> node(int index) {
    // assert isElementIndex(index);

    if (index < (size >> 1)) {
        Node<E> x = first;
        for (int i = 0; i < index; i++)
            x = x.next;
        return x;
    } else {
        Node<E> x = last;
        for (int i = size - 1; i > index; i--)
            x = x.prev;
        return x;
    }
}
void linkBefore(E e, Node<E> succ) {
    // assert succ != null;
    final Node<E> pred = succ.prev;
    final Node<E> newNode = new Node<>(pred, e, succ);
    succ.prev = newNode;
    if (pred == null)
        first = newNode;
    else
        pred.next = newNode;
    size++;
    modCount++;
}

删除元素

public E remove() {
    return removeFirst();
}
public E removeFirst() {
    final Node<E> f = first;
    if (f == null)
        throw new NoSuchElementException();
    return unlinkFirst(f);
}

private E unlinkFirst(Node<E> f) {
    // assert f == first && f != null;
    final E element = f.item;
    final Node<E> next = f.next;
    f.item = null;
    f.next = null; // help GC
    first = next;
    if (next == null)
        last = null;
    else
        next.prev = null;
    size--;
    modCount++;
    return element;
}

public E removeLast() {
    final Node<E> l = last;
    if (l == null)
        throw new NoSuchElementException();
    return unlinkLast(l);
}

private E unlinkLast(Node<E> l) {
    // assert l == last && l != null;
    final E element = l.item;
    final Node<E> prev = l.prev;
    l.item = null;
    l.prev = null; // help GC
    last = prev;
    if (prev == null)
        first = null;
    else
        prev.next = null;
    size--;
    modCount++;
    return element;
}

public E remove(int index) {
    checkElementIndex(index);
    return unlink(node(index));
}

E unlink(Node<E> x) {
    // assert x != null;
    final E element = x.item;
    final Node<E> next = x.next;
    final Node<E> prev = x.prev;

    if (prev == null) {
        first = next;
    } else {
        prev.next = next;
        x.prev = null;
    }

    if (next == null) {
        last = prev;
    } else {
        next.prev = prev;
        x.next = null;
    }

    x.item = null;
    size--;
    modCount++;
    return element;
}

栈

public void push(E e) {
    addFirst(e);
}
public E pop() {
    return removeFirst();
}

总结

双向链表实现的list
实现了Queue、Deque接口具有队列和双端队列、栈的特性
队列首尾添加、删除元素非常高效时间复杂度为O(1)
在中间添加、删除元素需要遍历list 比较低效时间复杂度为O(n)
不支持随机访问故访问非首尾节点比较低效
功能上等于ArrayList+ArrayDeque