LinkedList 源码浅析

LinkedList类定义

public class LinkedList
    extends AbstractSequentialList
    implements List, Deque, Cloneable, java.io.Serializable{}

可以重点关注下Deque这个接口，最后介绍这个接口。

主要成员变量

 transient int size = 0;

    /**
     * Pointer to first node.
     * Invariant: (first == null && last == null) ||
     *            (first.prev == null && first.item != null)
     */
    transient Node first;

    /**
     * Pointer to last node.
     * Invariant: (first == null && last == null) ||
     *            (last.next == null && last.item != null)
     */
    transient Node last;
private static class Node {
        E item;
        Node next;
        Node prev;

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

静态内部类Node是LinkedList的内部链表节点结构，包含了存储元素的item，以及前一个节点prev，后一个节点next。size代表节点数量，first是head节点，last是tail节点。

构造方法

 /**
     * Constructs an empty list.
     */
    public LinkedList() {
    }

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param  c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public LinkedList(Collection c) {
        this();
        addAll(c);
    }

主要有两个构造方法，第一个是建立一个空的list，第二个是用Collection参数来建立list。

主要方法

 /**
     * Appends the specified element to the end of this list.
     *
     * This method is equivalent to {@link #addLast}.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        linkLast(e);
        return true;
    }
/**
     * Links e as last element.
     */
    void linkLast(E e) {
        final Node l = last;
        final Node newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
    }

添加一个元素到队尾，实质是新产生一个元素，然后修改其next prev指向，同时修改first和last指针，最后更新size和modCount

/**
     * Inserts the specified element at the beginning of this list.
     *
     * @param e the element to add
     */
    public void addFirst(E e) {
        linkFirst(e);
    }
/**
     * Links e as first element.
     */
    private void linkFirst(E e) {
        final Node f = first;
        final Node newNode = new Node<>(null, e, f);
        first = newNode;
        if (f == null)
            last = newNode;
        else
            f.prev = newNode;
        size++;
        modCount++;
    }

这个方法是在队头添加元素，也是操作链表。
其他的poll remove addLast offer 等都是类似的，均是操作链表，更新size值和modCount，不再赘述。