源码阅读 - LinkedList

0. 什么是LinkedList

• 双向链表
• 非线程安全

1. 实现的本质

链表，Node<E> first指向链表头部，Node<E> last指向链表尾部

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;
    }
}

2. 主要api解析

2.1 构造函数

1. 无参，构建空链表
2. 参数是一个集合，会将集合中的数据放到链表中

/**
 * 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<? extends E> c) {
    this();
    addAll(c);
}

2.2 add方法

add方法有以下几种

//在最后添加
public boolean add(E e)
//在指定位置添加
public void add(int index, E element)
public boolean addAll(Collection<? extends E> c)
public boolean addAll(int index, Collection<? extends E> c)
public void addFirst(E e)
public void addLast(E e)

/**
 * Appends the specified element to the end of this list.
 *
 * <p>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<E> l = last;
    final Node<E> newNode = new Node<>(l, e, null);
    last = newNode;
    if (l == null)
        first = newNode;
    else
        l.next = newNode;
    size++;
    modCount++;
}
//===========================================================================
/**
 * Inserts the specified element at the specified position in this list.
 * Shifts the element currently at that position (if any) and any
 * subsequent elements to the right (adds one to their indices).
 *
 * @param index index at which the specified element is to be inserted
 * @param element element to be inserted
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public void add(int index, E element) {
    checkPositionIndex(index);
    if (index == size)
        linkLast(element);
    else
        linkBefore(element, node(index));
}

/**
 * Inserts element e before non-null Node succ.
 */
//将节点插入index位置
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++;
}

/**
 * Returns the (non-null) Node at the specified element index.
 */
//返回指定位置的node，由于是双向链表，所以如果index在前半段从first开始查找，如果在后半段则从last开始查找
//类中所有关于index的定位都是通过此方法进行
Node<E> node(int index) {
    // assert isElementIndex(index);
    //前半段，从first开始搜
    if (index < (size >> 1)) {
        Node<E> x = first;
        for (int i = 0; i < index; i++)
            x = x.next;
        return x;
    //后半段，从last开始搜
    } else {
        Node<E> x = last;
        for (int i = size - 1; i > index; i--)
            x = x.prev;
        return x;
    }
}

2.3 remove方法

//和revmoeFirst()相同
public E remove()
//删除指定位置节点
public E remove(int index)
//删除从first开始的第一个data为o的节点，这里使用equals进行比较
public boolean remove(Object o)
public E removeFirst()
//和remove(Object o)相同
public boolean removeFirstOccurrence(Object o)
public E removeLast()
//删除从last往前的第一个data为o的节点
public boolean removeLastOccurrence(Object o)

2.4 get方法

使用node方法进行定位，然后返回值

/**
 * Returns the element at the specified position in this list.
 *
 * @param index index of the element to return
 * @return the element at the specified position in this list
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public E get(int index) {
    checkElementIndex(index);
    return node(index).item;
}

2.5 遍历

可使用listIterator进行遍历。
还提供了一个类只做倒序遍历，也是通过ListItr实现的。

/**
 * Adapter to provide descending iterators via ListItr.previous
 */
private class DescendingIterator implements Iterator<E> {
    private final ListItr itr = new ListItr(size());
    public boolean hasNext() {
        return itr.hasPrevious();
    }
    public E next() {
        return itr.previous();
    }
    public void remove() {
        itr.remove();
    }
}

3. 参考

1. 源码 build 1.8.0_121-b13版本
2. LinkedList源码解析 http://www.cnblogs.com/ITtangtang/p/3948610.html