jdk源码阅读之LinkedList

和ArrayList相似，LinkedList也是实现了List接口，但是LinkedList是用链表实现的，而ArrayList是用数组实现的。两者的优缺点基本就是链表和数组的优缺点。
先看LinkedList声明

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable

• 域

transient int size = 0;
transient Node<E> first;
transient Node<E> last;

LinkedList是双向两头插入式链表，first域引用的是链表头，last域引用的是链表尾.
链表是以节点为单位进行操作的，因此在其内部肯定封装了一个节点的类

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;
        }
    }

LinkedList不是循环链表，因此first.prev=null，last.next=null.item域是我们想存放的数据。
LinkedList的增删查改方法：

1. 增

public boolean add(E e) {
        linkLast(e);
        return true;
    }
public void add(int index, E element) {
        checkPositionIndex(index);

        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }
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++;
    }
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++;
    }

对于链表，无需扩容，也不需要考虑到容器大小的问题。添加操作基本上是一些指针操作，速度较快。

2. 删

public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }
public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }
    public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

3. 查

public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }
public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }
    public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

4. 改

public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

5. 遍历

public ListIterator<E> listIterator(int index) {
        checkPositionIndex(index);
        return new ListItr(index);
    }
public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }
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();
        }
    }

LinkedList有两个迭代器，顺序相反，其中DescendingIterator 实际上是利用listIterator来实现的。

6. 与ArrayList的转换

public Object[] toArray() {
        Object[] result = new Object[size];
        int i = 0;
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;
        return result;
}

这个方法可以将LinkedList转换成ArrayList