先摆上JDK1.8中hashMap的类注释；我翻译了一下
/**
* Doubly-linked list implementation of the {@code List} and {@code Deque}
* interfaces. Implements all optional list operations, and permits all
* elements (including {@code null}).
* 双向链表实现了List和Deque接口。实现了List所有的操作,双向链表允许所有元素,包括null
*
* <p>All of the operations perform as could be expected for a doubly-linked
* list. Operations that index into the list will traverse the list from
* the beginning or the end, whichever is closer to the specified index.
* 对于LinkedList所有的操作都是可以被预期的。操作LinkedList将会遍历LinkedList的指针.
* 解读：通过set(i,e),get(i)访问LinkedList的元素时,要遍历指针,如果i>LinkedList容量的一半,
* 就从尾部开始遍历.
*
* <p><strong>Note that this implementation is not synchronized.</strong>
* If multiple threads access a linked list concurrently, and at least
* one of the threads modifies the list structurally, it <i>must</i> be
* synchronized externally. (A structural modification is any operation
* that adds or deletes one or more elements; merely setting the value of
* an element is not a structural modification.) This is typically
* accomplished by synchronizing on some object that naturally
* encapsulates the list.
* 注意,LinkedList是线程不同步的.如果多线程同时访问LinkedList,此时如果有一个线程修改LinkedList结构,
* 那么就必须在外层进行同步操作处理(这里的修改结构包括添加元素,删除元素)。
* 解读：ArrayList同样有此特性.
*
* If no such object exists, the list should be "wrapped" using the
* {@link Collections#synchronizedList Collections.synchronizedList}
* method. This is best done at creation time, to prevent accidental
* unsynchronized access to the list:<pre>
* List list = Collections.synchronizedList(new LinkedList(...));</pre>
* 考虑线程同步问题,可以用Collections.synchronizedList替代LindedList
*
* <p>The iterators returned by this class's {@code iterator} and
* {@code listIterator} methods are <i>fail-fast</i>: if the list is
* structurally modified at any time after the iterator is created, in
* any way except through the Iterator's own {@code remove} or
* {@code add} methods, the iterator will throw a {@link
* ConcurrentModificationException}. Thus, in the face of concurrent
* modification, the iterator fails quickly and cleanly, rather than
* risking arbitrary, non-deterministic behavior at an undetermined
* time in the future.
* 在迭代一个LinkedList时,任何修改LinkedList的操作,迭代器都会终止,并抛出
* ConcurrentModificationException异常.
*
* <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
* as it is, generally speaking, impossible to make any hard guarantees in the
* presence of unsynchronized concurrent modification. Fail-fast iterators
* throw {@code ConcurrentModificationException} on a best-effort basis.
* Therefore, it would be wrong to write a program that depended on this
* exception for its correctness: <i>the fail-fast behavior of iterators
* should be used only to detect bugs.</i>
* 快速失败机制,是一种错误检测机制。它只能被用来检测错误，因为JDK并不保证fail-fast机制一定会发生.
* 参考我的博客 ”Iterator fail-fast“
*
* @author Josh Bloch
* @see List
* @see ArrayList
* @since 1.2
* @param <E> the type of elements held in this collection
*/
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
总结
1. LinkedList是基于链表结构实现，所以在类中包含了first和last两个指针(Node)。Node中包含了上一个节点和下一个节点的引用，这样就构成了双向的链表。每个Node只能
知道自己的前一个节点和后一个节点，但对于链表来说，这已经足够了
在此看一下LinkedList的数据结构，立体感受一下这个特性
所以LinkedList插入和删除元素效率很高，比ArrayList高。通过set(i,e),get(i)访问效率低，因为要遍历指针，如果i>size/2,那么就从尾部开始遍历。
翻阅get(i),set(i,E)的源码
/**
* Replaces the element at the specified position in this list with the
* specified element.
*
* @param index index of the element to replace
* @param element element to be stored at the specified position
* @return the element previously at the specified position
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index, E element) {
checkElementIndex(index);
Node<E> x = node(index);
E oldVal = x.item;
x.item = element;
return oldVal;
} /**
* 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;
} /**
* Returns the (non-null) Node at the specified element index.
*/
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;
}
}
很明显，在进行set和get操作时，并不是每次都从头部开始遍历指针的，而是调用了node(int index)这个方法，就是用来判断当前index的大致位置，如果i<(size>>1)也就是i<size/2,那么从first开始遍历，否则从last开始遍历。此时set，get操作的时间复杂度也就由O(n)变成了O(n/2)
2. 链表没有容量限制，但是双向链表本身使用了更多的空间.每插入一个元素都要构造一个Node对象
3. LinkedList线程不同步，采用fail-fast机制