Doubly-linked list implementation of the List and Deque interfaces. Implements all optional list operations, and permits all elements (including null).

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.

Note that this implementation is not synchronized. If multiple threads access a linked list concurrently, and at least one of the threads modifies the list structurally, it must 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. If no such object exists, the list should be "wrapped" using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:

   List list = Collections.synchronizedList(new LinkedList(...));

The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the Iterator's own remove or add methods, the iterator will throw a 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.

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 ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs. 

以双向链表实现。链表无容量限制,但双向链表本身使用了更多空间,每插入一个元素都要构造一个额外的Node对象,也需要额外的链表指针操作。

按下标访问元素－get(i)、set(i,e) 要悲剧的部分遍历链表将指针移动到位 (如果i>数组大小的一半,会从末尾移起)。

插入、删除元素时修改前后节点的指针即可,不再需要复制移动。但还是要部分遍历链表的指针才能移动到下标所指的位置。

只有在链表两头的操作－add()、addFirst()、removeLast()或用iterator()上的remove()倒能省掉指针的移动。

Apache Commons 有个TreeNodeList,里面是棵二叉树,可以快速移动指针到位。

/**

 * 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);

}

public class Test {

    public static void main(String[] args) {

        List<String> list = new LinkedList<String>();

        list.add("语文: 1");

        list.add("数学: 2");

        list.add("英语: 3");

    }

}

java.lang.Object

   ↳     java.util.AbstractCollection<E>

         ↳     java.util.AbstractList<E>

               ↳     java.util.AbstractSequentialList<E>

                     ↳     java.util.LinkedList<E>

public class LinkedList<E>

    extends AbstractSequentialList<E>

    implements List<E>, Deque<E>, Cloneable, java.io.Serializable {}

transient int size = 0;

/**

 * Pointer to first node.

 * Invariant: (first == null && last == null) ||

 *            (first.prev == null && first.item != null)

 */

transient Node<E> first;

/**

 * Pointer to last node.

 * Invariant: (first == null && last == null) ||

 *            (last.next == null && last.item != null)

 */

transient Node<E> last;

//每一个元素节点

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;

    }

}

/**

 * 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++;

}

/**

 * 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;

}    

private void checkElementIndex(int index) {

    if (!isElementIndex(index))

        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

}

    /**

 * Tells if the argument is the index of an existing element.

 */

private boolean isElementIndex(int index) {

    return index >= 0 && index < size;

}

/**

 * 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;

    }

}

        第一个元素（头部）                 最后一个元素（尾部）

        抛出异常        特殊值            抛出异常        特殊值

插入    addFirst(e)    offerFirst(e)    addLast(e)        offerLast(e)

移除    removeFirst()  pollFirst()      removeLast()    pollLast()

检查    getFirst()     peekFirst()      getLast()        peekLast()

队列方法       等效方法

add(e)        addLast(e)

offer(e)      offerLast(e)

remove()      removeFirst()

poll()        pollFirst()

element()     getFirst()

peek()        peekFirst()

栈方法        等效方法

push(e)      addFirst(e)

pop()        removeFirst()

peek()       peekFirst()

//不可取

for (int i = 0; i < linkList.size(); i++) {

    link = linkList.get(i);

}