先看看类定义：
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
.......
}
LinkedList与ArrayList相比，LinkedList不再实现RandomAccess接口，表明不支持快速随机访问数据，但实现了Deque接口，可当队列使用，Deque继承自Queue,接口定义：
public interface Queue<E> extends Collection<E> {
void addFirst(E e);
void addLast(E e);
boolean offerFirst(E e);
boolean offerLast(E e);
E removeFirst();
E removeLast();
E pollFirst();
E pollLast();
E getFirst();
E getLast();
E peekFirst();
E peekLast();
boolean removeFirstOccurrence(Object o);
boolean removeLastOccurrence(Object o);
boolean add(E e);
boolean offer(E e);
E remove();
E poll();
E element();
E peek();
void push(E e);
boolean remove(Object o);
boolean contains(Object o);
public int size();
Iterator<E> iterator();
Iterator<E> descendingIterator();
28 }
看看LinkedList的两个属性，一个头节点，一个末节点：
/**
* 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;
在看看Node的定义：
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;
}
}
Node中保存着存入集合的对象，同时也保存着上一个节点和下一个节点，由此可知，LinkedList 内部采用的是双向链表结构。
再看看CRUD操作:
add:
/**
* 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); //last节点后拼接一个节点
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; // 将last节点指向当前新创建的节点，新节点变为last节点，原last节点的下一个节点指向最新的last节点
if (l == null)
first = newNode;
else
l.next = newNode;
size++;
modCount++;
}
与linkLast对应的还有linkFirst:
/**
* Links e as first element.
*/将元素添加到首节点
private void linkFirst(E e) {
final Node<E> f = first;
final Node<E> newNode = new Node<>(null, e, f);
first = newNode;
if (f == null)
last = newNode;
else
f.prev = newNode;
size++;
modCount++;
}
既然存在在对尾、队尾添加元素，那么是不是也应该存在在指定某个元素之前或之后添加元素呢，是的，的确存在：
/**
* Inserts element e before non-null Node succ.
*/
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++;
}
与此同时也还存在另外两个方法，一个是在指定节点位置添加元素，另一个是将指定位置的元素修改成当前元素：
/**
* 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) { //指定位置的元素修改成当前元素
checkElementIndex(index);
Node<E> x = node(index);
E oldVal = x.item;
x.item = element;
return oldVal;
} /**
* 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) { //指定节点位置添加元素
checkPositionIndex(index); if (index == size)
linkLast(element);
else
linkBefore(element, node(index));
}
linkedList 虽然提供了get(index) 方法，但并不没有ArrayList那样高效的获取，而是变量整个数据结构，此时，做了一个优化，根据index判断元素是在链表的前端还是后端，如果是后端，则从队尾开始遍历：
/**
* 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;
}
}
获取对首或队尾元素就简单了，直接返回对首或队尾元素就ok了：
/**
* Returns the first element in this list.
*
* @return the first element in this list
* @throws NoSuchElementException if this list is empty
*/
public E getFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return f.item;
} /**
* Returns the last element in this list.
*
* @return the last element in this list
* @throws NoSuchElementException if this list is empty
*/
public E getLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return l.item;
}
LinkedList 无参remove方法，是直接删除队首元素：
/**
* Retrieves and removes the head (first element) of this list.
*
* @return the head of this list
* @throws NoSuchElementException if this list is empty
* @since 1.5
*/
public E remove() {
return removeFirst();
}
/**
* Removes and returns the first element from this list.
*
* @return the first element from this list
* @throws NoSuchElementException if this list is empty
*/
public E removeFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return unlinkFirst(f);
}
/**
* Unlinks non-null first node f.
*/
private E unlinkFirst(Node<E> f) { //将对首元素设置null,将对首的下一元素作为对首元素
// assert f == first && f != null;
final E element = f.item;
final Node<E> next = f.next;
f.item = null;
f.next = null; // help GC
first = next;
if (next == null)
last = null;
else
next.prev = null;
size--;
modCount++;
return element;
}
remove(Object),变量整个数据结构删除：
/**
* Removes the first occurrence of the specified element from this list,
* if it is present. If this list does not contain the element, it is
* unchanged. More formally, removes the element with the lowest index
* {@code i} such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
* (if such an element exists). Returns {@code true} if this list
* contained the specified element (or equivalently, if this list
* changed as a result of the call).
*
* @param o element to be removed from this list, if present
* @return {@code true} if this list contained the specified element
*/
public boolean remove(Object o) {
if (o == null) {//先判断是否是空值
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null) {
unlink(x);
return true;
}
}
} else {
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item)) {
unlink(x);
return true;
}
}
}
return false;
}
linkedList实现了队列接口，使用中也可以直接当队列用：
/**
* Adds the specified element as the tail (last element) of this list.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Queue#offer})
* @since 1.5
*/
public boolean offer(E e) {
return add(e);
} // Deque operations
/**
* Inserts the specified element at the front of this list.
*
* @param e the element to insert
* @return {@code true} (as specified by {@link Deque#offerFirst})
* @since 1.6
*/
public boolean offerFirst(E e) {
addFirst(e);
return true;
} /**
* Inserts the specified element at the end of this list.
*
* @param e the element to insert
* @return {@code true} (as specified by {@link Deque#offerLast})
* @since 1.6
*/
public boolean offerLast(E e) {
addLast(e);
return true;
}
linkedList 还提供了几个方法，可以直接通过peek方法获取元素，需要注意的是获取之后并不会从队列中删除该元素：
/**
* Retrieves, but does not remove, the first element of this list,
* or returns {@code null} if this list is empty.
*
* @return the first element of this list, or {@code null}
* if this list is empty
* @since 1.6
*/
public E peekFirst() {
final Node<E> f = first;
return (f == null) ? null : f.item;
} /**
* Retrieves, but does not remove, the last element of this list,
* or returns {@code null} if this list is empty.
*
* @return the last element of this list, or {@code null}
* if this list is empty
* @since 1.6
*/
public E peekLast() {
final Node<E> l = last;
return (l == null) ? null : l.item;
}
与peek对应的还有poll方法，同样是获取元素，不同点在于，获取元素之后，该元素将会从队列中删除。
/**
* Retrieves and removes the first element of this list,
* or returns {@code null} if this list is empty.
*
* @return the first element of this list, or {@code null} if
* this list is empty
* @since 1.6
*/
public E pollFirst() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
} /**
* Retrieves and removes the last element of this list,
* or returns {@code null} if this list is empty.
*
* @return the last element of this list, or {@code null} if
* this list is empty
* @since 1.6
*/
public E pollLast() {
final Node<E> l = last;
return (l == null) ? null : unlinkLast(l);
}
LinkedList提供了多种操作数据集合的方法，但最重要的一点就是，整个是实现是非线程安全的，在多线程下需要进行同步处理，或者使用并发包中的对应实现类。