LinkedList
类定义
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
和 ArrayList 的接口实现还是有区别的。
实现了 Deque 接口，继承自 AbstractSequentialList。
Deque 接口
/**
* A linear collection that supports element insertion and removal at
* both ends. The name <i>deque</i> is short for "double ended queue"
* and is usually pronounced "deck". Most {@code Deque}
* implementations place no fixed limits on the number of elements
* they may contain, but this interface supports capacity-restricted
* deques as well as those with no fixed size limit.
*
* <p>This interface defines methods to access the elements at both
* ends of the deque. Methods are provided to insert, remove, and
* examine the element. Each of these methods exists in two forms:
* one throws an exception if the operation fails, the other returns a
* special value (either {@code null} or {@code false}, depending on
* the operation). The latter form of the insert operation is
* designed specifically for use with capacity-restricted
* {@code Deque} implementations; in most implementations, insert
* operations cannot fail.
*/
public interface Deque<E> extends Queue<E> {
double ended queue：支持线性时间在两头改变集合。
AbstractSequentialList
/**
* This class provides a skeletal implementation of the <tt>List</tt>
* interface to minimize the effort required to implement this interface
* backed by a "sequential access" data store (such as a linked list). For
* random access data (such as an array), <tt>AbstractList</tt> should be used
* in preference to this class.<p>
*
* This class is the opposite of the <tt>AbstractList</tt> class in the sense
* that it implements the "random access" methods (<tt>get(int index)</tt>,
* <tt>set(int index, E element)</tt>, <tt>add(int index, E element)</tt> and
* <tt>remove(int index)</tt>) on top of the list's list iterator, instead of
* the other way around.<p>
*
* To implement a list the programmer needs only to extend this class and
* provide implementations for the <tt>listIterator</tt> and <tt>size</tt>
* methods. For an unmodifiable list, the programmer need only implement the
* list iterator's <tt>hasNext</tt>, <tt>next</tt>, <tt>hasPrevious</tt>,
* <tt>previous</tt> and <tt>index</tt> methods.<p>
*
* For a modifiable list the programmer should additionally implement the list
* iterator's <tt>set</tt> method. For a variable-size list the programmer
* should additionally implement the list iterator's <tt>remove</tt> and
* <tt>add</tt> methods.<p>
*
* The programmer should generally provide a void (no argument) and collection
* constructor, as per the recommendation in the <tt>Collection</tt> interface
* specification.<p>
*/
public abstract class AbstractSequentialList<E> extends AbstractList<E> {
这个的访问性能是 O(N) 的 ，相比于 arraylist 的 O(1) 访问性能。
基于迭代器，遍历整个 list，进行相关处理。
内部变量
// 链表大小
transient int size = 0;
/**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
* (first.prev == null && first.item != null)
* 首节点
*/
transient Node<E> first;
/**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
* (last.next == null && last.item != null)
*
* 尾节点
*/
transient Node<E> last;
Node 节点
定义如下：
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 中的节点是双向的。
构造器
/**
* 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);
}
addAll()
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}
具体实现如下：
public boolean addAll(int index, Collection<? extends E> c) {
// 检测下标
checkPositionIndex(index);
// 转化为数组
Object[] a = c.toArray();
// 数组为0，直接返回 false
int numNew = a.length;
if (numNew == 0)
return false;
// 声明两个节点，前继和后继
Node<E> pred, succ;
// 如果是列表元素的末尾
if (index == size) {
succ = null;
pred = last;
} else {
succ = node(index);
pred = succ.prev;
}
// 遍历元素，执行插入操作。
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
Node<E> newNode = new Node<>(pred, e, null);
// 处理前继节点为 null 的情况。
if (pred == null)
first = newNode;
else
pred.next = newNode;
pred = newNode;
}
// 如果后继为空
if (succ == null) {
last = pred;
} else {
pred.next = succ;
succ.prev = pred;
}
// 更新大小
// 更新变化次数
size += numNew;
modCount++;
return true;
}
检测下标 checkPositionIndex
private void checkPositionIndex(int index) {
if (!isPositionIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
/**
* Tells if the argument is the index of a valid position for an
* iterator or an add operation.
*/
private boolean isPositionIndex(int index) {
return index >= 0 && index <= size;
}
直接将 index 和数组的大小进行范围比较。
核心方法
add
在指定位置插入一个元素：
public void add(int index, E element) {
// 位置校验
checkPositionIndex(index);
// 等于最后
if (index == size)
linkLast(element);
else
linkBefore(element, node(index));
}
node(index)
获取指定位置的元素，实现如下：
/**
* Returns the (non-null) Node at the specified element index.
*
* 如果超过了范围呢？
*/
Node<E> node(int index) {
// assert isElementIndex(index);
// 这里因为是双向节点。
// 所以只判断了一半的位置，如果在前一半，从前往后比较快；反之，则从后往前遍历比较快。
// 也就是 TC 为 O(N/2)
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;
}
}
linkLast
/**
* Links e as last element.
*/
void linkLast(E e) {
// 最后一个元素
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null);
last = newNode;
// 处理为空的情况
if (l == null)
first = newNode;
else
l.next = newNode;
// 更新 size + modCount
size++;
modCount++;
}
linkBefore
/**
* 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;
// 前继为空
if (pred == null)
first = newNode;
else
// 新的节点放在前继节点之后
pred.next = newNode;
size++;
modCount++;
}
set
set 的思路其实非常简单粗暴，找到对应的节点，然后直接更换即可。
最后返回以前的节点。
/**
* 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;
}
get
获取方法对比之下就非常的简单。
校验范围，范围节点元素。
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}
remove
指定索引删除一个元素：
/**
* Removes the element at the specified position in this list. Shifts any
* subsequent elements to the left (subtracts one from their indices).
* Returns the element that was removed from the list.
*
* @param index the index of the element to be removed
* @return the element previously at the specified position
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
// 校验范围
checkElementIndex(index);
// 取消对应节点
return unlink(node(index));
}
unlink
a<=>b<=>c，如果删除 b，就会变成 a<=>c
/**
* Unlinks non-null node x.
*/
E unlink(Node<E> x) {
// assert x != null;
final E element = x.item;
final Node<E> next = x.next;
final Node<E> prev = x.prev;
if (prev == null) {
first = next;
} else {
prev.next = next;
x.prev = null;
}
if (next == null) {
last = prev;
} else {
next.prev = prev;
x.next = null;
}
x.item = null;
size--;
modCount++;
return element;
}
小结
比较有收获的就是获取元素时，根据下标的位置，判断是在前面还是在后面。
从而节省大概一半的时间。
参考资料
jdk8
