1. LinkedList概述
LinkedList 和 ArrayList 一樣，都實現(xiàn)了 List 接口宪卿，但其內(nèi)部的數(shù)據(jù)結構有本質的不同。LinkedList 是基于鏈表實現(xiàn)的万栅，所以它的插入和刪除操作比 ArrayList 更加高效佑钾。但也是由于其為基于鏈表的，所以隨機訪問的效率要比 ArrayList 差烦粒。
2. LinkedList結構
（1）LinkedList繼承關系
LinkedList類圖關系
- LinkedList 是一個繼承于AbstractSequentialList的雙向鏈表休溶。它也可以被當作堆棧、隊列或雙端隊列進行操作撒遣。
- LinkedList 實現(xiàn) List 接口邮偎，能對它進行隊列操作。
- LinkedList 實現(xiàn) Deque 接口义黎，即能將LinkedList當作雙端隊列使用。
- LinkedList 實現(xiàn)了Cloneable接口豁跑，即覆蓋了函數(shù)clone()廉涕，能克隆。
- LinkedList 實現(xiàn)java.io.Serializable接口艇拍，這意味著LinkedList支持序列化狐蜕，能通過序列化去傳輸。
- LinkedList 是非同步的卸夕。
LinkedList 為什么要繼承 AbstractSequentialList ?
- AbstractSequentialList 實現(xiàn)了get(int index)层释、set(int index, E element)、add(int index, E element) 和 remove(int index)這些骨干性函數(shù)快集。降低了List接口的復雜度贡羔。這些接口都是隨機訪問List的廉白，LinkedList是雙向鏈表，既然它繼承于AbstractSequentialList乖寒，就相當于已經(jīng)實現(xiàn)了“get(int index)這些接口”猴蹂。
- 此外，我們?nèi)粜枰ㄟ^AbstractSequentialList自己實現(xiàn)一個列表楣嘁，只需要擴展此類磅轻，并提供 listIterator() 和 size() 方法的實現(xiàn)即可。若要實現(xiàn)不可修改的列表逐虚，則需要實現(xiàn)列表迭代器的 hasNext聋溜、next、hasPrevious叭爱、previous 和 index 方法即可撮躁。
（2）LinkedList類定義
LinkedList中定義了三個屬性：
- first 指向第一個節(jié)點的指針
- last 指向最后一個節(jié)點的指針
- size 是雙向鏈表中節(jié)點實例的個數(shù)，默認為0
節(jié)點 Node ：雙向鏈表中節(jié)點對應為Node類的實例涤伐。Node 中包含成員變量： prev, next, element馒胆。其中，prev是該節(jié)點的上一個節(jié)點凝果，next是該節(jié)點的下一個節(jié)點祝迂，element是該節(jié)點所包含的值。
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
// 鏈表中節(jié)點數(shù)量
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;
}
（3）LinkedList數(shù)據(jù)結構
LinkedList底層的數(shù)據(jù)結構是基于雙向循環(huán)鏈表的器净，且頭結點中不存放數(shù)據(jù)型雳，如下：
雙向鏈表
節(jié)點信息
對應的源碼中為Node內(nèi)部類：
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;
}
}
3. LinkedList實現(xiàn)
（1）構造方法
LinkedList提供了兩個構造方法：
- 1）無參構造的空集合：前一節(jié)點和后一節(jié)點均為null纠俭，這樣整個鏈表其實就只有header一個節(jié)點，用于表示一個空的鏈表浪慌。
/**
* Constructs an empty list.
*/
public LinkedList() {
}
- 2）接收一個Collection參數(shù)c糟把，調(diào)用第一個構造方法構造一個空的鏈表费奸，之后通過addAll將c中的元素全部添加到鏈表中。
/**
* 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);
}
（2）添加元素
1）add(E e)
將元素添加到鏈表的尾部，linkLast 方法分三步：
① 添加元素到尾部
② 集合長度加1（size++）
③ 集合修改次數(shù)加1（modCount++）
/**
* 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++;
}
2）add(int index, E element)
將元素添加到指定位置
① 檢查要添加的元素的位置是否越界
② 如果要添加的位置index=size虐急，直接在鏈表尾部添加粤蝎，等價于add(E e)方法
③ 否則蟀苛，索引到index位置的元素腺律，在其前面添加該元素
/**
* 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) {
checkPositionIndex(index);
if (index == size)
linkLast(element);
else
linkBefore(element, node(index));
}
node(int index)：二分查找某個位置的元素，如果index小于總長度size的一半古掏，從前往后查找损话，否則從后往前查找
/**
* 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;
}
}
找到對應位置的元素，執(zhí)行方法 linkBefore(E e, Node<E> succ) ：將該節(jié)點插入到index位置的元素的前面槽唾，即調(diào)整index位置元素的頭指針指向要插入的元素丧枪，同時修改index位置前一個元素的尾指針指向新添加的元素
/**
* Inserts element e before non-null Node succ.
*/
void linkBefore(E e, Node<E> succ) {
// assert succ != null;
// 將index-1位置元素存起來
final Node<E> pred = succ.prev;
// 創(chuàng)建要插入的新節(jié)點光涂，其頭指針指向index-1位置的元素，尾指針指向index位置元素
final Node<E> newNode = new Node<>(pred, e, succ);
// 調(diào)整指針位置
succ.prev = newNode;
if (pred == null)
first = newNode;
else
pred.next = newNode;
size++;
modCount++;
}
插入步驟如下：
①找到index位置元素
② 創(chuàng)建要插入的節(jié)點
實例化插入的節(jié)點
③ 調(diào)整指針指向
調(diào)整指針
3）addAll(int index, Collection<? extends E> c)
將collection對象轉換成數(shù)組鏈表豪诲，插入到指定位置顶捷，同上的在指定位置插入元素類似
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}
/**
* Inserts all of the elements in the specified collection into this
* list, starting at the specified position. Shifts the element
* currently at that position (if any) and any subsequent elements to
* the right (increases their indices). The new elements will appear
* in the list in the order that they are returned by the
* specified collection's iterator.
*
* @param index index at which to insert the first element
* from the specified collection
* @param c collection containing elements to be added to this list
* @return {@code true} if this list changed as a result of the call
* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index);
Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;
Node<E> pred, succ;
if (index == size) {
succ = null;
pred = last;
} else {
succ = node(index);
pred = succ.prev;
}
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
Node<E> newNode = new Node<>(pred, e, null);
if (pred == null)
first = newNode;
else
pred.next = newNode;
pred = newNode;
}
if (succ == null) {
last = pred;
} else {
pred.next = succ;
succ.prev = pred;
}
size += numNew;
modCount++;
return true;
}
（3）獲取元素
get(int index)：獲取指定位置的元素
① 檢查是否越界
② 遍歷鏈表的元素
從頭開始遍歷還是從結尾開始遍歷，取決于 index 與當前鏈表長度的 size/2 比較屎篱，如果索引位置小于當前鏈表長度的一半服赎，從頭往后遍歷，否則從尾向前開始遍歷交播。
注意細節(jié)：位運算與直接做除法的區(qū)別重虑。
/**
* 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;
}
}
（4）刪除元素
1）remove()
本質是調(diào)用removeFirst()方法，刪除鏈表頭節(jié)點元素
/**
* 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();
}
2）removeFirst()
移除第一個節(jié)點秦士，將第一個節(jié)點置為null（讓GC回收）缺厉，將下一個節(jié)點變成第一個節(jié)點，鏈表長度減1隧土，修改次數(shù)加1提针。
/**
* 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) {
// 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;
}
3）removeLast()
移除最后一個節(jié)點，將最后一個節(jié)點置為null（讓GC回收）曹傀，將最后一個節(jié)點的上一節(jié)點變成最后一個節(jié)點辐脖，鏈表長度減1，修改次數(shù)加1皆愉。
/**
* Removes and returns the last element from this list.
*
* @return the last element from this list
* @throws NoSuchElementException if this list is empty
*/
public E removeLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return unlinkLast(l);
}
/**
* Unlinks non-null last node l.
*/
private E unlinkLast(Node<E> l) {
// assert l == last && l != null;
final E element = l.item;
final Node<E> prev = l.prev;
l.item = null;
l.prev = null; // help GC
last = prev;
if (prev == null)
first = null;
else
prev.next = null;
size--;
modCount++;
return element;
}
4）remove(int index)
刪除指定位置元素：根據(jù)索引index獲取需要移除的節(jié)點嗜价，將移除的節(jié)點置空，調(diào)整其上一個節(jié)點和下一個節(jié)點指針的指向幕庐，鏈表長度減1久锥，修改次數(shù)加1。异剥。
/**
* 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) {
checkElementIndex(index);
return unlink(node(index));
}
/**
* 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;
}
（4）清空集合
clear() ：遍歷鏈表瑟由，將所有元素置空，然后將鏈表長度修改成0冤寿，修改次數(shù)加1错妖。
/**
* Removes all of the elements from this list.
* The list will be empty after this call returns.
*/
public void clear() {
// Clearing all of the links between nodes is "unnecessary", but:
// - helps a generational GC if the discarded nodes inhabit
// more than one generation
// - is sure to free memory even if there is a reachable Iterator
for (Node<E> x = first; x != null; ) {
Node<E> next = x.next;
x.item = null;
x.next = null;
x.prev = null;
x = next;
}
first = last = null;
size = 0;
modCount++;
}
（5）包含元素
contains(Object o) ：集合中是否包含某個元素
查找方式：從前向后查找，返回元素值為查找的值o的索引疚沐，不存在返回-1
indexOf(Object o)判斷鏈表中是否存在節(jié)點的element和o相等，若相等則返回該節(jié)點在鏈表中的索引位置潮模，若不存在則放回-1亮蛔。
contains(Object o)方法通過判斷indexOf(Object o)方法返回的值是否是-1來判斷鏈表中是否包含對象o。
/**
* Returns {@code true} if this list contains the specified element.
* More formally, returns {@code true} if and only if this list contains
* at least one element {@code e} such that
* <tt>(o==null ? e==null : o.equals(e))</tt>.
*
* @param o element whose presence in this list is to be tested
* @return {@code true} if this list contains the specified element
*/
public boolean contains(Object o) {
return indexOf(o) != -1;
}
/**
* Returns the index of the first occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the lowest index {@code i} such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
* or -1 if there is no such index.
*
* @param o element to search for
* @return the index of the first occurrence of the specified element in
* this list, or -1 if this list does not contain the element
*/
public int indexOf(Object o) {
int index = 0;
if (o == null) {
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null)
return index;
index++;
}
} else {
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item))
return index;
index++;
}
}
return -1;
}
（6）復制元素
1）Object clone()
調(diào)用父類的clone()方法初始化對象鏈表clone擎厢，將clone構造成一個空的雙向循環(huán)鏈表究流，之后將first的下一個節(jié)點開始將逐個節(jié)點添加到clone中辣吃，最后返回克隆的clone對象。
/**
* Returns a shallow copy of this {@code LinkedList}. (The elements
* themselves are not cloned.)
*
* @return a shallow copy of this {@code LinkedList} instance
*/
public Object clone() {
LinkedList<E> clone = superClone();
// Put clone into "virgin" state
clone.first = clone.last = null;
clone.size = 0;
clone.modCount = 0;
// Initialize clone with our elements
for (Node<E> x = first; x != null; x = x.next)
clone.add(x.item);
return clone;
}
private LinkedList<E> superClone() {
try {
return (LinkedList<E>) super.clone();
} catch (CloneNotSupportedException e) {
throw new InternalError(e);
}
}
2）Object[] toArray()
將集合轉換為數(shù)組：創(chuàng)建大小和LinkedList相等的數(shù)組result芬探，遍歷鏈表神得，將每個節(jié)點的元素element復制到數(shù)組中，返回數(shù)組偷仿。
/**
* Returns an array containing all of the elements in this list
* in proper sequence (from first to last element).
*
* <p>The returned array will be "safe" in that no references to it are
* maintained by this list. (In other words, this method must allocate
* a new array). The caller is thus free to modify the returned array.
*
* <p>This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all of the elements in this list
* in proper sequence
*/
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;
}
3）<T> T[] toArray(T[] a)
將集合轉換為指定類型的數(shù)組：
① 先判斷數(shù)組a的大小是否足夠哩簿，若大小不夠則拓展。
② 這里用到了反射的方法酝静，重新實例化了一個大小為size的數(shù)組节榜。③ 之后將數(shù)組a賦值給數(shù)組result，遍歷鏈表向result中添加的元素别智。
④ 最后判斷數(shù)組a的長度是否大于size宗苍，若大于則將size位置的內(nèi)容設置為null，返回a薄榛。
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
a = (T[])java.lang.reflect.Array.newInstance(
a.getClass().getComponentType(), size);
int i = 0;
Object[] result = a;
for (Node<E> x = first; x != null; x = x.next)
result[i++] = x.item;
if (a.length > size)
a[size] = null;
return a;
}
從代碼中可以看出讳窟，數(shù)組a的length小于等于size時，a中所有元素被覆蓋敞恋，被拓展來的空間存儲的內(nèi)容都是null丽啡；若數(shù)組a的length的length大于size，則0至size-1位置的內(nèi)容被覆蓋耳舅，size位置的元素被設置為null碌上，size之后的元素不變。
（7）遍歷元素
listIterator：這是一個內(nèi)部類浦徊，用于遍歷當前的鏈表元素馏予，但是由于LinkedList也是非線程安全的類，也可能會產(chǎn)生多線程修改的異常盔性。
public ListIterator<E> listIterator(int index) {
checkPositionIndex(index);
return new ListItr(index);
}
// ListItr實現(xiàn)了ListIterator接口霞丧，可知它是一個迭代器，通過它可以遍歷修改LinkedList冕香。
// 在LinkedList中提供了獲取ListItr對象的方法：listIterator(int index)蛹尝。
private class ListItr implements ListIterator<E> {
private Node<E> lastReturned;
private Node<E> next;
private int nextIndex;
private int expectedModCount = modCount;
ListItr(int index) {
// assert isPositionIndex(index);
next = (index == size) ? null : node(index);
nextIndex = index;
}
......
