LinkedList几个核心方法是linkFirst(E),linkLast(E),linkBefore(E),unlinkFirst(),unlinkLast(),unlink(Node)
LinkedList是双向链表,四个重要的成员变量是pre,next,first,last
pre是前驱节点,next是后继节点,first是第一个节点,last是最后一个节点
linkFirst(E)是把节点添加到链表头部
linkLast(E)是把节点添加到链表尾部
linkBefore(succ)是把节点添加到指定节点的前面
unlinkFirst()是移除链表第一个节点
unlinkLast()是移除链表最后一个节点
unlink(Node)是移除指定节点
插入操作
/**
* Links e as first element. 元素添加到链表头部 头插法
*/
private void linkFirst(E e) {
final Node<E> f = first; //头结点
final Node<E> newNode = new Node<>(null, e, f); //插入节点指向头结点
first = newNode; //插入节点为新的头结点
if (f == null) //链表为空设置插入节点为尾节点
last = newNode;
else //否则旧的头结点指向插入节点
f.prev = newNode;
size++;
modCount++;
}
/**
* Links e as last element. 元素添加到链表尾部 尾插法
*/
void linkLast(E e) {
final Node<E> l = last;//尾节点
final Node<E> newNode = new Node<>(l, e, null);//新节点前驱指向l即尾节点
last = newNode; //自身设置为尾结点
if (l == null) //为空的话,说明原来的LinkedList为空,所以同时也需要把新节点设置为头节点
first = newNode;
else //不空就把l的next设置为newNode 尾节点后继指向新节点
l.next = newNode;
size++;
modCount++;
}
/**
* Inserts element e before non-null Node succ. 在非空节点succ之前插入元素e 插入操作
*/
void linkBefore(E e, Node<E> succ) {
// assert succ != null;
final Node<E> pred = succ.prev; //获取succ节点的前驱节点 0->1->2->4 succ是4 e是3 pred是2
final Node<E> newNode = new Node<>(pred, e, succ); //用e新建节点前驱指向2 后继指向4
succ.prev = newNode; //succ前驱指向pred 4指向2
if (pred == null) //pred为null说明该节点插入在头节点之前,要重置first头节点
first = newNode;
else
pred.next = newNode; //否则pred后继指向新建节点
size++;
modCount++;
}
删除操作
/**
* Unlinks non-null first node f. 删除头结点
*/
private E unlinkFirst(Node<E> f) {
// assert f == first && f != null;
final E element = f.item; //头结点元素
final Node<E> next = f.next; //获取头结点的下一个节点
f.item = null; //头结点元素置空
f.next = null; // help GC 防止内存泄漏
first = next; //头结点的下一个节点设为新的头结点
if (next == null)//链表只有一个结点
last = null;
else
next.prev = null;//头结点置空
size--;
modCount++;
return element;
}
/**
* Unlinks non-null last node l. 删除尾结点
*/
private E unlinkLast(Node<E> l) {
// assert l == last && l != null;
final E element = l.item;//取出尾结点元素
final Node<E> prev = l.prev;//尾节点前一个节点
l.item = null;//元素置空
l.prev = null; // help GC 防止内存泄漏
last = prev; //尾节点前一个节点成为新的尾节点
if (prev == null)//链表只有一个节点
first = null;
else
prev.next = null; //清空原来的尾节点
size--;
modCount++;
return element;
}
/**
* Unlinks non-null node x.删除指定节点 返回节点里的元素
*/
E unlink(Node<E> x) {
// assert x != null;
final E element = x.item;
final Node<E> next = x.next;//指定节点的后继节点
final Node<E> prev = x.prev;//指定节点的前驱节点
//没有前驱即删除的是头结点,指定节点的后继节点成为新的头结点
if (prev == null) {
first = next;
} else {
prev.next = next;//指定节点的前驱节点指向指定节点的后继节点
x.prev = null;//防止内存泄漏
}
//没有前驱即删除的是尾结点,指定节点的前驱节点成为新的尾结点
if (next == null) {
last = prev;
} else {
next.prev = prev;//指定节点的后继节点指向指定节点的前驱节点
x.next = null;//防止内存泄漏
}
x.item = null;//清空节点元素
size--;
modCount++;
return element;
}
这个方法是根据索引获取节点
/**
* Returns the (non-null) Node at the specified element index.
*/
Node<E> node(int index) {
// assert isElementIndex(index);
//index小于size的一半就从头开始找
if (index < (size >> 1)) {
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
//index大于size的一半就从尾开始找
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}
其他方法都是基于linkFirst(),linkLast(),linkBefore(),unlinkFirst(),unlinkLast(),unlink()这几个方法的
offer,add,addLast基于linkLast(E),add(index)基于linkBefore()
poll(),remove()基于unlinkFirst(),remove(index)基于unlink(Node)
addFirst基于linkFirst(E)
removeLast()基于unlinkLast()
poll和remove的区别是元素不存在poll会返回null,remove抛异常
//这个方法是把集合c添加到指定的位置
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index);//判断传进来的参数是否合法
Object[] a = c.toArray();//先把集合转化为数组,然后为该数组添加一个新的引用
int numNew = a.length;
if (numNew == 0)//如果待添加的集合为空,直接返回,无需进行后面的步骤。后面都是用来把集合中的元素添加到
return false;
//1->4->5 在4插入2 3 pred是1 succ是4 index为1因为是从0开始
Node<E> pred, succ;//succ是待添加节点的位置 pred是待添加节点的前一个节点 succ是4 pred是1
if (index == size) {
//如果index==size;说明此时需要添加LinkedList中的集合中的每一个元素都是在LinkedList的最后面
succ = null; //所以把succ设置为空,pred指向尾节点。
pred = last;
} else {
succ = node(index);//否则的话succ指向插入待插入位置的节点 node(index) 根据index找到待插入位置
pred = succ.prev;
}
//接着遍历数组中的每个元素
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
Node<E> newNode = new Node<>(pred, e, null);//使用当前值新建一个节点然后指向pred 1<-2
if (pred == null) //pred为空即LinkedList为空,把新建节点设为头结点
first = newNode;
else
pred.next = newNode; //否则pred指向新建节点 1->2
pred = newNode; //最后把pred指向当前节点,即pred后移一位,以便后续新节点的添加
}
// 如果是从尾部开始插入的,则把last置为最后一个插入的元素
if (succ == null) {
last = pred;//此时pred指向的是LinkedList中的最后一个元素,所以把last指向pred指向的节点。
} else {
// 如果不是从尾部插入的,则把尾部的数据和之前的节点连起来 1->4->5 在4插入2 3
pred.next = succ; //pred指向succ pred是3 3->4
succ.prev = pred; //succ指向pred 4指向3 3<-4
}
size += numNew;//最后把集合的大小设置为新的大小。
modCount++;
return true;
}
LinkedList源码文件
/*
* Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
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*/
package java.util;
import java.util.function.Consumer;
/**
* Doubly-linked list implementation of the {@code List} and {@code Deque}
* interfaces. Implements all optional list operations, and permits all
* elements (including {@code 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.
*
* <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.
*
* 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>
*
* <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.
*
* <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>
*
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @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
{
transient int size = 0;
/**
* Pointer to first node.
* Invariant: (first == null && last == null) || 头结点
* (first.prev == null && first.item != null)
*/
transient Node<E> first;
/**
* Pointer to last node.
* Invariant: (first == null && last == null) || 尾节点
* (last.next == null && last.item != null)
*/
transient Node<E> last;
/**
* 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);
}
/**
* Links e as first element. 元素添加到链表头部 头插法
*/
private void linkFirst(E e) {
final Node<E> f = first; //头结点
final Node<E> newNode = new Node<>(null, e, f); //插入节点指向头结点
first = newNode; //插入节点为新的头结点
if (f == null) //链表为空设置插入节点为尾节点
last = newNode;
else //否则旧的头结点指向插入节点
f.prev = newNode;
size++;
modCount++;
}
/**
* Links e as last element. 元素添加到链表尾部 尾插法
*/
void linkLast(E e) {
final Node<E> l = last;//尾节点
final Node<E> newNode = new Node<>(l, e, null);//新节点前驱指向l即尾节点
last = newNode; //自身设置为尾结点
if (l == null) //为空的话,说明原来的LinkedList为空,所以同时也需要把新节点设置为头节点
first = newNode;
else //不空就把l的next设置为newNode 尾节点后继指向新节点
l.next = newNode;
size++;
modCount++;
}
/**
* Inserts element e before non-null Node succ. 在非空节点succ之前插入元素e 插入操作
*/
void linkBefore(E e, Node<E> succ) {
// assert succ != null;
final Node<E> pred = succ.prev; //获取succ节点的前驱节点 0->1->2->4 succ是4 e是3 pred是2
final Node<E> newNode = new Node<>(pred, e, succ); //用e新建节点前驱指向2 后继指向4
succ.prev = newNode; //succ前驱指向pred 4指向2
if (pred == null) //pred为null说明该节点插入在头节点之前,要重置first头节点
first = newNode;
else
pred.next = newNode; //否则pred后继指向新建节点
size++;
modCount++;
}
/**
* Unlinks non-null first node f. 删除头结点
*/
private E unlinkFirst(Node<E> f) {
// assert f == first && f != null;
final E element = f.item; //头结点元素
final Node<E> next = f.next; //获取头结点的下一个节点
f.item = null; //头结点元素置空
f.next = null; // help GC 防止内存泄漏
first = next; //头结点的下一个节点设为新的头结点
if (next == null)//链表只有一个结点
last = null;
else
next.prev = null;//头结点置空
size--;
modCount++;
return element;
}
/**
* Unlinks non-null last node l. 删除尾结点
*/
private E unlinkLast(Node<E> l) {
// assert l == last && l != null;
final E element = l.item;//取出尾结点元素
final Node<E> prev = l.prev;//尾节点前一个节点
l.item = null;//元素置空
l.prev = null; // help GC 防止内存泄漏
last = prev; //尾节点前一个节点成为新的尾节点
if (prev == null)//链表只有一个节点
first = null;
else
prev.next = null; //清空原来的尾节点
size--;
modCount++;
return element;
}
/**
* Unlinks non-null node x.删除指定节点 返回节点里的元素
*/
E unlink(Node<E> x) {
// assert x != null;
final E element = x.item;
final Node<E> next = x.next;//指定节点的后继节点
final Node<E> prev = x.prev;//指定节点的前驱节点
//没有前驱即删除的是头结点,指定节点的后继节点成为新的头结点
if (prev == null) {
first = next;
} else {
prev.next = next;//指定节点的前驱节点指向指定节点的后继节点
x.prev = null;//防止内存泄漏
}
//没有前驱即删除的是尾结点,指定节点的前驱节点成为新的尾结点
if (next == null) {
last = prev;
} else {
next.prev = prev;//指定节点的后继节点指向指定节点的前驱节点
x.next = null;//防止内存泄漏
}
x.item = null;//清空节点元素
size--;
modCount++;
return element;
}
/**
* 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;
}
/**
* 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);
}
/**
* 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);
}
/**
* Inserts the specified element at the beginning of this list.
*
* @param e the element to add
*/
public void addFirst(E e) {
linkFirst(e);
}
/**
* Appends the specified element to the end of this list.
*
* <p>This method is equivalent to {@link #add}.
*
* @param e the element to add
*/
public void addLast(E e) {
linkLast(e);
}
/**
* 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 number of elements in this list.
*
* @return the number of elements in this list
*/
public int size() {
return size;
}
/**
* 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);//将元素添加到链表尾部
return true;
}
/**
* 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).
* 删除指定元素
* @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) {
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;
}
/**
* Appends all of the elements in the specified collection to the end of
* this list, in the order that they are returned by the specified
* collection's iterator. The behavior of this operation is undefined if
* the specified collection is modified while the operation is in
* progress. (Note that this will occur if the specified collection is
* this list, and it's nonempty.)
*
* @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 NullPointerException if the specified collection is null
*/
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);//判断传进来的参数是否合法
Object[] a = c.toArray();//先把集合转化为数组,然后为该数组添加一个新的引用
int numNew = a.length;
if (numNew == 0)//如果待添加的集合为空,直接返回,无需进行后面的步骤。后面都是用来把集合中的元素添加到
return false;
//1->4->5 在4插入2 3 pred是1 succ是4 index为1因为是从0开始
Node<E> pred, succ;//succ是待添加节点的位置 pred是待添加节点的前一个节点 succ是4 pred是1
if (index == size) {
//如果index==size;说明此时需要添加LinkedList中的集合中的每一个元素都是在LinkedList的最后面
succ = null; //所以把succ设置为空,pred指向尾节点。
pred = last;
} else {
succ = node(index);//否则的话succ指向插入待插入位置的节点 node(index) 根据index找到待插入位置
pred = succ.prev;
}
//接着遍历数组中的每个元素
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
Node<E> newNode = new Node<>(pred, e, null);//使用当前值新建一个节点然后指向pred 1<-2
if (pred == null) //pred为空即LinkedList为空,把新建节点设为头结点
first = newNode;
else
pred.next = newNode; //否则pred指向新建节点 1->2
pred = newNode; //最后把pred指向当前节点,即pred后移一位,以便后续新节点的添加
}
// 如果是从尾部开始插入的,则把last置为最后一个插入的元素
if (succ == null) {
last = pred;//此时pred指向的是LinkedList中的最后一个元素,所以把last指向pred指向的节点。
} else {
// 如果不是从尾部插入的,则把尾部的数据和之前的节点连起来 1->4->5 在4插入2 3
pred.next = succ; //pred指向succ pred是3 3->4
succ.prev = pred; //succ指向pred 4指向3 3<-4
}
size += numNew;//最后把集合的大小设置为新的大小。
modCount++;
return true;
}
/**
* Removes all of the elements from this list. 删除链表所有元素
* 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; ) {
//从头结点开始遍历
Node<E> next = x.next;
x.item = null;
x.next = null;
x.prev = null;
x = next; //遍历下一节点
}
first = last = null;
size = 0;
modCount++;
}
// Positional Access Operations
/**
* Returns the element at the specified position in this list.
* 根据索引获取元素
* @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);//检查越界
return node(index).item;
}
/**
* Replaces the element at the specified position in this list with the
* specified element.
* 往指定位置设置值,更新操作 返回旧值
* @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;
}
/**
* 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).
* 往指定位置添加值,新增操作 返回旧值
* @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) //在链表尾部添加
linkLast(element);
else
linkBefore(element, node(index));
}
/**
* 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.
* 移除指定位置的节点
* @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));
}
/**
* Tells if the argument is the index of an existing element. 参数是否越界
*/
private boolean isElementIndex(int index) {
return index >= 0 && index < size;
}
/**
* Tells if the argument is the index of a valid position for an 参数是否能作为添加操作的索引 等于size时是添加到尾部
* iterator or an add operation.
*/
private boolean isPositionIndex(int index) {
return index >= 0 && index <= size;
}
/**
* Constructs an IndexOutOfBoundsException detail message.
* Of the many possible refactorings of the error handling code,
* this "outlining" performs best with both server and client VMs.
*/
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
private void checkElementIndex(int index) {
if (!isElementIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private void checkPositionIndex(int index) {
if (!isPositionIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
/**
* Returns the (non-null) Node at the specified element index.
*/
Node<E> node(int index) {
// assert isElementIndex(index);
//index小于size的一半就从头开始找
if (index < (size >> 1)) {
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
//index大于size的一半就从尾开始找
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}
// Search Operations
/**
* 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.
* 根据值返回第一个索引,从头开始找
* @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;
}
/**
* Returns the index of the last occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the highest index {@code i} such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
* or -1 if there is no such index.
* 根据值返回最后一个索引,从尾开始找
* @param o element to search for
* @return the index of the last occurrence of the specified element in
* this list, or -1 if this list does not contain the element
*/
public int lastIndexOf(Object o) {
int index = size;
if (o == null) {
for (Node<E> x = last; x != null; x = x.prev) {
index--;
if (x.item == null)
return index;
}
} else {
for (Node<E> x = last; x != null; x = x.prev) {
index--;
if (o.equals(x.item))
return index;
}
}
return -1;
}
// Queue operations.
/**
* Retrieves, but does not remove, the head (first element) of this list.
* 返回队头元素,不存在返回null
* @return the head of this list, or {@code null} if this list is empty
* @since 1.5
*/
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}
/**
* Retrieves, but does not remove, the head (first element) of this list.
* 返回队头元素,不存在抛NoSuchElementException
* @return the head of this list
* @throws NoSuchElementException if this list is empty
* @since 1.5
*/
public E element() {
return getFirst();
}
/**
* Retrieves and removes the head (first element) of this list.
* 移除队头元素,不存在返回null和remove区别是会remove会抛异常
* @return the head of this list, or {@code null} if this list is empty
* @since 1.5
*/
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}
/**
* Retrieves and removes the head (first element) of this list.
* 移除队头元素,不存在抛NoSuchElementException
* @return the head of this list
* @throws NoSuchElementException if this list is empty
* @since 1.5
*/
public E remove() {
return removeFirst();
}
/**
* Adds the specified element as the tail (last element) of this list.
* 添加元素到队尾
* @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.
* 添加元素到队头
* @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.
* 添加元素到队尾 和offer效果一样,实现了Deque接口
* @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;
}
/**
* Retrieves, but does not remove, the first element of this list,
* or returns {@code null} if this list is empty.
* 返回队头元素
* @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.
* 返回队尾元素
* @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;
}
/**
* Retrieves and removes the first element of this list,
* or returns {@code null} if this list is empty.
* 移除队头元素,不存在返回null,remove会抛异常
* @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.
* 移除队尾元素,不存在返回null
* @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);
}
/**
* Pushes an element onto the stack represented by this list. In other
* words, inserts the element at the front of this list.
*
* <p>This method is equivalent to {@link #addFirst}.
* 添加元素到队头
* @param e the element to push
* @since 1.6
*/
public void push(E e) {
addFirst(e);
}
/**
* Pops an element from the stack represented by this list. In other
* words, removes and returns the first element of this list.
*
* <p>This method is equivalent to {@link #removeFirst()}.
* 移除队头元素
* @return the element at the front of this list (which is the top
* of the stack represented by this list)
* @throws NoSuchElementException if this list is empty
* @since 1.6
*/
public E pop() {
return removeFirst();
}
/**
* Removes the first occurrence of the specified element in this
* list (when traversing the list from head to tail). If the list
* does not contain the element, it is unchanged.
* 删除第一个出现的元素
* @param o element to be removed from this list, if present
* @return {@code true} if the list contained the specified element
* @since 1.6
*/
public boolean removeFirstOccurrence(Object o) {
return remove(o);
}
/**
* Removes the last occurrence of the specified element in this
* list (when traversing the list from head to tail). If the list
* does not contain the element, it is unchanged.
* 删除最后一个出现的元素
* @param o element to be removed from this list, if present
* @return {@code true} if the list contained the specified element
* @since 1.6
*/
public boolean removeLastOccurrence(Object o) {
if (o == null) {
for (Node<E> x = last; x != null; x = x.prev) {
if (x.item == null) {
unlink(x);
return true;
}
}
} else {
for (Node<E> x = last; x != null; x = x.prev) {
if (o.equals(x.item)) {
unlink(x);
return true;
}
}
}
return false;
}
/**
* Returns a list-iterator of the elements in this list (in proper
* sequence), starting at the specified position in the list.
* Obeys the general contract of {@code List.listIterator(int)}.<p>
*
* The list-iterator is <i>fail-fast</i>: if the list is structurally
* modified at any time after the Iterator is created, in any way except
* through the list-iterator's own {@code remove} or {@code add}
* methods, the list-iterator will throw a
* {@code 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.
* 链表迭代器
* @param index index of the first element to be returned from the
* list-iterator (by a call to {@code next})
* @return a ListIterator of the elements in this list (in proper
* sequence), starting at the specified position in the list
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see List#listIterator(int)
*/
public ListIterator<E> listIterator(int index) {
checkPositionIndex(index);
return new ListItr(index);
}
private class ListItr implements ListIterator<E> {
private Node<E> lastReturned;
private Node<E> next; //当前遍历元素 1->2->3 index为0 next就为1
private int nextIndex;
private int expectedModCount = modCount;
ListItr(int index) {
// assert isPositionIndex(index);
next = (index == size) ? null : node(index);
nextIndex = index;
}
public boolean hasNext() {
return nextIndex < size;
}
public E next() {
checkForComodification();
if (!hasNext())
throw new NoSuchElementException();
lastReturned = next;
next = next.next; //遍历下一节点
nextIndex++;
return lastReturned.item;
}
public boolean hasPrevious() {
return nextIndex > 0;
}
public E previous() {
checkForComodification();
if (!hasPrevious())
throw new NoSuchElementException();
lastReturned = next = (next == null) ? last : next.prev;
nextIndex--;
return lastReturned.item;
}
public int nextIndex() {
return nextIndex;
}
public int previousIndex() {
return nextIndex - 1;
}
public void remove() {
checkForComodification();
if (lastReturned == null)
throw new IllegalStateException();
Node<E> lastNext = lastReturned.next;
unlink(lastReturned);
if (next == lastReturned)
next = lastNext;
else
nextIndex--;
lastReturned = null;
expectedModCount++;
}
public void set(E e) {
if (lastReturned == null)
throw new IllegalStateException();
checkForComodification();
lastReturned.item = e;
}
public void add(E e) {
checkForComodification();
lastReturned = null;
if (next == null)
linkLast(e);
else
linkBefore(e, next);
nextIndex++;
expectedModCount++;
}
public void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
while (modCount == expectedModCount && nextIndex < size) {
action.accept(next.item);
lastReturned = next;
next = next.next;
nextIndex++;
}
checkForComodification();
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
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;
}
}
/**
* @since 1.6
*/
public Iterator<E> descendingIterator() {
return new DescendingIterator();
}
/**
* Adapter to provide descending iterators via ListItr.previous
*/
private class DescendingIterator implements Iterator<E> {
private final ListItr itr = new ListItr(size());
public boolean hasNext() {
return itr.hasPrevious();
}
public E next() {
return itr.previous();
}
public void remove() {
itr.remove();
}
}
@SuppressWarnings("unchecked")
private LinkedList<E> superClone() {
try {
return (LinkedList<E>) super.clone();
} catch (CloneNotSupportedException e) {
throw new InternalError(e);
}
}
/**
* 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;
}
/**
* 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.
* 链表转Object数组
* @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;
}
/**
* Returns an array containing all of the elements in this list in
* proper sequence (from first to last element); the runtime type of
* the returned array is that of the specified array. If the list fits
* in the specified array, it is returned therein. Otherwise, a new
* array is allocated with the runtime type of the specified array and
* the size of this list.
*
* <p>If the list fits in the specified array with room to spare (i.e.,
* the array has more elements than the list), the element in the array
* immediately following the end of the list is set to {@code null}.
* (This is useful in determining the length of the list <i>only</i> if
* the caller knows that the list does not contain any null elements.)
*
* <p>Like the {@link #toArray()} method, this method acts as bridge between
* array-based and collection-based APIs. Further, this method allows
* precise control over the runtime type of the output array, and may,
* under certain circumstances, be used to save allocation costs.
* 链表转泛型数组
* <p>Suppose {@code x} is a list known to contain only strings.
* The following code can be used to dump the list into a newly
* allocated array of {@code String}:
*
* <pre>
* String[] y = x.toArray(new String[0]);</pre>
*
* Note that {@code toArray(new Object[0])} is identical in function to
* {@code toArray()}.
*
* @param a the array into which the elements of the list are to
* be stored, if it is big enough; otherwise, a new array of the
* same runtime type is allocated for this purpose.
* @return an array containing the elements of the list
* @throws ArrayStoreException if the runtime type of the specified array
* is not a supertype of the runtime type of every element in
* this list
* @throws NullPointerException if the specified array is null
*/
@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;
}
private static final long serialVersionUID = 876323262645176354L;
/**
* Saves the state of this {@code LinkedList} instance to a stream
* (that is, serializes it).
* 序列化
* @serialData The size of the list (the number of elements it
* contains) is emitted (int), followed by all of its
* elements (each an Object) in the proper order.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
// Write out any hidden serialization magic
s.defaultWriteObject();
// Write out size
s.writeInt(size);
// Write out all elements in the proper order.
for (Node<E> x = first; x != null; x = x.next)
s.writeObject(x.item);
}
/**
* Reconstitutes this {@code LinkedList} instance from a stream
* (that is, deserializes it). 反序列化
*/
@SuppressWarnings("unchecked")
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// Read in any hidden serialization magic
s.defaultReadObject();
// Read in size
int size = s.readInt();
// Read in all elements in the proper order.
for (int i = 0; i < size; i++)
linkLast((E)s.readObject());
}
/**
* Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
* and <em>fail-fast</em> {@link Spliterator} over the elements in this
* list.
*
* <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
* {@link Spliterator#ORDERED}. Overriding implementations should document
* the reporting of additional characteristic values.
*
* @implNote
* The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}
* and implements {@code trySplit} to permit limited parallelism..
*
* @return a {@code Spliterator} over the elements in this list
* @since 1.8
*/
@Override
public Spliterator<E> spliterator() {
return new LLSpliterator<E>(this, -1, 0);
}
/** A customized variant of Spliterators.IteratorSpliterator */
static final class LLSpliterator<E> implements Spliterator<E> {
static final int BATCH_UNIT = 1 << 10; // batch array size increment
static final int MAX_BATCH = 1 << 25; // max batch array size;
final LinkedList<E> list; // null OK unless traversed
Node<E> current; // current node; null until initialized
int est; // size estimate; -1 until first needed
int expectedModCount; // initialized when est set
int batch; // batch size for splits
LLSpliterator(LinkedList<E> list, int est, int expectedModCount) {
this.list = list;
this.est = est;
this.expectedModCount = expectedModCount;
}
final int getEst() {
int s; // force initialization
final LinkedList<E> lst;
if ((s = est) < 0) {
if ((lst = list) == null)
s = est = 0;
else {
expectedModCount = lst.modCount;
current = lst.first;
s = est = lst.size;
}
}
return s;
}
public long estimateSize() {
return (long) getEst(); }
public Spliterator<E> trySplit() {
Node<E> p;
int s = getEst();
if (s > 1 && (p = current) != null) {
int n = batch + BATCH_UNIT;
if (n > s)
n = s;
if (n > MAX_BATCH)
n = MAX_BATCH;
Object[] a = new Object[n];
int j = 0;
do {
a[j++] = p.item; } while ((p = p.next) != null && j < n);
current = p;
batch = j;
est = s - j;
return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
}
return null;
}
public void forEachRemaining(Consumer<? super E> action) {
Node<E> p; int n;
if (action == null) throw new NullPointerException();
if ((n = getEst()) > 0 && (p = current) != null) {
current = null;
est = 0;
do {
E e = p.item;
p = p.next;
action.accept(e);
} while (p != null && --n > 0);
}
if (list.modCount != expectedModCount)
throw new ConcurrentModificationException();
}
public boolean tryAdvance(Consumer<? super E> action) {
Node<E> p;
if (action == null) throw new NullPointerException();
if (getEst() > 0 && (p = current) != null) {
--est;
E e = p.item;
current = p.next;
action.accept(e);
if (list.modCount != expectedModCount)
throw new ConcurrentModificationException();
return true;
}
return false;
}
public int characteristics() {
return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
}
}
}