List - LinkedList 源码解析
大约 14 分钟
List - LinkedList 源码解析
介绍
LinkedList 是一个同时实现了 List 和 Deque 接口的双向链表，既可以被看做是一个顺序集合，也可以看做是一个队列，又可以看做为一个栈。
双向链表结构
- LinkedList 内部使用双向链表来存储元素，每个节点包含了对前一个节点和后一个节点的引用。
- 这种结构使得在任意位置进行删除和插入操作的时间复杂度是 O(1)。
实现接口
- 实现了 List 接口，故可以存储一组有序元素，并且支持通过索引访问、插入、删除等操作。
- 实现了 Deque 接口，提供了双端队列的功能，故可以从队列的两端进行元素的插入和删除操作。
特点和优势
- 节点逻辑有序：LinkedList 中的所有数据节点，是通过引用来连接起来，在物理存储上是随机分配的。
- 支持动态增长：LinkedList 的容量可以动态增长，不必要像数组预先分配指定大小且连续的空间。
- 插入和删除效率高：因为其底层是通过引用连接起来的，故对其操作时仅需更改与其相连接的结点的引用即可。
- 线程不安全的：是非线程安全的，在并发环境下使用，需要考虑同步操作或者改用其他线程安全的类，如
java.util.concurrent.ConcurrentLinkedDeque。
适用场景
- 不需要频繁的随机访问数据，需要频繁的添加、修改、删除操作，特别是在中间位置时推荐使用。
- 对存储占用不敏感的情况，因为其是通过引用连接起来的，所以相较于数组而言，存储了较多的辅助信息，故存储消耗比 ArrayList 大。
常用API
- 添加元素
- add(E e)：向链表尾部添加元素 e。
- add(int index, E element)：在指定位置 index 插入元素 element。
- 获取元素
- get(int index)：获取指定位置 index 的元素。
- getFirst()：获取链表的第一个元素。
- getLast()：获取链表的最后一个元素。
- 移除元素
- remove(int index)：移除指定位置 index 的元素。
- removeFirst()：移除链表的第一个元素。
- removeLast()：移除链表的最后一个元素。
- 替换元素
- set(int index, E element)：替换指定位置 index 的元素为 element。
- 查询元素位置
- indexOf(Object o)：返回元素 o 在链表中第一次出现的位置。
- lastIndexOf(Object o)：返回元素 o 在链表中最后一次出现的位置。
- 链表操作
- addFirst(E e)：在链表头部添加元素 e。
- addLast(E e)：在链表尾部添加元素 e。
- offer(E e)：向链表尾部添加元素，返回是否成功。
- offerFirst(E e)：向链表头部添加元素，返回是否成功。
- offerLast(E e)：向链表尾部添加元素，返回是否成功。
- poll()：移除并返回链表头部的元素。
- pollFirst()：移除并返回链表的第一个元素。
- pollLast()：移除并返回链表的最后一个元素。
- peek()：返回链表头部的元素，不移除。
- peekFirst()：返回链表的第一个元素，不移除。
- peekLast()：返回链表的最后一个元素，不移除。
- push(E e)：在链表头部添加元素 e，等效于 addFirst。
- pop()：移除并返回链表头部的元素，等效于 removeFirst。
- 其他方法
- isEmpty()：判断链表是否为空。
- size()：返回链表的大小（元素个数）。
- clear()：清空链表中的所有元素。
- contains(Object o)：判断链表是否包含元素 o。
实现方式
底层存储
LinkedList 的底层通过双向链表实现，底层主要是通过头指针、尾指针、元素个数这三个值来进行存储的。
- size：元素个数，或链表长度
- first：指向链表的头节点
- last：指向链表的末尾节点
注意
这里要注意的是，当链表为空的时候 first 和 last 引用都指向null。
- 底层实现源码
transient int size;
/**
* 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;
- Node类：其是 LinkedList 的静态私有内部类
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;
}
}
构造方法
LinkedList 中提供了两个构造方法：
LinkedList()：创建一个空集合LinkedList(Collection<? extends E> var1)：创建一个指定元素列表的集合
/**
* Constructs an empty list.
*/
public LinkedList() {
this.size = 0;
}
/**
* 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();
this.addAll(c);
}
add方法
LinkedList 中向集合中添加元素的操作，提供了 6 个不同的实现方法：add(E e)、add(int index, E element)、addAll(Collection<? extends E> c)、addAll(int index, Collection<? extends E> c)、addFirst(E e)、addLast(E e)
- add(E e)：向链表尾部添加元素 e。
/**
* 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;
}
- add(int index, E element)：在指定位置 index 插入元素 element。
/**
* 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));
}
- addAll(Collection<? extends E> c)
/**
* 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);
}
- addAll(int index, Collection<? extends E> 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;
}
- addFirst(E e)：向链表头部插入元素 e
/**
* Inserts the specified element at the beginning of this list.
*
* @param e the element to add
*/
public void addFirst(E e) {
linkFirst(e);
}
- addLast(E e)：向链表尾部插入元素 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);
}
从上面和 Add 操作相关的方法源码不难看出，绝大多数的实现都是基于
linkFirst(E e)（向链表头部插入元素 e）linkLast(E e)：（向链表尾部插入元素 e）
这两个方法实现的。下面对这两个方法的源码进行解析：
/**
* Links e as first element.
*/
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++;
}
/**
* 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++;
}
get方法
LinkedList 中提供了 3 个关于获取元素的方法：get(int index)、getFirst()、getLast()
- get(int index)：获取指定下标的元素
/**
* 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;
}
- getFirst()：获取首元素
/**
* 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;
}
- getLast()：获取末尾元素
/**
* 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;
}
remove方法
LinkedList 中提供了 7 个关于删除元素的方法：remove()、remove(int index)、remove(Object o)、removeFirst()、removeLast()、removeFirstOccurrence(Object o)、removeLastOccurrence(Object o)
- remove()：删除头节点引用
/**
* 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();
}
- remove(int index)：删除索引为 index 的引用
/**
* 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));
}
- remove(Object o)：删除列表中元素 o 第一次出现的引用
/**
* 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) {
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;
}
- removeFirst()：删除列表中第一个元素
/**
* 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);
}
- removeLast()：删除列表中的最后一个元素
/**
* 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);
}
- removeFirstOccurrence(Object o)：从头向后遍历，删除此列表中指定元素的第一个引用，如果不包含该元素，则列表保持不变。
/**
* 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.
*
* @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);
}
- removeLastOccurrence(Object o)：从头向后遍历，删除此列表中指定元素的最后一个引用，如果不包含该元素，则列表保持不变。
/**
* 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.
*
* @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;
}
set方法
LindedList 中修改指定索引位置的元素为 element。
- set(int index,E element)
/**
* 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;
}
clear方法
LinkedList 中提供的 clear 方法清空该链表/集合中的所有元素。
- clear()
/**
* 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++;
}
index方法
LinkedList 中提供的 indexOf(Object o) 方法可以查找链表中第一次出现元素 o 的索引下标。
- indexOf(Object o)
/**
* 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;
}
Queue接口方法
Queue 接口是单向队列的接口，提供了对队列的主要操作方法：
- peek()：查看队列顶端元素，也就是出口处元素
- poll()：返回队列顶端元素，并且将该元素出队列
- remove()：移除队首元素
- offer(E e)：将元素 e 入队
/**
* Retrieves, but does not remove, the head (first element) of this list.
*
* @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.
*
* @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.
*
* @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.
*
* @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.
*
* @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接口方法
Deque 双向队列接口继承自 Queue 队列接口，主要提供了双向队列相关的一些操作方法：
- offerFirst(E e)：从队头插入元素
- offerLast(E e)：从队位插入元素
- peekFirst()：查看队头元素
- peekLast()：查看队尾元素
- pollFirst()：弹出对头元素
- pollLast()：弹出队尾元素
- push(E e)：从队头将元素 e 入队
- pop()：出队一个元素
- removeFirstOccurrence(Object o)：删除元素 o 第一次出现的引用
- removeLastOccurrence(Object o)：删除元素 o 最后一次出现的引用
/**
* 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;
}
/**
* 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;
}
/**
* 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);
}
/**
* 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}.
*
* @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()}.
*
* @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.
*
* @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.
*
* @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;
}