ArrayList和LinkedList是List接口的两种实现，具有相同的查找、插入、删除操作，只是底层的实现方式不一样。LinkedList是以双向链表形式实现的集合类。
其增删操作由于不需要移动底层数组数据，只需要修改链表节点指针，所以效率较高。但是随机访问时的定位操作效率较低，需要遍历链表节点。（ArrayList与之相反）
目录
- 数据结构
- 初始化
- 扩容
- 查找操作
- 插入操作
- 删除操作
- 队列操作
- 栈操作
- 转化成数组
- 迭代器实现
数据结构
底层基于双向链表实现的。并且元素允许 null 的存在。
其源码如下：
//没有空头节点的双向链表
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;
}
}
//表的长度
transient int size = 0;
/**
* Pointer to first node.表的头节点指针
* Invariant: (first == null && last == null) ||
* (first.prev == null && first.item != null)
*/
transient Node<E> first;
/**
* Pointer to last node.表的尾节点指针
* Invariant: (first == null && last == null) ||
* (last.next == null && last.item != null)
*/
transient Node<E> last;
初始化
由于链表，初始化时不需要指定表的长度。
/**
* 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);
}
扩容
由于链表，扩容时只需要新建节点。
查找操作
1、通过索引查找元素（随机访问）：
注意node(int index)函数：通过索引找到对应的节点。（后面很多方法都会用的）
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}
/**
* Returns the (non-null) Node at the specified element index.
*/
//时间复杂度为n/2
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;
}
}
2、直接查找元素：
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;
}
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;
}
插入操作
在介绍插入操作前，首先介绍根据插入链表位置的不同，几种基本的插入链表方式：
- （1）元素作为头节点插入链表：
/**
* 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++;
}
public void addFirst(E e) {
linkFirst(e);
}
- （2）元素作为尾节点插入链表：
/**
* 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++;
}
public void addLast(E e) {
linkLast(e);
}
- （3）元素作为节点e的前驱节点插入链表（相当于元素作为链表的中间节点）：
/**
* 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++;
}
1、插入单个元素
public boolean add(E e) {
linkLast(e);
return true;
}
public void add(int index, E element) {
checkPositionIndex(index);
if (index == size)
linkLast(element);
else
//在node(index)节点之前插入
linkBefore(element, node(index));
}
2、插入集合：
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}
//插入成第index个节点(= 即index节点后移 = 即在index节点之前插入)(index从0开始)
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index);
Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;
//开始遍历之前的前驱节点，最终的后继节点
Node<E> pred, succ;
//特殊情况：当索引index=size时，通过node(index)找不到节点，超过范围
//(那为什么不把索引设置为index-1，即插入到index-1节点后？那插入到0节点之前也是特殊情况)
if (index == size) {
succ = null;
pred = last;
} else {
//找到第index个节点
succ = node(index);
pred = succ.prev;
}
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
//后继待定，指向下个遍历的节点
Node<E> newNode = new Node<>(pred, e, null);
if (pred == null)
first = newNode;
else
//新节点左边的所有链接成功
pred.next = newNode;
pred = newNode;
}
//连接最后一个节点pred右边的所有链接
if (succ == null) {
last = pred;
} else {
pred.next = succ;
succ.prev = pred;
}
size += numNew;
modCount++;
return true;
}
删除操作
首先介绍根据删除节点在链表位置的不同，几种基本的删除节点方式：
- （1）删除头节点：
/**
* 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;
}
public E removeFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return unlinkFirst(f);
}
- （2）删除尾节点：
/**
* 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;
}
public E removeLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return unlinkLast(l);
}
- （3）删除中间节点：
/**
* 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;
}
1、通过索引删除元素：
public E remove(int index) {
checkElementIndex(index);
return unlink(node(index));
}
2、删除指定元素：
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;
}
队列操作
主要操作：
//取队列头节点，但不删除
//若first为空时，不抛出错误
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}
//若first为空时，抛出错误
public E element() {
return getFirst();
}
//出队列（头删）
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}
//入队列（尾插）
public boolean offer(E e) {
return add(e);
}
栈操作
主要操作：
//入栈，头插
public void push(E e) {
addFirst(e);
}
//出栈，头删
public E pop() {
return removeFirst();
}
转化成数组
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;
}
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
//为什么用到反射？泛型类在运行时无法new实例化，只能在运行时得到class对象
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;
}
迭代器实现
首先了解什么是迭代器：
迭代器中fail-fast机制的实现：
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;
}
public boolean hasNext() {
return nextIndex < size;
}
public E next() {
checkForComodification();
if (!hasNext())
throw new NoSuchElementException();
lastReturned = next;
next = next.next;
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();
}
}
