LinkedList是什么？
在上一章节中我们讲到了数组集合 ArrayList ，这节我们接着讲集合中的另一个成员 LinkedList ，就像它的名字说的一样，这是一个链表，在C语言中我们知道，链表是我们通过结构体实现的，那么在 Java 中我们应该怎样实现呢？
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;
}
}这就是 LinkedList 用来保存元素的节点，Node类是 LinkedList 中的静态内部类，每一个 Node 对象代表一个元素，它具有三个属性，分别是用来存储元素的 item 对象，用来存储它上一个节点的 prev 对象，用来存储它下一个节点的 next 对象，那么我们就知道了 LinkedList 的内部是一个双链表。现在我们只需要知道 LinkedList 是怎样操作这些节点就行了。
首先我们来看看它的构造器：
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);
}LinkedList 不像 ArrayList 那样在初始化的时候传入集合容量，因为它的容量是可变的，与不可变的数组不同。在第二个构造器中我们看到当传入一个集合对象的时候会调用 addAll() 方法，那么我们接着看看 addAll() 的源码：
/**
* 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);
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;
}我们从源码中看到：在 addAll() 方法中和 ArrayList 一样，首先调用传入集合的 toArray() 方法将集合中的元素转到数组中便于遍历，然后回先判断一下插入的位置是否在链表尾部（如果刚开始链表为空的话，那么也就等于是在链表尾部插入），然后根据两种情况进行不同的操作。最后将 size 值更新。
成员属性：
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;我们再来看看 LinkedList 中的成员属性：size 值表示链表中的实际元素个数，跟 ArrayList的 size 值一样。
first 对象代表的是整个链表的头节点，last 对象代表的是整个链表的尾节点。
方法add()：
/**
* 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++;
}
/**
* 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;
}再调用 add() 方法的时候，其实就是在调用 linkLast() 方法，定义两个节点常量分别对应链表的的尾节点和新插入的节点，然后将新节点插入尾部，判断链表是否为空，若为空则将链表的手节点和尾节点都指向新加入的节点，否则将原来尾节点与新节点相连。
在 LinkedList 中还有一个方法和 add() 方法相同：
方法 offer()、offerFirst()、offerLast():
/**
* 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 operations
/**
* 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;
}我们在 offer() 方法中就可以看到，它内部就是调用了 add() 方法，那为什么有了 add() 方法还要一个和它功能相同的 offer() 方法呢？这是因为这两个方法继承自不同的接口：add() 方法继承自 Collection 接口，而 offer() 方法继承自 Deque 接口，Collection 是集合接口，而 Deque 接口代表双向队列，LinkedList 在继承了这两个接口后必须重写这两个接口中的方法，所以就写的一样咯。offerFirst() 方法表示在链表首部加入一个节点元素，而offerLast() 方法就和 add() 方法一样，在尾部添加节点元素，我们看看这两个方法的执行流程：
/**
* 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);
}
/**
* 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++;
}它们最终调用的还是 linkFirst() 和 linkLast() 方法。
方法 remove(int index)、remove(Object o)：
LinkedList提供两种删除指定节点的方式：
（1）按节点下标删除：
/**
* 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));
}
private void checkElementIndex(int index) {
if (!isElementIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
/**
* Tells if the argument is the index of an existing element.
*/
private boolean isElementIndex(int index) {
return index >= 0 && index < size;
}
/**
* 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;
}
}
/**
* 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;
}可以看到调用 remove(int index) 方法时会先用 checkElementIndex() 和 isElementIndex() 方法检查给定下标的合法性，如果给定的下标 小于0 或 超出链表的长度 checkElementIndex() 方法就会抛出一 IndexOutOfBoundsException(outOfBoundsMsg(index)) 异常（下标越界异常）。如果下标合法的话则继续调用 node(int index) 方法，方法中会首先判断给定的下标在链表的前半段还是后半段（index << 1 等于 index/2），若在前半段：从链表头部遍历；若在后半段：从链表尾部向前遍历（可能这就是为什么用双链表的原因吧），遍历到给定下表的节点后返回此节点给 unlink(Node<E> x) 方法删除节点：分删除头节点和其他节点两种情况，在这里就不做赘述了。
（2）按节点值删除：
/**
* 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;
}按节点值删除因为 null 值的特性，所以需要先判断给定的节点值是否是 null 值，若是则遍历链表找到第一个值为 null 的节点删除，若不是，则用equals() 对比节点值和给定值。此中删除方法只删除第一个匹配到的节点。
方法 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++;
}此方法负责清空链表所有节点，从头部遍历链表节点，一个一个清空 Node 对象，代码中 x.item = null;x.next = null;x.prev = null都是负责 help to gc的。
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