public class LinkedList<E> extends AbstractSequentialList<E> implements List<E>, Deque<E>, Cloneable, java.io.Serializable
其中Deque为双端队列
add(E e)方法如下，就是在链表的最后一项，在其next项附上要添加的数据
/**
* 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)
//如果last节点为空，说明链表是空的，那么新增的数据既是first又是last
first = newNode;
else
//如果last不为空，把新增的元素链接到原来的last的next
l.next = newNode;
size++;
modCount++;
}
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;
}
}
add(int index, E e)方法如下，就是在指定位置，把新节点插入到链表，并对应更新其前驱和后续节点的链接关系。
/**
* 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) {
//检查索引是否越界
checkPositionIndex(index);
if (index == size)
//在链表末端添加新节点
linkLast(element);
else
//在指定位置插入新节点
linkBefore(element, node(index));
}
private void checkPositionIndex(int index) {
if (!isPositionIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
/**
* Tells if the argument is the index of a valid position for an
* iterator or an add operation.
*/
private boolean isPositionIndex(int index) {
return index >= 0 && index <= size;
}
/**
* Inserts element e before non-null Node succ.
*/
void linkBefore(E e, Node<E> succ) {
// assert succ != null;
//保留要插入节点位置的前驱节点
final Node<E> pred = succ.prev;
//新建节点
final Node<E> newNode = new Node<>(pred, e, succ);
//把新建节点作为原节点的前驱节点
succ.prev = newNode;
//把新建节点作为pred的后续节点，或者作为链表头结点
if (pred == null)
first = newNode;
else
pred.next = newNode;
size++;
modCount++;
}
get方法如下：
/**
* 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) {
//检查索引是否越界
checkElementIndex(index);
//根据索引，遍历链表找到数据
return node(index).item;
}
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)) {
//索引值小于链表总长度的一半，从前往后遍历链表
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
//索引值大于等于链表总长度的一半，从后往前遍历链表
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}
根据索引获取元素时，有个小技巧，就是把链表对半分，当索引在前半段时，从前往后遍历，否则反向遍历，该算法可以节省遍历次数，从一半，减为四分之一。
有个需要注意的地方，Linkedlist里面有两个很相似的方法，一个是获取数据时，检查索引下标是否越界；另外一个是添加数据时，添加位置索引小标是否越界。前者边界条件比后者小1，代码如下：
/**
* Tells if the argument is the index of an existing element.
*/
private boolean isElementIndex(int index) {
return index >= 0 && index < size;
}
/**
* Tells if the argument is the index of a valid position for an
* iterator or an add operation.
*/
private boolean isPositionIndex(int index) {
return index >= 0 && index <= size;
}
remove(Object o)函数，该函数从前往后遍历链表，删除第一个满足条件的数据，到此结束。
/**
* 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;
}
/**
* 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;
//前驱节点解绑，如果前驱节点为空，则后续节点成为first
if (prev == null) {
first = next;
} else {
prev.next = next;
x.prev = null;
}
//后续节点解绑，如果后续节点为空，则前驱节点成为last
if (next == null) {
last = prev;
} else {
next.prev = prev;
x.next = null;
}
//节点数值释放，方便gc回收
x.item = null;
size--;
modCount++;
return element;
}
remove(int index)方法，先检查索引是否越界，然后根据索引获取node，调用unlink方法解绑该节点。
/**
* 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));
}