#来自ゾフィー（佐菲）
1 简介
LinkedList 的底层数据结构是双向链表。可以当作链表、栈、队列、双端队列来使用。有以下特点：
- 在插入或删除数据时，性能好；
- 允许有 null 值；
- 查询效率不高；
- 线程不安全；
public class LinkedList<E>extends AbstractSequentialList<E>implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{}
2 源码
LinkedList 数据结构：
private static class Node<E> {E item; //结点值Node<E> next; //后驱节点Node<E> prev; //前驱节点Node(Node<E> prev, E element, Node<E> next) {this.item = element;this.next = next;this.prev = prev;}
}
LinkedList 两个构造函数：
public LinkedList() {}public LinkedList(Collection<? extends E> c) {this();addAll(c);
}
addAll()
public boolean addAll(int index, Collection<? extends E> c) {//校验 index 是否合理checkPositionIndex(index);Object[] a = c.toArray();int numNew = a.length;if (numNew == 0)return false;//succ：待添加节点的位置。//pred：待添加节点的前驱节点。 Node<E> pred, succ;if (index == size) {//在末尾插入succ = null;pred = last;} else { //不在末尾插入succ = node(index); //这个方法 会折半pred = succ.prev;}for (Object o : a) {//创建新节点@SuppressWarnings("unchecked") E e = (E) o;Node<E> newNode = new Node<>(pred, e, null);if (pred == null)first = newNode;elsepred.next = newNode;pred = newNode;}if (succ == null) {last = pred;} else {pred.next = succ;succ.prev = pred;}//把集合的大小设置为新的大小 size += numNew;modCount++;return true;}
get() -> 会有折半
public E get(int index) {//校验 index 是否越界checkElementIndex(index);return node(index).item;
}Node<E> node(int index) {// assert isElementIndex(index);//分一半查找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;}}
add()
public boolean add(E e) {//在末尾追加元素的方法。linkLast(e);return true;
}void linkLast(E e) {final Node<E> l = last;final Node<E> newNode = new Node<>(l, e, null);last = newNode;if (l == null) //为空链表first = newNode;elsel.next = newNode;size++;//size 自增modCount++;
}
remove()
public boolean remove(Object o) {if (o == null) {for (Node<E> x = first; x != null; x = x.next) {if (x.item == null) {//移除节点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;
}//删除节点
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;//1 -> 2 -> 3 1 -> 3if (prev == null) { //移除的是头节点first = next;} else {prev.next = next;x.prev = null;}if (next == null) { //移除的是尾节点last = prev;} else {next.prev = prev;x.next = null;}x.item = null;size--;modCount++;return element;}
toArray()
public Object[] toArray() {//创建一个新数组 然后遍历链表，将每个元素存在数组里，返回Object[] result = new Object[size];int i = 0;for (Node<E> x = first; x != null; x = x.next)result[i++] = x.item;return result;
}
