1.概览
LinkedList是java的动态数组另一种实现方式,底层是基于双向链表,而不是数组。
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
LinkedList实现了动态数组与双向队列两个接口,提供了两种方法集合,可以用来实现队列、栈之类的功能。
2. 成员变量
先来看成员变量
transient int size = 0;
transient Node<E> first;
transient Node<E> last;
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;
}
}
链表一般就是有个head的节点就能完成对应的工作。LinkedList实现了双向链表,除了head,还有一个last节点和一个size参数,这主要是为了效率考虑,不然查询一次长度或者尾都得来一次全链路迭代,太慢了。Node内部类就不说了,非常简单的一个节点类。
3. 方法
3.1 构造方法
public LinkedList() {
// 此时first=last=null,size=0
}
public LinkedList(Collection<? extends E> c) {
this();
addAll(c);
}
3.2 添加元素
添加一个元素
public boolean add(E e) {
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)
first = newNode;
else
l.next = newNode;
size++;
modCount++;
}
很简单的添加逻辑,再来看一下addAll的实现
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}
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;
}
3.3 删除元素
public E remove(int index) {
checkElementIndex(index);
return unlink(node(index));
}
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;
}
3.4 修改元素
public E set(int index, E element) {
checkElementIndex(index);
Node<E> x = node(index);
E oldVal = x.item;
x.item = element;
return oldVal;
}
3.5 检索元素
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}
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;
}
}
检索是LinkedList比较值得看的一个方法,java的实现很简单,先判断index是大于当前size的一半还是小于,如果是大于则从尾节点往前否则从首结点往后检索。从代码上看,虽然双向链表的实现让性能快了一点,但还是O(n)的耗时,我觉得后续版本的优化可以向HashMap那样,当判断LinkedList的size大于一个阈值时可以将双向链接改造为红黑树或者跳表,从而实现O(lgn)的性能,当然这样也对空间消耗更多一点。
3.6 清空元素
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++;
}
从代码上看,LinkedList的clear方法是没有内存泄漏问题的,注意有个for循环,这里是为了gc优化。