jdk源码分析(3)-LinkedList
一、LinkedList简介
LinkedList是基于双向链表实现的，它也可以被当作堆栈、队列或双端队列进行操作。
LinkedList的源码大致分三个部分，双向循环链表的实现、List的API和Deque的API。
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
从类定义和图中也能很清晰的看到，LinkedList的结构大致分为三个部分；同时和ArrayList相比，他并没有实现RandomAccess接口，所以他并不支持随机访问操作；另外可以看到他的List接口是通过AbstractSequentialList实现的，同时还实现了多个迭代器，表明他的访问操作时通过迭代器完成的.
二、链表结构
常见链表
LinkedList是基于双向循环链表实现的，所以如图所示，当对链表进行插入、删除等操作时，
- 首先需要区分操作节点是否为首尾节点，并区分是否为空，
- 然后再变更相应
pre和next的引用即可；
void linkFirst(E e)
void linkLast(E e)
void linkBefore(E e, Node<E> succ)
E unlinkFirst(Node<E> f)
E unlinkLast(Node<E> l)
E unlink(Node<E> x)
/**
* 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;
}
}上面所列的方法封装了对双向循环链表常用操作，其中node(int index)是随机查询方法，这里通过判断index是前半段还是后半段，来确定遍历的方向以增加效率。
同时在LinkedList中有关List和Deque的API也是基于上面的封装的方法完成的
三、LinkedList的成员变量
private transient Entry<E> header = new Entry<E>(null, null, null);
private transient int size = 0;LinkedList有两个成员变量，表头 header 和长度size.
四、LinkedList的构造函数
public LinkedList() {//构造一个空链表
header.next = header.previous = header;
}
public LinkedList(Collection<? extends E> c) {//将一个数据类型相同的集合添加到LinkedList的尾部
this();
addAll(c);
}
五、LinkedList的内部类。
Entry
是LinkedList的节点类，节点类包含：当前节点的值,前一节点，后一节点。该节点类是一个静态内部类：当内部类对象不需要访问外围类对象时，应该声明为静态内部类。 private static class Entry<E> { E element; Entry<E> next; Entry<E> previous; Entry(E element, Entry<E> next, Entry<E> previous) { this.element = element; this.next = next; this.previous = previous; } }ListItr 是 LinkedList 的迭代器类
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private class ListItr implements ListIterator<E> {
private Entry<E> lastReturned = header;//上一次返回的节点
private Entry<E> next;//下一节点
private int nextIndex;//下一节点的索引
private int expectedModCount = modCount;//!!!!!!期望的改变次数~ Java的 fail-fast 机制。
ListItr(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException(“Index: “+index+
“, Size: “+size);
if (index < (size >> 1)) {//size>>1是右移一位，即size/2 ,若索引值小于 size/2则从前开始
next = header.next;
for (nextIndex=0; nextIndex<index; nextIndex++)
next = next.next;
} else {//否则从后开始
next = header;
for (nextIndex=size; nextIndex>index; nextIndex–)
next = next.previous;
}
}
public boolean hasNext() {//是否存在下一个元素
return nextIndex != size;//通过下一节点索引值是否等于size来判断是否到了最末尾
}
public E next() {
checkForComodification();
if (nextIndex == size)//!!
throw new NoSuchElementException();
lastReturned <span class="token operator">=</span> next<span class="token punctuation">;</span>
next <span class="token operator">=</span> next<span class="token punctuation">.</span>next<span class="token punctuation">;</span>
nextIndex<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> lastReturned<span class="token punctuation">.</span>element<span class="token punctuation">;</span>
}
public boolean hasPrevious() {//是否存在上一个
return nextIndex != 0;//通过下一节点的索引值是否等于0来判断是否在最前面即头节点，由此来判断是否有前节点
}
public E previous() {//取得上一元素
if (nextIndex == 0)
throw new NoSuchElementException();
lastReturned <span class="token operator">=</span> next <span class="token operator">=</span> next<span class="token punctuation">.</span>previous<span class="token punctuation">;</span> <span class="token comment">//??????</span>
nextIndex<span class="token operator">--</span><span class="token punctuation">;</span>
<span class="token function">checkForComodification</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> lastReturned<span class="token punctuation">.</span>element<span class="token punctuation">;</span>
}
public int nextIndex() {
return nextIndex;
}
public int previousIndex() {//上一元素的索引
return nextIndex-1;
}
public void remove() {//删除当前节点！！
checkForComodification();
Entry<E> lastNext = lastReturned.next;
try {
LinkedList.this.remove(lastReturned);
} catch (NoSuchElementException e) {
throw new IllegalStateException();
}
if (next==lastReturned)
next = lastNext;
else
nextIndex–;
lastReturned = header;
expectedModCount++;
}
public void set(E e) {
if (lastReturned == header)
throw new IllegalStateException();
checkForComodification();
lastReturned.element = e;
}
public void add(E e) {//讲e添加到当前节点前面
checkForComodification();
lastReturned = header;
addBefore(e, next);
nextIndex++;
expectedModCount++;
}
final void checkForComodification() {//!!!!!判断 modCount是否等于 expectedModCount来实现fail-fast机制。:hexoPostRenderEscape–>
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
DescendingIterator
private class DescendingIterator implements Iterator { final ListItr itr = new ListItr(size()); public boolean hasNext() { return itr.hasPrevious(); } public E next() { return itr.previous(); } public void remove() { itr.remove(); } }
六、LinkedList的成员函数
1.get类型的函数
public E getFirst() {//取得第一个节点的值 if (size==0) throw new NoSuchElementException();//时刻注意特殊情况的考虑 return header.next.element; } public E getLast() { if (size==0) throw new NoSuchElementException(); return header.previous.element;//获得最后一个是 header.previous.element } public E removeFirst() {//移除第一个节点 return remove(header.next);//remove函数下面单独介绍 } public E removeLast() {//移除最后一个节点 return remove(header.previous); } public void addFirst(E e) {//在 addBefore(e, header.next); } public void addLast(E e) { addBefore(e, header); } public boolean contains(Object o) { return indexOf(o) != -1; } public int size() { return size; } public boolean add(E e) {//在最末尾添加值为e的节点，添加在header前即最末尾 addBefore(e, header); return true;2.boolean remove(Object o) / E remove() / E remove(Entry
e) public boolean remove(Object o) { if (o==null) {//即使是null也要查找到然后再移除 for (Entry<E> e = header.next; e != header; e = e.next) { if (e.element==null) { remove(e);//调用下面的方法 return true; } } } else { for (Entry<E> e = header.next; e != header; e = e.next) { if (o.equals(e.element)) { remove(e); return true; } } } return false; } private E remove(Entry<E> e) { if (e == header) throw new NoSuchElementException();//考虑头指针的特殊情况 E result = e.element; e.previous.next = e.next;//!!! e.next.previous = e.previous;//!!! e.next = e.previous = null;// ???不是特别理解 e.element = null; size--; modCount++; return result; }3.增删改查的一些方法
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public void clear() {//清空LinkedList<span class="token class-name">Entry</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">E</span><span class="token punctuation">></span></span> e <span class="token operator">=</span> header<span class="token punctuation">.</span>next<span class="token punctuation">;</span> <span class="token keyword">while</span> <span class="token punctuation">(</span>e <span class="token operator">!=</span> header<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token class-name">Entry</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">E</span><span class="token punctuation">></span></span> next <span class="token operator">=</span> e<span class="token punctuation">.</span>next<span class="token punctuation">;</span> e<span class="token punctuation">.</span>next <span class="token operator">=</span> e<span class="token punctuation">.</span>previous <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span> e<span class="token punctuation">.</span>element <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span> e <span class="token operator">=</span> next<span class="token punctuation">;</span> <span class="token punctuation">}</span> header<span class="token punctuation">.</span>next <span class="token operator">=</span> header<span class="token punctuation">.</span>previous <span class="token operator">=</span> header<span class="token punctuation">;</span> size <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>modCount++;
}public E get(int index) {//获得某索引对应的节点值
<span class="token keyword">return</span> <span class="token function">entry</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">.</span>element<span class="token punctuation">;</span>}
public E set(int index, E element) {//设置某索引的节点值
<span class="token class-name">Entry</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">E</span><span class="token punctuation">></span></span> e <span class="token operator">=</span> <span class="token function">entry</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">E</span> oldVal <span class="token operator">=</span> e<span class="token punctuation">.</span>element<span class="token punctuation">;</span> e<span class="token punctuation">.</span>element <span class="token operator">=</span> element<span class="token punctuation">;</span> <span class="token keyword">return</span> oldVal<span class="token punctuation">;</span>}
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">,</span> <span class="token class-name">E</span> element<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">addBefore</span><span class="token punctuation">(</span>element<span class="token punctuation">,</span> <span class="token punctuation">(</span>index<span class="token operator">==</span>size <span class="token operator">?</span> header <span class="token operator">:</span> <span class="token function">entry</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">E</span> <span class="token function">remove</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token comment">//移除节点</span>
<span class="token keyword">return</span> <span class="token function">remove</span><span class="token punctuation">(</span><span class="token function">entry</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
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4.Entry
entry(int index) private Entry<E> entry(int index) { if (index < 0 || index >= size) throw new IndexOutOfBoundsException("Index: "+index+ ", Size: "+size); Entry<E> e = header; if (index < (size >> 1)) {//若小于size/2，则从头遍历 for (int i = 0; i <= index; i++) e = e.next; } else {//否则从尾遍历 for (int i = size; i > index; i--) e = e.previous; } return e; }
七、方法归类
a.LinkedList可以作为FIFO(先进先出)的队列，作为FIFO的队列时，下表的方法等价：
队列方法 等效方法
add(e) addLast(e)
offer(e) offerLast(e)
remove() removeFirst()
poll() pollFirst()
element() getFirst()
peek() peekFirst()
b.LinkedList可以作为LIFO(后进先出)的栈，作为LIFO的栈时，下表的方法等价：
栈方法 等效方法
push(e) addFirst(e)
pop() removeFirst()
peek() peekFirst()
八、总结
LinkedList是以双链表的形式实现的。
LinkedList即可以作为链表，还可以作为队列和栈。
LinkedList是 非 线程安全的。
LinkedList 基于双向循环链表实现，随机访问比较慢，所以在遍历 List 的时候一定要注意。
LinkedList 可以添加重复元素，可以添加 null。
