JDK源码学习系列05----LinkedList
1.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。
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable LinkedList继承了AbstractSequentialList<E>，并实现了List<E>, Deque<E>, Cloneable, java.io.Serializable等接口。AbstractSequentialList 实现了get(int
index)、set(int index, E element)、add(int index, E element) 和 remove(int index)这些函数.
2.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.
3.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);
}
4.LinkedList的内部类。
a.Entry<E>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;
}
}b.ListItr 是 LinkedList 的迭代器类 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 = next;
next = next.next;
nextIndex++;
return lastReturned.element;
}
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 = next = next.previous; //??????
nextIndex--;
checkForComodification();
return lastReturned.element;
}
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。
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
c.
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();
}
}5.LinkedList的成员函数
由于LinkedList的成员函数很多，就不单独每一个做为一部分，把一些类似的函数放在一起，经常被调用的比较复杂的函数再单独介绍。
a.
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;
}b. boolean remove(Object o) / E remove() / E remove(Entry<E> 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;
}c.boolean addAll(Collection<? extends E> c) / boolean addAll(int index, Collection<? extends E> c)
d.
public void clear() {//清空LinkedList
Entry<E> e = header.next;
while (e != header) {
Entry<E> next = e.next;
e.next = e.previous = null;
e.element = null;
e = next;
}
header.next = header.previous = header;
size = 0;
modCount++;
}
public E get(int index) {//获得某索引对应的节点值
return entry(index).element;
}
public E set(int index, E element) {//设置某索引的节点值
Entry<E> e = entry(index);
E oldVal = e.element;
e.element = element;
return oldVal;
}
public void add(int index, E element) {
addBefore(element, (index==size ? header : entry(index)));
}
public E remove(int index) {//移除节点
return remove(entry(index));
}
e.Entry<E> entry(int index)
此方法得到某索引对应的节点对象
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;
}
f.
public E peek() {//获取第一个元素的值
if (size==0)
return null;
return getFirst();
}
public E element() {//获取第一个元素的值
return getFirst();
}
public E poll() {//移除第一个元素
if (size==0)
return null;
return removeFirst();
}
public E remove() {//移除第一个元素
return removeFirst();
}
public boolean offer(E e) {
return add(e);
}
public boolean offerFirst(E e) {
addFirst(e);
return true;
}
public boolean offerLast(E e) {
addLast(e);
return true;
}
public E peekFirst() {
if (size==0)
return null;
return getFirst();
}
public E peekLast() {
if (size==0)
return null;
return getLast();
}
public E pollFirst() {
if (size==0)
return null;
return removeFirst();
}
public E pollLast() {
if (size==0)
return null;
return removeLast();
}
public void push(E e) {
addFirst(e);
}
public E pop() {
return removeFirst();
}g.Entry<E> addBefore(E e, Entry<E> entry)
private Entry<E> addBefore(E e, Entry<E> entry) {
Entry<E> newEntry = new Entry<E>(e, entry, entry.previous);
newEntry.previous.next = newEntry;
newEntry.next.previous = newEntry;
size++;
modCount++;
return newEntry;
}h.
public Object[] toArray() {
Object[] result = new Object[size];
int i = 0;
for (Entry<E> e = header.next; e != header; e = e.next)
result[i++] = e.element;
return result;
}
public <T> T[] toArray(T[] a) {
if (a.length < size)
a = (T[])java.lang.reflect.Array.newInstance(
a.getClass().getComponentType(), size);
int i = 0;
Object[] result = a;
for (Entry<E> e = header.next; e != header; e = e.next)
result[i++] = e.element;
if (a.length > size)
a[size] = null;
return a;
}i.
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
// Write out any hidden serialization magic
s.defaultWriteObject();
// Write out size
s.writeInt(size);
// Write out all elements in the proper order.
for (Entry e = header.next; e != header; e = e.next)
s.writeObject(e.element);
}
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// Read in any hidden serialization magic
s.defaultReadObject();
// Read in size
int size = s.readInt();
// Initialize header
header = new Entry<E>(null, null, null);
header.next = header.previous = header;
// Read in all elements in the proper order.
for (int i=0; i<size; i++)
addBefore((E)s.readObject(), header);
}
5.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()
6.总结
a.LinkedList是以双链表的形式实现的。
b.LinkedList即可以作为链表，还可以作为队列和栈。
c.LinkedList是 非 线程安全的。