LinkedList类似C语言的双向链表，但是java中没有指针如何实现呢，看完LinkedList
你将对java中的引用类型有更深入的理解.
LindedList的声明如下：
public class LinkedList extends AbstractSequentialList implements List, Cloneable, java.io.Serializable
有关AbstractSequentialList：http://blog.csdn.net/treeroot/archive/2004/09/18/108696.aspx
有关List： http://blog.csdn.net/treeroot/archive/2004/09/14/104638.aspx
有关Cloneable：http://blog.csdn.net/treeroot/archive/2004/09/07/96936.aspx
下面分析一下这个链表的实现，这里只重点分析某些方法。
private transient Entry header = new Entry(null, null, null);
private transient int size = 0;
public LinkedList() {
header.next = header.previous = header;
}
header相当于C中的头指针，而且这个头指针不是链表的元素，有关Entry将在下面介绍。
public LinkedList(Collection c) {
this();
addAll(c);
}
public Object getFirst() {
if (size==0)
throw new NoSuchElementException();
return header.next.element;
}
头指针的下一个元素就是第一个元素
public Object getLast() {
if (size==0)
throw new NoSuchElementException();
return header.previous.element;
}
头指针的前一个当然就是最后一个了
public Object removeFirst() {
Object first = header.next.element;
remove(header.next);
return first;
}
public Object removeLast() {
Object last = header.previous.element;
remove(header.previous);
return last;
}
public void addFirst(Object o) {
addBefore(o, header.next);
}
public void addLast(Object o) {
addBefore(o, header);
}
这个方法在链表末尾插入新的元素，功能和add方法一样，这个方法完全是为了对称性(因为有addFirst)
public boolean contains(Object o) {
return indexOf(o) != -1;
}
public int size() {
return size;
}
public boolean add(Object o) {
addBefore(o, header);
return true;
}
public boolean remove(Object o) {
if (o==null) {
for (Entry e = header.next; e != header; e = e.next) {
if (e.element==null) {
remove(e);
return true;
}
}
} else {
for (Entry e = header.next; e != header; e = e.next) {
if (o.equals(e.element)) {
remove(e);
return true;
}
}
}
return false;
}
用过C的人应该感到很熟悉了，这里就是通过指针后移来查找相同的元素，注意这里最多只删除一个
元素，符合List接口中的说明。
public boolean addAll(Collection c) {
return addAll(size, c);
}
public boolean addAll(int index, Collection c) {
int numNew = c.size();
if (numNew==0)
return false;
modCount++;
Entry successor = (index==size ? header : entry(index));
Entry predecessor = successor.previous;
Iterator it = c.iterator();
for (int i=0; i Entry e = new Entry(it.next(), successor, predecessor);
predecessor.next = e;
predecessor = e;
}
successor.previous = predecessor;
size += numNew;
return true;
}
这里又看到熟悉的面孔了，在数据结构里面的链表中插入元素不就是这样吗？
successor代表后一个指针，就是说插入的数据在他的前面，predecessor代表前一个指针，也就是要插入数据之前的指针。如果对数据结构比较了解的话，应该比较容易理解，这里我可以把代码改一下，但是不能算是优化：
for (int i=0; i Entry e = new Entry(it.next(), null, predecessor);
predecessor.next = e;
predecessor = e;
}
predecessor.next = successor;
successor.previous = predecessor;
这样修改和原来是一样的，如果Entry有一个这样的构造函数Entry(Object element,Entry previous)那就
好了，那就可以用修改后的代码优化了(并没有多大的价值)。如果可以看明白为什么可以这样修改，那就完全理解了，如果对这种数据结构不熟悉的话，可以画一个链表，然后按代码执行修改你的链表，这个方法很有效的。
public void clear() {
modCount++;
header.next = header.previous = header;
size = 0;
}
public Object get(int index) {
return entry(index).element;
}
public Object set(int index, Object element) {
Entry e = entry(index);
Object oldVal = e.element;
e.element = element;
return oldVal;
}
public void add(int index, Object element) {
addBefore(element, (index==size ? header : entry(index)));
}
public Object remove(int index) {
Entry e = entry(index);
remove(e);
return e.element;
}
private Entry entry(int index) {
if (index < 0 || index >= size)
throw new IndexOutOfBoundsException("Index: "+index+", Size: "+size);
Entry e = header;
if (index < (size >> 1)) {
for (int i = 0; i <= index; i++)
e = e.next;
} else {
for (int i = size; i > index; i--)
e = e.previous;
}
return e;
}
这个方法返回一个Entry，这里注意注意当index为０时返回的是head.next，不可能返回head。因为index>=0而且 所以循环语句至少要执行一次。这里指针移动进行了优化，因为是一个双向链表，可以朝不同方向移动，size>>2相当于size=size/2
public int indexOf(Object o) {
int index = 0;
if (o==null) {
for (Entry e = header.next; e != header; e = e.next) {
if (e.element==null)
return index;
index++;
}
} else {
for (Entry e = header.next; e != header; e = e.next) {
if (o.equals(e.element))
return index;
index++;
}
}
return -1;
}
这里唯一注意的就是index++的位置
public int lastIndexOf(Object o) {
int index = size;
if (o==null) {
for (Entry e = header.previous; e != header; e = e.previous) {
index--;
if (e.element==null)
return index;
}
} else {
for (Entry e = header.previous; e != header; e = e.previous) {
index--;
if (o.equals(e.element))
return index;
}
}
return -1;
}
注意index--的位置
public ListIterator listIterator(int index) {
return new ListItr(index);
}
以下是一个私有内部类
private class ListItr implements ListIterator {
private Entry lastReturned = header;
private Entry next;
//调用next()方法的节点
private int nextIndex;
private int expectedModCount = modCount;
ListItr(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: "+index+", Size: "+size);
if (index < (size >> 1)) {
next = header.next;
for (nextIndex=0; nextIndex next = next.next;
} else {
next = header;
for (nextIndex=size; nextIndex>index; nextIndex--)
next = next.previous;
}
}
public boolean hasNext() {
return nextIndex != size;
}
public Object next() {
checkForComodification();
if (nextIndex == size)
throw new NoSuchElementException();
lastReturned = next;
next = next.next;
nextIndex++;
return lastReturned.element;
}
public boolean hasPrevious() {
return nextIndex != 0;
}
public Object previous() {
if (nextIndex == 0)
throw new NoSuchElementException();
lastReturned = next = next.previous;
nextIndex--;
checkForComodification();
return lastReturned.element;
}
public int nextIndex() {
return nextIndex;
}
public int previousIndex() {
return nextIndex-1;
}
public void remove() {
checkForComodification();
try {
LinkedList.this.remove(lastReturned);
} catch (NoSuchElementException e) {
throw new IllegalStateException();
}
if (next==lastReturned) //这里表示删除的是调用previous()返回的元素。
next = lastReturned.next; //next被删除，所以next要后移，索引不变。
else
nextIndex--; //删除的是next.previous,所以索引要减１。
lastReturned = header; //这里很重要：１.释放资源。２.不允许连续调用remove。
expectedModCount++;
}
public void set(Object o) {
if (lastReturned == header)
throw new IllegalStateException();
checkForComodification();
lastReturned.element = o;
}
public void add(Object o) {
checkForComodification();
lastReturned = header;
addBefore(o, next);
nextIndex++;
expectedModCount++;
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
以下是Entry的定义：
private static class Entry {
Object element;
Entry next;
Entry previous;
Entry(Object element, Entry next, Entry previous) {
this.element = element;
this.next = next;
this.previous = previous;
}
}
很简单，就是一个双向链表的接点，只有一个构造函数而已，没有其他方法。这里的next和previous不就是一个引用吗？其实不就是一个C里面的指针吗！不过C语言不会处理空指针，直接让操作系统处理了，Java确实抛出一个系统异常NullPointerException,根本不给他破坏系统的机会。
private Entry addBefore(Object o, Entry e) {
Entry newEntry = new Entry(o, e, e.previous);
newEntry.previous.next = newEntry;
newEntry.next.previous = newEntry;
size++;
modCount++;
return newEntry;
}
private void remove(Entry e) {
if (e == header)
throw new NoSuchElementException();
e.previous.next = e.next;
e.next.previous = e.previous;
size--;
modCount++;
}
public Object clone() {
LinkedList clone = null;
try {
clone = (LinkedList)super.clone();
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
// Put clone into "virgin" state
clone.header = new Entry(null, null, null);
clone.header.next = clone.header.previous = clone.header;
clone.size = 0;
clone.modCount = 0;
// Initialize clone with our elements
for (Entry e = header.next; e != header; e = e.next)
clone.add(e.element);
return clone;
}
public Object[] toArray() {
Object[] result = new Object[size];
int i = 0;
for (Entry e = header.next; e != header; e = e.next)
result[i++] = e.element;
return result;
}
}
public Object[] toArray(Object a[]) {
if (a.length < size)
a = (Object[])java.lang.reflect.Array.newInstance(a.getClass().getComponentType(), size);
int i = 0;
for (Entry e = header.next; e != header; e = e.next)
a[i++] = e.element;
if (a.length > size)
a[size] = null;
return a;
}
private static final long serialVersionUID = 876323262645176354L;
private synchronized 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 synchronized 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(null, null, null);
header.next = header.previous = header;
// Read in all elements in the proper order.
for (int i=0; i add(s.readObject());
}
这里和ArrayList有一个区别就是size被声明为 transient的，因为这里调用的是add方法，这样
size会自动增加，而在ArrayList是直接给数组赋值(效率更高)。
这里比较一下ArrayList和LinkedList：
1.ArrayList是基于数组，LinkedList基于链表实现。
2.对于随机访问get和set，ArrayList觉得优于LinkedList，因为LinkedList要移动指针。
3.对于新增和删除操作add和remove，LinedList比较占优势，因为ArrayList要移动数据。
4.查找操作indexOf,lastIndexOf,contains等，两者差不多。
这里只是理论上分析，事实上也不一定，比如ArrayList在末尾插入和删除数据就不设计到数据移动，不过还是
有这么个建议：随机访问比较多的话一定要用ArrayList而不是LinkedList，如果需要频繁的插入和删除应该
考虑用LinkedList来提高性能。
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