LinkedList類似C語言的雙向鏈表，但是java中沒有指針如何實現呢，看完LinkedList
你將對java中的引用類型有更深入的理解。LindedList的聲明如下：
public class LinkedList extends AbstractSequentialList implements List, Cloneable, java.io.Serializable
有關AbstractSequentialList：http://blog.csdn.net/treeroot/archive/2004/09/18/108696.aspx
有關List： http://blog.csdn.net/treeroot/archive/2004/09/14/104638.aspx
有關Cloneable：http://blog.csdn.net/treeroot/archive/2004/09/07/96936.aspx
下面分析一下這個鏈表的實現，這里只重點分析某些方法。
private transient Entry header = new Entry(null, null, null);
private transient int size = 0;
public LinkedList() {
header.next = header.previous = header;
}
header相當于C中的頭指針，而且這個頭指針不是鏈表的元素，有關Entry將在下面介紹。
public LinkedList(Collection c) {
this();
addAll(c);
}
public Object getFirst() {
if (size==0)
throw new NoSuchElementException();
return header.next.element;
}
頭指針的下一個元素就是第一個元素
public Object getLast() {
if (size==0)
throw new NoSuchElementException();
return header.previous.element;
}
頭指針的前一個當然就是最后一個了
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);
}
這個方法在鏈表末尾插入新的元素，功能和add方法一樣，這個方法完全是為了對稱性(因為有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;
}
用過C的人應該感到很熟悉了，這里就是通過指針后移來查找相同的元素，注意這里最多只刪除一個
元素，符合List接口中的說明。
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;
}
這里又看到熟悉的面孔了，在數據結構里面的鏈表中插入元素不就是這樣嗎？
successor代表后一個指針，就是說插入的數據在他的前面，predecessor代表前一個指針，也就是要插入數據之前的指針。如果對數據結構比較了解的話，應該比較容易理解，這里我可以把代碼改一下，但是不能算是優化：
for (int i=0; i
Entry e = new Entry(it.next(), null, predecessor);
predecessor.next = e;
predecessor = e;
}
predecessor.next = successor;
successor.previous = predecessor;
這樣修改和原來是一樣的，如果Entry有一個這樣的構造函數Entry(Object element,Entry previous)那就
好了，那就可以用修改后的代碼優化了(并沒有多大的價值)。如果可以看明白為什么可以這樣修改，那就完全理解了，如果對這種數據結構不熟悉的話，可以畫一個鏈表，然后按代碼執行修改你的鏈表，這個方法很有效的。
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;
}
這個方法返回一個Entry，這里注意注意當index為０時返回的是head.next，不可能返回head。因為index>=0而且
所以循環語句至少要執行一次。這里指針移動進行了優化，因為是一個雙向鏈表，可以朝不同方向移動，size>>2相當于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;
}
這里唯一注意的就是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);
}
以下是一個私有內部類
private class ListItr implements ListIterator {
private Entry lastReturned = header;
private Entry next;
//調用next()方法的節點
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) //這里表示刪除的是調用previous()返回的元素。
next = lastReturned.next; //next被刪除，所以next要后移，索引不變。
else
nextIndex--; //刪除的是next.previous,所以索引要減１。
lastReturned = header; //這里很重要：１.釋放資源。２.不允許連續調用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的定義：
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;
}
}
很簡單，就是一個雙向鏈表的接點，只有一個構造函數而已，沒有其他方法。這里的next和previous不就是一個引用嗎？其實不就是一個C里面的指針嗎！不過C語言不會處理空指針，直接讓操作系統處理了，Java確實拋出一個系統異常NullPointerException,根本不給他破壞系統的機會。
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());
}
這里和ArrayList有一個區別就是size被聲明為 transient的，因為這里調用的是add方法，這樣
size會自動增加，而在ArrayList是直接給數組賦值(效率更高)。
這里比較一下ArrayList和LinkedList：
1.ArrayList是基于數組，LinkedList基于鏈表實現。
2.對于隨機訪問get和set，ArrayList覺得優于LinkedList，因為LinkedList要移動指針。
3.對于新增和刪除操作add和remove，LinedList比較占優勢，因為ArrayList要移動數據。
4.查找操作indexOf,lastIndexOf,contains等，兩者差不多。
這里只是理論上分析，事實上也不一定，比如ArrayList在末尾插入和刪除數據就不設計到數據移動，不過還是
有這么個建議：隨機訪問比較多的話一定要用ArrayList而不是LinkedList，如果需要頻繁的插入和刪除應該
考慮用LinkedList來提高性能。
引自：
http://www.wangchao.net.cn/bbsshowlist.jsp?area_id=02&board_id=01&parent_id=43264