public class LinkedList<E>
     extends AbstractSequentialList<E>
     implements List<E>, Deque<E>, Cloneable, java.io.Serializable

1 private transient Entry<E> header = new Entry<E>(null, null, null);
2 private transient int size = 0;

1 private static class Entry<E> {
 2    E element;
 3     Entry<E> next;
 4     Entry<E> previous;
 5 
 6     Entry(E element, Entry<E> next, Entry<E> previous) {
 7         this.element = element;
 8         this.next = next;
 9         this.previous = previous;
10    }
11 }

1 public LinkedList() {
2     header.next = header.previous = header;
3 }
4 public LinkedList(Collection<? extends E> c) {
5     this();
6    addAll(c);
7 }

 1 public boolean addAll(Collection<? extends E> c) {
 2     return addAll(size, c);
 3 }
 4 // index參數指(zhi)定collection中(zhong)插入(ru)的第(di)一個元素的位置
  5 public boolean addAll(int index, Collection<? extends E> c) {
 6     // 插入位置超過了鏈表的長度或(huo)小(xiao)于(yu)0，報(bao)IndexOutOfBoundsException異常
  7     if (index < 0 || index > size)
 8         throw new IndexOutOfBoundsException("Index: "+index+
  9                                                 ", Size: "+size);
10     Object[] a = c.toArray();
11    int numNew = a.length;
12    // 若需要插(cha)入的節點(dian)個數(shu)為(wei)0則返回(hui)false，表示(shi)沒(mei)有插(cha)入元素
13     if (numNew==0)
14         return false;
15     modCount++;//否則，插入對象，鏈表(biao)修改次(ci)數加(jia)1
16     // 保存index處(chu)的節(jie)點。插(cha)入位置如果是size，則在頭結(jie)點前(qian)面插(cha)入，否則在獲(huo)取index處(chu)的節(jie)點插(cha)入
17     Entry<E> successor = (index==size ? header : entry(index));
18     // 獲取(qu)前(qian)一個(ge)(ge)節點(dian)，插入時需(xu)要修(xiu)改這(zhe)個(ge)(ge)節點(dian)的next引(yin)用
19     Entry<E> predecessor = successor.previous;
20     // 按順序將a數組中(zhong)的(de)第(di)一個(ge)元(yuan)素插入(ru)到index處，將之(zhi)后的(de)元(yuan)素插在這個(ge)元(yuan)素后面
21     for (int i=0; i<numNew; i++) {
22         // 結合Entry的構造方法，這條(tiao)語(yu)句是插入(ru)操作(zuo)，相當于C語(yu)言中(zhong)鏈(lian)表中(zhong)插入(ru)節(jie)點并修改(gai)指針
23         Entry<E> e = new Entry<E>((E)a[i], successor, predecessor);
24         // 插入節(jie)(jie)點(dian)后將前(qian)(qian)一(yi)節(jie)(jie)點(dian)的(de)(de)next指(zhi)向當(dang)前(qian)(qian)節(jie)(jie)點(dian)，相(xiang)當(dang)于修改前(qian)(qian)一(yi)節(jie)(jie)點(dian)的(de)(de)next指(zhi)針
25         predecessor.next = e;
26         // 相(xiang)當于C語言中成功插入元素后(hou)將(jiang)指(zhi)針向后(hou)移(yi)動一個(ge)位置以實現循環的功能(neng)
27         predecessor = e;
28   }
29     // 插入元(yuan)素前(qian)index處的元(yuan)素鏈接到插入的Collection的最后一個節點(dian)
30     successor.previous = predecessor;
31     // 修改size
32     size += numNew;
33     return true;
34 }

1 private Entry<E> entry(int index) {
 2         if (index < 0 || index >= size)
 3             throw new IndexOutOfBoundsException("Index: "+index+
 4                                                 ", Size: "+size);
 5         Entry<E> e = header;
 6         // 根據這個(ge)判斷決(jue)定從哪個(ge)方向遍歷這個(ge)鏈表
 7         if (index < (size >> 1)) {
 8             for (int i = 0; i <= index; i++)
 9                 e = e.next;
10         } else {
11             // 可以通過header節點(dian)(dian)向前遍歷，說明這個(ge)(ge)一(yi)個(ge)(ge)循(xun)環雙(shuang)向鏈(lian)表(biao)，header的previous指向鏈(lian)表(biao)的最后一(yi)個(ge)(ge)節點(dian)(dian)，這也驗證了構造方法(fa)中對于header節點(dian)(dian)的前后節點(dian)(dian)均指向自己的解(jie)釋
12             for (int i = size; i > index; i--)
13                 e = e.previous;
14        }
15         return e;
16     }

     // 將元素(E)添加到LinkedList中(zhong)
     public boolean add(E e) {
         // 將(jiang)節點(節點數據是e)添加到表頭(tou)(header)之前。
         // 即，將(jiang)節(jie)點(dian)添加(jia)到雙向鏈(lian)表的末端(duan)。
         addBefore(e, header);
         return true;
     }

     public void add(int index, E element) {
         addBefore(element, (index==size ? header : entry(index)));
     }
    
    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;
    }

 public void addFirst(E e) {
     addBefore(e, header.next);
 }

 public void addLast(E e) {
     addBefore(e, header);
 }

 1 public void clear() {
 2     Entry<E> e = header.next;
 3     // e可以理解為(wei)一(yi)個(ge)移動的“指針”，因為(wei)是循環鏈表，所以回到header的時候說明已經沒有節(jie)點了
 4      while (e != header) {
 5        // 保留(liu)e的下一(yi)個(ge)節(jie)點(dian)的引用
 6         Entry<E> next = e.next;
 7         // 解除(chu)節點(dian)(dian)e對(dui)前后(hou)節點(dian)(dian)的引用
 8         e.next = e.previous = null;
 9         // 將節(jie)點e的內容(rong)置空
10         e.element = null;
11         // 將e移動(dong)到下(xia)一(yi)個節點
12         e = next;
13  }
14     // 將header構造(zao)成一(yi)個(ge)循環鏈表，同構造(zao)方法構造(zao)一(yi)個(ge)空的(de)LinkedList
15     header.next = header.previous = header;
16     // 修改size
17     size = 0;
18     modCount++;
19 }

 public boolean contains(Object o) {
     return indexOf(o) != -1;
 }
 // 從(cong)前向后查(cha)找，返(fan)回“值為對象(o)的(de)(de)節(jie)點對應的(de)(de)索引”  不存(cun)在就(jiu)返(fan)回-1 
 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;
 }

 1 private E remove(Entry<E> e) {
 2     if (e == header)
 3         throw new NoSuchElementException();
 4     // 保留將被移除的節點e的內(nei)容
 5     E result = e.element;
 6    // 將前一節點的(de)next引(yin)用賦值(zhi)為(wei)e的(de)下一節點
 7     e.previous.next = e.next;
 8    // 將e的(de)下(xia)一節點的(de)previous賦值為e的(de)上(shang)一節點
 9     e.next.previous = e.previous;
10    // 上面兩條語句的(de)執行(xing)已經導致了無法在鏈(lian)表中(zhong)訪問到e節(jie)(jie)點，而(er)下面解除了e節(jie)(jie)點對前后節(jie)(jie)點的(de)引用
11    e.next = e.previous = null;
12   // 將被移(yi)除的節點的內容(rong)設為null
13   e.element = null;
14   // 修改(gai)size大小
15   size--;
16   modCount++;
17   // 返回移除(chu)節(jie)點e的內容
18   return result;
19 }

// 獲取(qu)雙向鏈表中指(zhi)定位置的(de)節點    
    private Entry<E> entry(int index) {    
        if (index < 0 || index >= size)    
            throw new IndexOutOfBoundsException("Index: "+index+    
                                                ", Size: "+size);    
        Entry<E> e = header;    
        // 獲取(qu)index處(chu)的節點(dian)。    
        // 若(ruo)index < 雙(shuang)向鏈表長(chang)度的1/2,則從(cong)前先后(hou)查找;    
        // 否(fou)則，從后向前(qian)查找(zhao)。    
        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;    
    }

 1 public Object clone() {
 2     LinkedList<E> clone = null;
 3     try {
 4         clone = (LinkedList<E>) super.clone();
 5     } catch (CloneNotSupportedException e) {
 6         throw new InternalError();
 7    }
 8     clone.header = new Entry<E>(null, null, null);
 9     clone.header.next = clone.header.previous = clone.header;
10     clone.size = 0;
11     clone.modCount = 0;
12     for (Entry<E> e = header.next; e != header; e = e.next)
13        clone.add(e.element);
14     return clone;
15 }

1 public Object[] toArray() {
2     Object[] result = new Object[size];
3     int i = 0;
4     for (Entry<E> e = header.next; e != header; e = e.next)
5         result[i++] = e.element;
6     return result;
7 }

 1 public <T> T[] toArray(T[] a) {
 2     if (a.length < size)
 3         a = (T[])java.lang.reflect.Array.newInstance(
 4                                a.getClass().getComponentType(), size);
 5     int i = 0;
 6     Object[] result = a;
 7     for (Entry<E> e = header.next; e != header; e = e.next)
 8         result[i++] = e.element;
 9     if (a.length > size)
10         a[size] = null;
11     return a;
12 }

1 public ListIterator<E> listIterator(int index) {
2     return new ListItr(index);
3 }

  1 private class ListItr implements ListIterator<E> {
  2 // 最(zui)近一次返回的節(jie)點(dian)，也是當前持有的節(jie)點(dian)
  3     private Entry<E> lastReturned = header;
  4     // 對下一(yi)個(ge)元(yuan)素(su)的引用
  5     private Entry<E> next;
  6     // 下(xia)一個節點的(de)index
  7     private int nextIndex;
  8     private int expectedModCount = modCount;
  9     // 構(gou)造(zao)方(fang)法，接(jie)收一個index參(can)數，返(fan)回一個ListItr對(dui)象(xiang)
 10     ListItr(int index) {
 11         // 如果index小于0或大于size，拋出IndexOutOfBoundsException異常
 12         if (index < 0 || index > size)
 13         throw new IndexOutOfBoundsException("Index: "+index+
 14                             ", Size: "+size);
 15         // 判斷遍歷方向(xiang)
 16         if (index < (size >> 1)) {
 17         // next賦值為第(di)一(yi)個(ge)節(jie)點
 18         next = header.next;
 19         // 獲取指定位置的節點(dian)
 20         for (nextIndex=0; nextIndex<index; nextIndex++)
 21             next = next.next;
 22         } else {
 23 // else中的處理(li)和(he)if塊(kuai)中的處理(li)一致，只是(shi)遍歷方(fang)向不同(tong)
 24         next = header;
 25         for (nextIndex=size; nextIndex>index; nextIndex--)
 26             next = next.previous;
 27        }
 28    }
 29     // 根(gen)據nextIndex是(shi)否等(deng)于size判斷時(shi)候還有下一(yi)個(ge)節點（也(ye)可(ke)以理(li)解為是(shi)否遍歷完了LinkedList）
 30     public boolean hasNext() {
 31         return nextIndex != size;
 32    }
 33     // 獲取(qu)下一個(ge)元素
 34     public E next() {
 35        checkForComodification();
 36         // 如果nextIndex==size,則已(yi)經遍(bian)歷完鏈表，即沒(mei)有(you)(you)下(xia)一(yi)個(ge)(ge)節(jie)(jie)點(dian)了（實際(ji)上是有(you)(you)的(de)，因為是循環鏈表，任何一(yi)個(ge)(ge)節(jie)(jie)點(dian)都會有(you)(you)上一(yi)個(ge)(ge)和下(xia)一(yi)個(ge)(ge)節(jie)(jie)點(dian)，這(zhe)里的(de)沒(mei)有(you)(you)下(xia)一(yi)個(ge)(ge)節(jie)(jie)點(dian)只是說所有(you)(you)節(jie)(jie)點(dian)都已(yi)經遍(bian)歷完了）
 37         if (nextIndex == size)
 38         throw new NoSuchElementException();
 39         // 設置(zhi)最近一次返回的(de)節點為next節點
 40         lastReturned = next;
 41         // 將next“向后移動(dong)一位”
 42         next = next.next;
 43         // index計(ji)數加1
 44         nextIndex++;
 45         // 返回lastReturned的(de)元(yuan)素
 46         return lastReturned.element;
 47    }
 48 
 49     public boolean hasPrevious() {
 50         return nextIndex != 0;
 51    }
 52     // 返回上一(yi)個節點，和next()方法相似
 53     public E previous() {
 54         if (nextIndex == 0)
 55         throw new NoSuchElementException();
 56 
 57         lastReturned = next = next.previous;
 58         nextIndex--;
 59        checkForComodification();
 60         return lastReturned.element;
 61    }
 62 
 63     public int nextIndex() {
 64         return nextIndex;
 65    }
 66 
 67     public int previousIndex() {
 68         return nextIndex-1;
 69    }
 70     // 移除當前(qian)Iterator持(chi)有的節點
 71     public void remove() {
 72            checkForComodification();
 73             Entry<E> lastNext = lastReturned.next;
 74             try {
 75                 LinkedList.this.remove(lastReturned);
 76             } catch (NoSuchElementException e) {
 77                 throw new IllegalStateException();
 78            }
 79         if (next==lastReturned)
 80                 next = lastNext;
 81             else
 82         nextIndex--;
 83         lastReturned = header;
 84         expectedModCount++;
 85    }
 86     // 修改當(dang)前節點的(de)內容(rong)
 87     public void set(E e) {
 88         if (lastReturned == header)
 89         throw new IllegalStateException();
 90        checkForComodification();
 91         lastReturned.element = e;
 92    }
 93     // 在當前持(chi)有節點(dian)后面插入新節點(dian)
 94     public void add(E e) {
 95        checkForComodification();
 96         // 將最近一次返回節點修(xiu)改(gai)為header
 97         lastReturned = header;
 98        addBefore(e, next);
 99         nextIndex++;
100         expectedModCount++;
101    }
102     // 判斷expectedModCount和modCount是否(fou)一致，以確(que)保(bao)通過ListItr的修(xiu)改操作正確(que)的反映在LinkedList中
103     final void checkForComodification() {
104         if (modCount != expectedModCount)
105         throw new ConcurrentModificationException();
106    }
107 }

 1 LinkedList<String> list = new LinkedList<String>();
 2         list.add("First");
 3         list.add("Second");
 4         list.add("Thrid");
 5        System.out.println(list);
 6         ListIterator<String> itr = list.listIterator();
 7         while (itr.hasNext()) {
 8            System.out.println(itr.next());
 9        }
10         try {
11             System.out.println(itr.next());// throw Exception
12         } catch (Exception e) {
13             // TODO: handle exception
14        }
15         itr = list.listIterator();
16        System.out.println(list);
17        System.out.println(itr.next());
18         itr.add("new node1");
19        System.out.println(list);
20         itr.add("new node2");
21        System.out.println(list);
22        System.out.println(itr.next());
23         itr.set("modify node");
24        System.out.println(list);
25        itr.remove();
26         System.out.println(list);

 1 結果：
 2 [First, Second, Thrid]
 3 First
 4 Second
 5 Thrid
 6 [First, Second, Thrid]
 7 First
 8 [First, new node1, Second, Thrid]
 9 [First, new node1, new node2, Second, Thrid]
10 Second
11 [First, new node1, new node2, modify node, Thrid]
12 [First, new node1, new node2, Thrid]

1 public Iterator<E> descendingIterator() {
2    return new DescendingIterator();
3 }

 1 private class DescendingIterator implements Iterator {
 2    // 獲取(qu)ListItr對象
 3 final ListItr itr = new ListItr(size());
 4 // hasNext其實是(shi)調用了(le)itr的(de)hasPrevious方法(fa)
 5    public boolean hasNext() {
 6        return itr.hasPrevious();
 7    }
 8 // next()其實(shi)是調用(yong)了itr的previous方法
 9    public E next() {
10        return itr.previous();
11    }
12    public void remove() {
13        itr.remove();
14    }
15 }