LinkedList的源码分析

  1 public class LinkedList<E>
  2     extends AbstractSequentialList<E>
  3     implements List<E>, Deque<E>, Cloneable, java.io.Serializable {
  4     
  5     /**
  6      * 记录当前链表中元素的个数
  7      */
  8     transient int size = 0;
  9 
 10     /**
 11      * Pointer to first node.
 12      * Invariant: (first == null && last == null) ||
 13      *            (first.prev == null && first.item != null)
 14      * 链表的头部节点
 15      */
 16     transient Node<E> first;
 17 
 18     /**
 19      * Pointer to last node.
 20      * Invariant: (first == null && last == null) ||
 21      *            (last.next == null && last.item != null)
 22      * 链表的尾部节点
 23      */
 24     transient Node<E> last;
 25 
 26     /**
 27      * Constructs an empty list.
 28      * 默认空构造器 -- 注意LinkedList并不提供指定容量的构造器
 29      */
 30     public LinkedList() {
 31     }
 32 
 33     /**
 34      * 支持将一个Collection对象转换为linkedList对象
 35      */
 36     public LinkedList(Collection<? extends E> c) {
 37         this();
 38         addAll(c);
 39     }
 40 
 41     /**
 42      * 从头部开始添加一个元素
 43      */
 44     private void linkFirst(E e) {
 45         //获取头部节点
 46         final Node<E> f = first; 
 47         
 48        //组建新的节点，新的节点的prev为null,并赋值给头部节点
 49         final Node<E> newNode = new Node<>(null, e, f); 
 50         first = newNode;
 51         
 52         //头节点==null.说明链表中没有元素
 53         if (f == null)
 54             last = newNode;
 55         else  
 56             f.prev = newNode; //设置旧的头节点的前一个节点为新建的节点
 57         size++; 
 58         modCount++;
 59     }
 60 
 61     /**
 62      * Links e as last element.
 63      * 从尾部开始添加一个元素
 64      */
 65     void linkLast(E e) {
 66         
 67         //获取尾部节点
 68         final Node<E> l = last;
 69         
 70         //组建新的节点，新节点的next节点为null,并将新的节点赋值给尾部节点
 71         final Node<E> newNode = new Node<>(l, e, null);
 72         last = newNode;
 73         
 74         //尾部节点为null,说明链表中没有元素
 75         if (l == null)
 76             first = newNode;
 77         else
 78             l.next = newNode; //设置旧的尾节点的next为新建的节点
 79         size++;
 80         modCount++;
 81     }
 82 
 83     /**
 84      * 在指定的节点之前添加元素
 85      */
 86     void linkBefore(E e, Node<E> succ) {
 87        
 88         //获取指定节点的prev
 89         final Node<E> pred = succ.prev;
 90         
 91         //构建新的节点
 92         final Node<E> newNode = new Node<>(pred, e, succ);
 93         //将新建的节点设置为指定节点的prev
 94         succ.prev = newNode;
 95         
 96         //判断prev是否为头结点
 97         if (pred == null)
 98             first = newNode;
 99         else
100             pred.next = newNode; //设置新节点为prev的next节点
101         size++;
102         modCount++;
103     }
104 
105     /**
106      * 移除头部节点
107      */
108     private E unlinkFirst(Node<E> f) {
109         // assert f == first && f != null;
110         final E element = f.item;
111         
112         //获取头节点的next，并赋值给first
113         final Node<E> next = f.next;
114         f.item = null;
115         f.next = null; // help GC
116         first = next;
117         
118         //next == null,说明链表中已经没有元素
119         if (next == null)
120             last = null;
121         else
122             next.prev = null;  //设置头结点的prev为null
123         size--;
124         modCount++;
125         return element;
126     }
127 
128     /**
129      * 移除尾部节点
130      */
131     private E unlinkLast(Node<E> l) {
132         // assert l == last && l != null;
133         final E element = l.item;
134         
135         //获取尾部节点的prev节点，并赋值为last节点
136         final Node<E> prev = l.prev;
137         l.item = null;
138         l.prev = null; // help GC
139         last = prev;
140         
141         //prev == null，说明链表中已经没有元素
142         if (prev == null)
143             first = null;
144         else
145             prev.next = null; //设置尾节点的next为空
146         size--;
147         modCount++;
148         return element;
149     }
150 
151     /**
152      *  移除链表中的某个节点
153      */
154     E unlink(Node<E> x) {
155 
156         //获取要移除的元素
157         final E element = x.item;
158         final Node<E> next = x.next;
159         final Node<E> prev = x.prev;
160         
161         //要移除的节点是否是头结点
162         if (prev == null) {
163             first = next;
164         } else {
165             //把当前节点的prev节点的next节点赋值为当前节点的next
166             prev.next = next;
167             //当前节点的prev置为null
168             x.prev = null;
169         }
170         
171         //要移除的节点是否是尾节点
172         if (next == null) {
173             last = prev;
174         } else {
175             //当前节点的next节点的prev节点赋值为当前节点的prev
176             next.prev = prev;
177             x.next = null;
178         }
179         
180         //将当前节点置为null
181         x.item = null;
182         size--;
183         modCount++;
184         return element;
185     }
186 
187     /**
188      * 获取链表中的第一个元素
189      */
190     public E getFirst() {
191         final Node<E> f = first;
192         if (f == null)
193             throw new NoSuchElementException();
194         return f.item;
195     }
196 
197     /**
198      * 获取链表中的最后一个元素
199      */
200     public E getLast() {
201         final Node<E> l = last;
202         if (l == null)
203             throw new NoSuchElementException();
204         return l.item;
205     }
206 
207     /**
208      * 移除链表中的第一个元素并返回
209      */
210     public E removeFirst() {
211         final Node<E> f = first;
212         if (f == null)
213             throw new NoSuchElementException();
214         
215         //执行移除头部节点的方法
216         return unlinkFirst(f);
217     }
218 
219     /**
220      * 移除链表中的最后一个元素并返回
221      */
222     public E removeLast() {
223         final Node<E> l = last;
224         if (l == null)
225             throw new NoSuchElementException();
226         
227         //执行移除尾部节点的方法
228         return unlinkLast(l);
229     }
230 
231     /**
232      * 在链表的头部添加节点
233      */
234     public void addFirst(E e) {
235         linkFirst(e);
236     }
237 
238     /**
239      * 在链表的尾部添加节点
240      */
241     public void addLast(E e) {
242         linkLast(e);
243     }
244 
245     /**
246      * 链表中是否包含某个元素
247      */
248     public boolean contains(Object o) {
249         return indexOf(o) != -1;
250     }
251 
252     /**
253      * 链表中元素的个数
254      */
255     public int size() {
256         return size;
257     }
258 
259     /**
260      * 添加一个元素到链表中
261      * 默认是尾部添加
262      * 成功则返回true
263      */
264     public boolean add(E e) {
265         linkLast(e);
266         return true;
267     }
268 
269     /**
270      *  移除链表中的某个元素
271      */
272     public boolean remove(Object o) {
273         if (o == null) {
274             //遍历查找null元素,并进行移除
275             for (Node<E> x = first; x != null; x = x.next) {
276                 if (x.item == null) {
277                     //执行移除的方法
278                     unlink(x);
279                     return true;
280                 }
281             }
282         } else {
283             //遍历找到要移除的元素
284             for (Node<E> x = first; x != null; x = x.next) {
285                 if (o.equals(x.item)) {
286                     //执行移除的方法
287                     unlink(x);
288                     return true;
289                 }
290             }
291         }
292         return false;
293     }
294 
295     /**
296      *  将一个collection对象转换为链表中的节点
297      */
298     public boolean addAll(Collection<? extends E> c) {
299         return addAll(size, c);
300     }
301 
302     /**
303      * 
304      */
305     public boolean addAll(int index, Collection<? extends E> c) {
306         
307         //检查索引的合理性
308         checkPositionIndex(index);
309         
310         //将collection对象转换为数组，数组为空则直接返回
311         Object[] a = c.toArray();
312         int numNew = a.length;
313         if (numNew == 0)
314             return false;
315         
316         //判断是在指定位置插入节点还是在尾部插入节点
317         Node<E> pred, succ;
318         if (index == size) {
319             succ = null;
320             pred = last;
321         } else {
322             succ = node(index);
323             pred = succ.prev;
324         }
325         
326         //遍历数组，进行插入
327         for (Object o : a) {
328             @SuppressWarnings("unchecked") E e = (E) o;
329             Node<E> newNode = new Node<>(pred, e, null);
330             //判断链表书否为空
331             if (pred == null)
332                 first = newNode;
333             else
334                 pred.next = newNode;
335             pred = newNode;
336         }
337         
338         //如果succ为null,则是在尾部进行节点的插入
339         if (succ == null) {
340             last = pred;
341         } else {
342             //将新插入的节点和后面的节点进行链接
343             pred.next = succ;
344             succ.prev = pred;
345         }
346 
347         size += numNew;
348         modCount++;
349         return true;
350     }
351 
352     /**
353      * 清空链表中的元素
354      */
355     public void clear() {
356         
357         //循环设置链表的值为null,加快gc
358         for (Node<E> x = first; x != null; ) {
359             Node<E> next = x.next;
360             x.item = null;
361             x.next = null;
362             x.prev = null;
363             x = next;
364         }
365         first = last = null;
366         size = 0;
367         modCount++;
368     }
369 
370     /**
371      * 获取指定位置的元素
372      */
373     public E get(int index) {
374         //检查索引的合理性
375         checkElementIndex(index);
376         return node(index).item;
377     }
378 
379     /**
380      * 替换指定位置上的元素并返回旧的元素
381      */
382     public E set(int index, E element) {
383         checkElementIndex(index);
384         //找到指定元素所在的节点
385         Node<E> x = node(index);
386         E oldVal = x.item;
387         x.item = element;
388         return oldVal;
389     }
390 
391     /**
392      * 在指定的位置添加元素
393      */
394     public void add(int index, E element) {
395         checkPositionIndex(index);
396 
397         if (index == size)
398             linkLast(element);
399         else
400             linkBefore(element, node(index));
401     }
402 
403     /**
404      *  移除指定位置的元素
405      */
406     public E remove(int index) {
407         //检查索引的合理性
408         checkElementIndex(index);
409         //进行指定位置上的节点移除
410         return unlink(node(index));
411     }
412 
413    
414     private boolean isElementIndex(int index) {
415         return index >= 0 && index < size;
416     }
417 
418     private boolean isPositionIndex(int index) {
419         return index >= 0 && index <= size;
420     }
421 
422     private String outOfBoundsMsg(int index) {
423         return "Index: "+index+", Size: "+size;
424     }
425     
426     /**
427      * 检查索引是否合理
428      * @param index
429      */
430     private void checkElementIndex(int index) {
431         if (!isElementIndex(index))
432             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
433     }
434     
435 
436     private void checkPositionIndex(int index) {
437         if (!isPositionIndex(index))
438             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
439     }
440 
441     /**
442      * 返回某个位置上的节点元素
443      */
444     Node<E> node(int index) {
445         
446         //二分查找
447         if (index < (size >> 1)) {
448             Node<E> x = first;
449             for (int i = 0; i < index; i++)
450                 x = x.next;
451             return x;
452         } else {
453             Node<E> x = last;
454             for (int i = size - 1; i > index; i--)
455                 x = x.prev;
456             return x;
457         }
458     }
459 
460     /**
461      * 元素存在于链表中，则返回所在的节点的位置
462      * 否则返回-1
463      */
464     public int indexOf(Object o) {
465         //初始化为0
466         int index = 0;
467         //如果元素为null,返回第一个查找到的null的位置
468         if (o == null) {
469             //遍历链表，查找null元素，并增加index的值
470             for (Node<E> x = first; x != null; x = x.next) {
471                 if (x.item == null)
472                     return index;
473                 index++;
474             }
475         } else {
476             //遍历链表，查找元素，并增加index的值
477             for (Node<E> x = first; x != null; x = x.next) {
478                 if (o.equals(x.item))
479                     return index;
480                 index++;
481             }
482         }
483         return -1;
484     }
485 
486     /**
487      * 从链表的尾节点开始查找指定的元素
488      * 并返回指定元素的位置
489      * 没有找到返回-1
490      */
491     public int lastIndexOf(Object o) {
492         int index = size;
493         //指定的元素是否为null
494         if (o == null) {
495             //遍历查找null元素
496             for (Node<E> x = last; x != null; x = x.prev) {
497                 index--;
498                 if (x.item == null)
499                     return index;
500             }
501         } else {
502             //遍历查找指定的元素
503             for (Node<E> x = last; x != null; x = x.prev) {
504                 index--;
505                 if (o.equals(x.item))
506                     return index;
507             }
508         }
509         return -1;
510     }
511 
512     /**
513      * 进行出队列的操作
514      */
515     public E peek() {
516         final Node<E> f = first;
517         return (f == null) ? null : f.item;
518     }
519 
520     /**
521      * 获取链表中的一个元素
522      * 默认返回头结点的元素
523      */
524     public E element() {
525         return getFirst();
526     }
527 
528     /**
529      * 进行出队列的操作
530      * 并移除元素
531      */
532     public E poll() {
533         final Node<E> f = first;
534         return (f == null) ? null : unlinkFirst(f);
535     }
536 
537     /**
538      * 移除链表中的元素
539      * 默认为头结点的元素
540      */
541     public E remove() {
542         return removeFirst();
543     }
544 
545     /**
546      *  添加元素，默认在尾部添加
547      */
548     public boolean offer(E e) {
549         return add(e);
550     }
551 
552    
553     /**
554      * 在链表的头部添加元素
555      * 成功则返回true
556      */
557     public boolean offerFirst(E e) {
558         addFirst(e);
559         return true;
560     }
561 
562     /**
563      * 在尾部添加元素
564      */
565     public boolean offerLast(E e) {
566         addLast(e);
567         return true;
568     }
569 
570     /**
571      * 获取第一个元素
572      */
573     public E peekFirst() {
574         final Node<E> f = first;
575         return (f == null) ? null : f.item;
576      }
577 
578     /**
579      * 获取最后一个元素
580      */
581     public E peekLast() {
582         final Node<E> l = last;
583         return (l == null) ? null : l.item;
584     }
585 
586     /**
587      * 获取第一个元素
588      * 并移除
589      */
590     public E pollFirst() {
591         final Node<E> f = first;
592         return (f == null) ? null : unlinkFirst(f);
593     }
594 
595     /**
596      * 获取最后一个元素
597      * 并移除
598      */
599     public E pollLast() {
600         final Node<E> l = last;
601         return (l == null) ? null : unlinkLast(l);
602     }
603 
604     /**
605      * 头部添加元素
606      */
607     public void push(E e) {
608         addFirst(e);
609     }
610 
611     /**
612      * 移除头部元素
613      */
614     public E pop() {
615         return removeFirst();
616     }
617 
618     /**
619      * 从头查找，并移除指定的元素
620      * 成功返回true
621      */
622     public boolean removeFirstOccurrence(Object o) {
623         return remove(o);
624     }
625 
626     /**
627      * 从尾部查找，并移除指定的元素
628      * 成功返回true
629      */
630     public boolean removeLastOccurrence(Object o) {
631         if (o == null) {
632             //遍历查找指定的null元素
633             for (Node<E> x = last; x != null; x = x.prev) {
634                 if (x.item == null) {
635                     unlink(x);
636                     return true;
637                 }
638             }
639         } else {
640             //遍历查找指定的元素
641             for (Node<E> x = last; x != null; x = x.prev) {
642                 if (o.equals(x.item)) {
643                     unlink(x);
644                     return true;
645                 }
646             }
647         }
648         return false;
649     }
650 
651     /**
652      * 从指定位置开始获取链表的迭代器
653      */
654     public ListIterator<E> listIterator(int index) {
655         checkPositionIndex(index);
656         return new ListItr(index);
657     }
658     
659     /**
660      * 链表的内部类
661      * 一个迭代器对象
662      * @author Administrator
663      *
664      */
665     private class ListItr implements ListIterator<E> {
666         private Node<E> lastReturned;
667         private Node<E> next;
668         private int nextIndex;
669         private int expectedModCount = modCount;
670 
671         ListItr(int index) {
672             // assert isPositionIndex(index);
673             next = (index == size) ? null : node(index);
674             nextIndex = index;
675         }
676 
677         public boolean hasNext() {
678             return nextIndex < size;
679         }
680 
681         public E next() {
682             checkForComodification();
683             if (!hasNext())
684                 throw new NoSuchElementException();
685 
686             lastReturned = next;
687             next = next.next;
688             nextIndex++;
689             return lastReturned.item;
690         }
691 
692         public boolean hasPrevious() {
693             return nextIndex > 0;
694         }
695 
696         public E previous() {
697             checkForComodification();
698             if (!hasPrevious())
699                 throw new NoSuchElementException();
700 
701             lastReturned = next = (next == null) ? last : next.prev;
702             nextIndex--;
703             return lastReturned.item;
704         }
705 
706         public int nextIndex() {
707             return nextIndex;
708         }
709 
710         public int previousIndex() {
711             return nextIndex - 1;
712         }
713 
714         public void remove() {
715             checkForComodification();
716             if (lastReturned == null)
717                 throw new IllegalStateException();
718 
719             Node<E> lastNext = lastReturned.next;
720             unlink(lastReturned);
721             if (next == lastReturned)
722                 next = lastNext;
723             else
724                 nextIndex--;
725             lastReturned = null;
726             expectedModCount++;
727         }
728 
729         public void set(E e) {
730             if (lastReturned == null)
731                 throw new IllegalStateException();
732             checkForComodification();
733             lastReturned.item = e;
734         }
735 
736         public void add(E e) {
737             checkForComodification();
738             lastReturned = null;
739             if (next == null)
740                 linkLast(e);
741             else
742                 linkBefore(e, next);
743             nextIndex++;
744             expectedModCount++;
745         }
746 
747         public void forEachRemaining(Consumer<? super E> action) {
748             Objects.requireNonNull(action);
749             while (modCount == expectedModCount && nextIndex < size) {
750                 action.accept(next.item);
751                 lastReturned = next;
752                 next = next.next;
753                 nextIndex++;
754             }
755             checkForComodification();
756         }
757 
758         final void checkForComodification() {
759             if (modCount != expectedModCount)
760                 throw new ConcurrentModificationException();
761         }
762     }
763     
764     /**
765      * 链表中保存的节点对象
766      * @param <E>
767      */
768     private static class Node<E> {
769         E item;  //当前元素
770         Node<E> next; //下一个元素
771         Node<E> prev; //上一个元素
772 
773         Node(Node<E> prev, E element, Node<E> next) {
774             this.item = element;
775             this.next = next;
776             this.prev = prev;
777         }
778     }
779 
780     /**
781      * @since 1.6
782      */
783     public Iterator<E> descendingIterator() {
784         return new DescendingIterator();
785     }
786 
787     /**
788      * Adapter to provide descending iterators via ListItr.previous
789      */
790     private class DescendingIterator implements Iterator<E> {
791         private final ListItr itr = new ListItr(size());
792         public boolean hasNext() {
793             return itr.hasPrevious();
794         }
795         public E next() {
796             return itr.previous();
797         }
798         public void remove() {
799             itr.remove();
800         }
801     }
802     
803     /**
804      * 调用父类进行链表的克隆
805      * @return
806      */
807     @SuppressWarnings("unchecked")
808     private LinkedList<E> superClone() {
809         try {
810             return (LinkedList<E>) super.clone();
811         } catch (CloneNotSupportedException e) {
812             throw new InternalError(e);
813         }
814     }
815 
816     /**
817      * 进行链表的克隆
818      */
819     public Object clone() {
820         LinkedList<E> clone = superClone();
821 
822         // Put clone into "virgin" state
823         clone.first = clone.last = null;
824         clone.size = 0;
825         clone.modCount = 0;
826 
827         // Initialize clone with our elements
828         for (Node<E> x = first; x != null; x = x.next)
829             clone.add(x.item);
830 
831         return clone;
832     }
833 
834     /**
835      * 将链表转换为Object类型的数组
836      */
837     public Object[] toArray() {
838         Object[] result = new Object[size];
839         int i = 0;
840         for (Node<E> x = first; x != null; x = x.next)
841             result[i++] = x.item;
842         return result;
843     }
844 
845     /**
846      * 将链表转换为指定类型的数组
847      */
848     @SuppressWarnings("unchecked")
849     public <T> T[] toArray(T[] a) {
850         if (a.length < size)
851             a = (T[])java.lang.reflect.Array.newInstance(
852                                 a.getClass().getComponentType(), size);
853         int i = 0;
854         Object[] result = a;
855         for (Node<E> x = first; x != null; x = x.next)
856             result[i++] = x.item;
857 
858         if (a.length > size)
859             a[size] = null;
860 
861         return a;
862     }
863 
864     private static final long serialVersionUID = 876323262645176354L;
865 
866     /**
867      * 序列化链表中的元素
868      */
869     private void writeObject(java.io.ObjectOutputStream s)
870         throws java.io.IOException {
871         // Write out any hidden serialization magic
872         s.defaultWriteObject();
873 
874         // Write out size
875         s.writeInt(size);
876 
877         // Write out all elements in the proper order.
878         for (Node<E> x = first; x != null; x = x.next)
879             s.writeObject(x.item);
880     }
881 
882     /**
883      * 反序列化链表中的元素
884      */
885     @SuppressWarnings("unchecked")
886     private void readObject(java.io.ObjectInputStream s)
887         throws java.io.IOException, ClassNotFoundException {
888         // Read in any hidden serialization magic
889         s.defaultReadObject();
890 
891         // Read in size
892         int size = s.readInt();
893 
894         // Read in all elements in the proper order.
895         for (int i = 0; i < size; i++)
896             linkLast((E)s.readObject());
897     }
898 
899     /**
900      * java8新增的返回
901      * 返回一个并行的分片迭代器
902      */
903     @Override
904     public Spliterator<E> spliterator() {
905         return new LLSpliterator<E>(this, -1, 0);
906     }
907 
908     /** A customized variant of Spliterators.IteratorSpliterator */
909     static final class LLSpliterator<E> implements Spliterator<E> {
910         static final int BATCH_UNIT = 1 << 10;  // batch array size increment
911         static final int MAX_BATCH = 1 << 25;  // max batch array size;
912         final LinkedList<E> list; // null OK unless traversed
913         Node<E> current;      // current node; null until initialized
914         int est;              // size estimate; -1 until first needed
915         int expectedModCount; // initialized when est set
916         int batch;            // batch size for splits
917 
918         LLSpliterator(LinkedList<E> list, int est, int expectedModCount) {
919             this.list = list;
920             this.est = est;
921             this.expectedModCount = expectedModCount;
922         }
923 
924         final int getEst() {
925             int s; // force initialization
926             final LinkedList<E> lst;
927             if ((s = est) < 0) {
928                 if ((lst = list) == null)
929                     s = est = 0;
930                 else {
931                     expectedModCount = lst.modCount;
932                     current = lst.first;
933                     s = est = lst.size;
934                 }
935             }
936             return s;
937         }
938 
939         public long estimateSize() { return (long) getEst(); }
940 
941         public Spliterator<E> trySplit() {
942             Node<E> p;
943             int s = getEst();
944             if (s > 1 && (p = current) != null) {
945                 int n = batch + BATCH_UNIT;
946                 if (n > s)
947                     n = s;
948                 if (n > MAX_BATCH)
949                     n = MAX_BATCH;
950                 Object[] a = new Object[n];
951                 int j = 0;
952                 do { a[j++] = p.item; } while ((p = p.next) != null && j < n);
953                 current = p;
954                 batch = j;
955                 est = s - j;
956                 return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
957             }
958             return null;
959         }
960 
961         public void forEachRemaining(Consumer<? super E> action) {
962             Node<E> p; int n;
963             if (action == null) throw new NullPointerException();
964             if ((n = getEst()) > 0 && (p = current) != null) {
965                 current = null;
966                 est = 0;
967                 do {
968                     E e = p.item;
969                     p = p.next;
970                     action.accept(e);
971                 } while (p != null && --n > 0);
972             }
973             if (list.modCount != expectedModCount)
974                 throw new ConcurrentModificationException();
975         }
976 
977         public boolean tryAdvance(Consumer<? super E> action) {
978             Node<E> p;
979             if (action == null) throw new NullPointerException();
980             if (getEst() > 0 && (p = current) != null) {
981                 --est;
982                 E e = p.item;
983                 current = p.next;
984                 action.accept(e);
985                 if (list.modCount != expectedModCount)
986                     throw new ConcurrentModificationException();
987                 return true;
988             }
989             return false;
990         }
991 
992         public int characteristics() {
993             return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
994         }
995     }
996 
997 }