Java多线程系列--【JUC集合06】- ConcurrentSkipListSet

2019年3月10日 238次阅读来源: JUC

参考：http://www.cnblogs.com/skywang12345/p/3498634.html

概要

本章对Java.util.concurrent包中的ConcurrentSkipListSet类进行详细的介绍。内容包括：
ConcurrentSkipListSet介绍
 ConcurrentSkipListSet原理和数据结构
 ConcurrentSkipListSet函数列表
 ConcurrentSkipListSet源码(JDK1.7.0_40版本)
ConcurrentSkipListSet示例

转载请注明出处：http://www.cnblogs.com/skywang12345/p/3498634.html

ConcurrentSkipListSet介绍

ConcurrentSkipListSet是线程安全的有序的集合，适用于高并发的场景。
ConcurrentSkipListSet和TreeSet，它们虽然都是有序的集合。但是，第一，它们的线程安全机制不同，TreeSet是非线程安全的，而ConcurrentSkipListSet是线程安全的。第二，ConcurrentSkipListSet是通过ConcurrentSkipListMap实现的，而TreeSet是通过TreeMap实现的。

ConcurrentSkipListSet原理和数据结构

ConcurrentSkipListSet的数据结构，如下图所示：

说明：
(01) ConcurrentSkipListSet继承于AbstractSet。因此，它本质上是一个集合。
(02) ConcurrentSkipListSet实现了NavigableSet接口。因此，ConcurrentSkipListSet是一个有序的集合。
(03) ConcurrentSkipListSet是通过ConcurrentSkipListMap实现的。它包含一个ConcurrentNavigableMap对象m，而m对象实际上是ConcurrentNavigableMap的实现类ConcurrentSkipListMap的实例。ConcurrentSkipListMap中的元素是key-value键值对；而ConcurrentSkipListSet是集合，它只用到了ConcurrentSkipListMap中的key！

ConcurrentSkipListSet函数列表

// 构造一个新的空 set，该 set 按照元素的自然顺序对其进行排序。
ConcurrentSkipListSet()
// 构造一个包含指定 collection 中元素的新 set，这个新 set 按照元素的自然顺序对其进行排序。
ConcurrentSkipListSet(Collection<? extends E> c)
// 构造一个新的空 set，该 set 按照指定的比较器对其元素进行排序。
ConcurrentSkipListSet(Comparator<? super E> comparator)
// 构造一个新 set，该 set 所包含的元素与指定的有序 set 包含的元素相同，使用的顺序也相同。
ConcurrentSkipListSet(SortedSet<E> s)

// 如果此 set 中不包含指定元素，则添加指定元素。
boolean add(E e)
// 返回此 set 中大于等于给定元素的最小元素；如果不存在这样的元素，则返回 null。
E ceiling(E e)
// 从此 set 中移除所有元素。
void clear()
// 返回此 ConcurrentSkipListSet 实例的浅表副本。
ConcurrentSkipListSet<E> clone()
// 返回对此 set 中的元素进行排序的比较器；如果此 set 使用其元素的自然顺序，则返回 null。
Comparator<? super E> comparator()
// 如果此 set 包含指定的元素，则返回 true。
boolean contains(Object o)
// 返回在此 set 的元素上以降序进行迭代的迭代器。
Iterator<E> descendingIterator()
// 返回此 set 中所包含元素的逆序视图。
NavigableSet<E> descendingSet()
// 比较指定对象与此 set 的相等性。
boolean equals(Object o)
// 返回此 set 中当前第一个（最低）元素。
E first()
// 返回此 set 中小于等于给定元素的最大元素；如果不存在这样的元素，则返回 null。
E floor(E e)
// 返回此 set 的部分视图，其元素严格小于 toElement。
NavigableSet<E> headSet(E toElement)
// 返回此 set 的部分视图，其元素小于（或等于，如果 inclusive 为 true）toElement。
NavigableSet<E> headSet(E toElement, boolean inclusive)
// 返回此 set 中严格大于给定元素的最小元素；如果不存在这样的元素，则返回 null。
E higher(E e)
// 如果此 set 不包含任何元素，则返回 true。
boolean isEmpty()
// 返回在此 set 的元素上以升序进行迭代的迭代器。
Iterator<E> iterator()
// 返回此 set 中当前最后一个（最高）元素。
E last()
// 返回此 set 中严格小于给定元素的最大元素；如果不存在这样的元素，则返回 null。
E lower(E e)
// 获取并移除第一个（最低）元素；如果此 set 为空，则返回 null。
E pollFirst()
// 获取并移除最后一个（最高）元素；如果此 set 为空，则返回 null。
E pollLast()
// 如果此 set 中存在指定的元素，则将其移除。
boolean remove(Object o)
// 从此 set 中移除包含在指定 collection 中的所有元素。
boolean removeAll(Collection<?> c)
// 返回此 set 中的元素数目。
int size()
// 返回此 set 的部分视图，其元素范围从 fromElement 到 toElement。
NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)
// 返回此 set 的部分视图，其元素从 fromElement（包括）到 toElement（不包括）。
NavigableSet<E> subSet(E fromElement, E toElement)
// 返回此 set 的部分视图，其元素大于等于 fromElement。
NavigableSet<E> tailSet(E fromElement)
// 返回此 set 的部分视图，其元素大于（或等于，如果 inclusive 为 true）fromElement。
NavigableSet<E> tailSet(E fromElement, boolean inclusive)

ConcurrentSkipListSet源码(JDK1.7.0_40版本)

ConcurrentSkipListSet.java的完整源码如下：

《Java多线程系列--【JUC集合06】- ConcurrentSkipListSet》

  1 /*
  2  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
  3  *
  4  *
  5  *
  6  *
  7  *
  8  *
  9  *
 10  *
 11  *
 12  *
 13  *
 14  *
 15  *
 16  *
 17  *
 18  *
 19  *
 20  *
 21  *
 22  *
 23  */
 24 
 25 /*
 26  *
 27  *
 28  *
 29  *
 30  *
 31  * Written by Doug Lea with assistance from members of JCP JSR-166
 32  * Expert Group and released to the public domain, as explained at
 33  * http://creativecommons.org/publicdomain/zero/1.0/
 34  */
 35 
 36 package java.util.concurrent;
 37 import java.util.*;
 38 import sun.misc.Unsafe;
 39 
 40 /**
 41  * A scalable concurrent {@link NavigableSet} implementation based on
 42  * a {@link ConcurrentSkipListMap}.  The elements of the set are kept
 43  * sorted according to their {@linkplain Comparable natural ordering},
 44  * or by a {@link Comparator} provided at set creation time, depending
 45  * on which constructor is used.
 46  *
 47  * <p>This implementation provides expected average <i>log(n)</i> time
 48  * cost for the <tt>contains</tt>, <tt>add</tt>, and <tt>remove</tt>
 49  * operations and their variants.  Insertion, removal, and access
 50  * operations safely execute concurrently by multiple threads.
 51  * Iterators are <i>weakly consistent</i>, returning elements
 52  * reflecting the state of the set at some point at or since the
 53  * creation of the iterator.  They do <em>not</em> throw {@link
 54  * ConcurrentModificationException}, and may proceed concurrently with
 55  * other operations.  Ascending ordered views and their iterators are
 56  * faster than descending ones.
 57  *
 58  * <p>Beware that, unlike in most collections, the <tt>size</tt>
 59  * method is <em>not</em> a constant-time operation. Because of the
 60  * asynchronous nature of these sets, determining the current number
 61  * of elements requires a traversal of the elements, and so may report
 62  * inaccurate results if this collection is modified during traversal.
 63  * Additionally, the bulk operations <tt>addAll</tt>,
 64  * <tt>removeAll</tt>, <tt>retainAll</tt>, <tt>containsAll</tt>,
 65  * <tt>equals</tt>, and <tt>toArray</tt> are <em>not</em> guaranteed
 66  * to be performed atomically. For example, an iterator operating
 67  * concurrently with an <tt>addAll</tt> operation might view only some
 68  * of the added elements.
 69  *
 70  * <p>This class and its iterators implement all of the
 71  * <em>optional</em> methods of the {@link Set} and {@link Iterator}
 72  * interfaces. Like most other concurrent collection implementations,
 73  * this class does not permit the use of <tt>null</tt> elements,
 74  * because <tt>null</tt> arguments and return values cannot be reliably
 75  * distinguished from the absence of elements.
 76  *
 77  * <p>This class is a member of the
 78  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 79  * Java Collections Framework</a>.
 80  *
 81  * @author Doug Lea
 82  * @param <E> the type of elements maintained by this set
 83  * @since 1.6
 84  */
 85 public class ConcurrentSkipListSet<E>
 86     extends AbstractSet<E>
 87     implements NavigableSet<E>, Cloneable, java.io.Serializable {
 88 
 89     private static final long serialVersionUID = -2479143111061671589L;
 90 
 91     /**
 92      * The underlying map. Uses Boolean.TRUE as value for each
 93      * element.  This field is declared final for the sake of thread
 94      * safety, which entails some ugliness in clone()
 95      */
 96     private final ConcurrentNavigableMap<E,Object> m;
 97 
 98     /**
 99      * Constructs a new, empty set that orders its elements according to
100      * their {@linkplain Comparable natural ordering}.
101      */
102     public ConcurrentSkipListSet() {
103         m = new ConcurrentSkipListMap<E,Object>();
104     }
105 
106     /**
107      * Constructs a new, empty set that orders its elements according to
108      * the specified comparator.
109      *
110      * @param comparator the comparator that will be used to order this set.
111      *        If <tt>null</tt>, the {@linkplain Comparable natural
112      *        ordering} of the elements will be used.
113      */
114     public ConcurrentSkipListSet(Comparator<? super E> comparator) {
115         m = new ConcurrentSkipListMap<E,Object>(comparator);
116     }
117 
118     /**
119      * Constructs a new set containing the elements in the specified
120      * collection, that orders its elements according to their
121      * {@linkplain Comparable natural ordering}.
122      *
123      * @param c The elements that will comprise the new set
124      * @throws ClassCastException if the elements in <tt>c</tt> are
125      *         not {@link Comparable}, or are not mutually comparable
126      * @throws NullPointerException if the specified collection or any
127      *         of its elements are null
128      */
129     public ConcurrentSkipListSet(Collection<? extends E> c) {
130         m = new ConcurrentSkipListMap<E,Object>();
131         addAll(c);
132     }
133 
134     /**
135      * Constructs a new set containing the same elements and using the
136      * same ordering as the specified sorted set.
137      *
138      * @param s sorted set whose elements will comprise the new set
139      * @throws NullPointerException if the specified sorted set or any
140      *         of its elements are null
141      */
142     public ConcurrentSkipListSet(SortedSet<E> s) {
143         m = new ConcurrentSkipListMap<E,Object>(s.comparator());
144         addAll(s);
145     }
146 
147     /**
148      * For use by submaps
149      */
150     ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {
151         this.m = m;
152     }
153 
154     /**
155      * Returns a shallow copy of this <tt>ConcurrentSkipListSet</tt>
156      * instance. (The elements themselves are not cloned.)
157      *
158      * @return a shallow copy of this set
159      */
160     public ConcurrentSkipListSet<E> clone() {
161         ConcurrentSkipListSet<E> clone = null;
162         try {
163             clone = (ConcurrentSkipListSet<E>) super.clone();
164             clone.setMap(new ConcurrentSkipListMap(m));
165         } catch (CloneNotSupportedException e) {
166             throw new InternalError();
167         }
168 
169         return clone;
170     }
171 
172     /* ---------------- Set operations -------------- */
173 
174     /**
175      * Returns the number of elements in this set.  If this set
176      * contains more than <tt>Integer.MAX_VALUE</tt> elements, it
177      * returns <tt>Integer.MAX_VALUE</tt>.
178      *
179      * <p>Beware that, unlike in most collections, this method is
180      * <em>NOT</em> a constant-time operation. Because of the
181      * asynchronous nature of these sets, determining the current
182      * number of elements requires traversing them all to count them.
183      * Additionally, it is possible for the size to change during
184      * execution of this method, in which case the returned result
185      * will be inaccurate. Thus, this method is typically not very
186      * useful in concurrent applications.
187      *
188      * @return the number of elements in this set
189      */
190     public int size() {
191         return m.size();
192     }
193 
194     /**
195      * Returns <tt>true</tt> if this set contains no elements.
196      * @return <tt>true</tt> if this set contains no elements
197      */
198     public boolean isEmpty() {
199         return m.isEmpty();
200     }
201 
202     /**
203      * Returns <tt>true</tt> if this set contains the specified element.
204      * More formally, returns <tt>true</tt> if and only if this set
205      * contains an element <tt>e</tt> such that <tt>o.equals(e)</tt>.
206      *
207      * @param o object to be checked for containment in this set
208      * @return <tt>true</tt> if this set contains the specified element
209      * @throws ClassCastException if the specified element cannot be
210      *         compared with the elements currently in this set
211      * @throws NullPointerException if the specified element is null
212      */
213     public boolean contains(Object o) {
214         return m.containsKey(o);
215     }
216 
217     /**
218      * Adds the specified element to this set if it is not already present.
219      * More formally, adds the specified element <tt>e</tt> to this set if
220      * the set contains no element <tt>e2</tt> such that <tt>e.equals(e2)</tt>.
221      * If this set already contains the element, the call leaves the set
222      * unchanged and returns <tt>false</tt>.
223      *
224      * @param e element to be added to this set
225      * @return <tt>true</tt> if this set did not already contain the
226      *         specified element
227      * @throws ClassCastException if <tt>e</tt> cannot be compared
228      *         with the elements currently in this set
229      * @throws NullPointerException if the specified element is null
230      */
231     public boolean add(E e) {
232         return m.putIfAbsent(e, Boolean.TRUE) == null;
233     }
234 
235     /**
236      * Removes the specified element from this set if it is present.
237      * More formally, removes an element <tt>e</tt> such that
238      * <tt>o.equals(e)</tt>, if this set contains such an element.
239      * Returns <tt>true</tt> if this set contained the element (or
240      * equivalently, if this set changed as a result of the call).
241      * (This set will not contain the element once the call returns.)
242      *
243      * @param o object to be removed from this set, if present
244      * @return <tt>true</tt> if this set contained the specified element
245      * @throws ClassCastException if <tt>o</tt> cannot be compared
246      *         with the elements currently in this set
247      * @throws NullPointerException if the specified element is null
248      */
249     public boolean remove(Object o) {
250         return m.remove(o, Boolean.TRUE);
251     }
252 
253     /**
254      * Removes all of the elements from this set.
255      */
256     public void clear() {
257         m.clear();
258     }
259 
260     /**
261      * Returns an iterator over the elements in this set in ascending order.
262      *
263      * @return an iterator over the elements in this set in ascending order
264      */
265     public Iterator<E> iterator() {
266         return m.navigableKeySet().iterator();
267     }
268 
269     /**
270      * Returns an iterator over the elements in this set in descending order.
271      *
272      * @return an iterator over the elements in this set in descending order
273      */
274     public Iterator<E> descendingIterator() {
275         return m.descendingKeySet().iterator();
276     }
277 
278 
279     /* ---------------- AbstractSet Overrides -------------- */
280 
281     /**
282      * Compares the specified object with this set for equality.  Returns
283      * <tt>true</tt> if the specified object is also a set, the two sets
284      * have the same size, and every member of the specified set is
285      * contained in this set (or equivalently, every member of this set is
286      * contained in the specified set).  This definition ensures that the
287      * equals method works properly across different implementations of the
288      * set interface.
289      *
290      * @param o the object to be compared for equality with this set
291      * @return <tt>true</tt> if the specified object is equal to this set
292      */
293     public boolean equals(Object o) {
294         // Override AbstractSet version to avoid calling size()
295         if (o == this)
296             return true;
297         if (!(o instanceof Set))
298             return false;
299         Collection<?> c = (Collection<?>) o;
300         try {
301             return containsAll(c) && c.containsAll(this);
302         } catch (ClassCastException unused)   {
303             return false;
304         } catch (NullPointerException unused) {
305             return false;
306         }
307     }
308 
309     /**
310      * Removes from this set all of its elements that are contained in
311      * the specified collection.  If the specified collection is also
312      * a set, this operation effectively modifies this set so that its
313      * value is the <i>asymmetric set difference</i> of the two sets.
314      *
315      * @param  c collection containing elements to be removed from this set
316      * @return <tt>true</tt> if this set changed as a result of the call
317      * @throws ClassCastException if the types of one or more elements in this
318      *         set are incompatible with the specified collection
319      * @throws NullPointerException if the specified collection or any
320      *         of its elements are null
321      */
322     public boolean removeAll(Collection<?> c) {
323         // Override AbstractSet version to avoid unnecessary call to size()
324         boolean modified = false;
325         for (Iterator<?> i = c.iterator(); i.hasNext(); )
326             if (remove(i.next()))
327                 modified = true;
328         return modified;
329     }
330 
331     /* ---------------- Relational operations -------------- */
332 
333     /**
334      * @throws ClassCastException {@inheritDoc}
335      * @throws NullPointerException if the specified element is null
336      */
337     public E lower(E e) {
338         return m.lowerKey(e);
339     }
340 
341     /**
342      * @throws ClassCastException {@inheritDoc}
343      * @throws NullPointerException if the specified element is null
344      */
345     public E floor(E e) {
346         return m.floorKey(e);
347     }
348 
349     /**
350      * @throws ClassCastException {@inheritDoc}
351      * @throws NullPointerException if the specified element is null
352      */
353     public E ceiling(E e) {
354         return m.ceilingKey(e);
355     }
356 
357     /**
358      * @throws ClassCastException {@inheritDoc}
359      * @throws NullPointerException if the specified element is null
360      */
361     public E higher(E e) {
362         return m.higherKey(e);
363     }
364 
365     public E pollFirst() {
366         Map.Entry<E,Object> e = m.pollFirstEntry();
367         return (e == null) ? null : e.getKey();
368     }
369 
370     public E pollLast() {
371         Map.Entry<E,Object> e = m.pollLastEntry();
372         return (e == null) ? null : e.getKey();
373     }
374 
375 
376     /* ---------------- SortedSet operations -------------- */
377 
378 
379     public Comparator<? super E> comparator() {
380         return m.comparator();
381     }
382 
383     /**
384      * @throws NoSuchElementException {@inheritDoc}
385      */
386     public E first() {
387         return m.firstKey();
388     }
389 
390     /**
391      * @throws NoSuchElementException {@inheritDoc}
392      */
393     public E last() {
394         return m.lastKey();
395     }
396 
397     /**
398      * @throws ClassCastException {@inheritDoc}
399      * @throws NullPointerException if {@code fromElement} or
400      *         {@code toElement} is null
401      * @throws IllegalArgumentException {@inheritDoc}
402      */
403     public NavigableSet<E> subSet(E fromElement,
404                                   boolean fromInclusive,
405                                   E toElement,
406                                   boolean toInclusive) {
407         return new ConcurrentSkipListSet<E>
408             (m.subMap(fromElement, fromInclusive,
409                       toElement,   toInclusive));
410     }
411 
412     /**
413      * @throws ClassCastException {@inheritDoc}
414      * @throws NullPointerException if {@code toElement} is null
415      * @throws IllegalArgumentException {@inheritDoc}
416      */
417     public NavigableSet<E> headSet(E toElement, boolean inclusive) {
418         return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
419     }
420 
421     /**
422      * @throws ClassCastException {@inheritDoc}
423      * @throws NullPointerException if {@code fromElement} is null
424      * @throws IllegalArgumentException {@inheritDoc}
425      */
426     public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
427         return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
428     }
429 
430     /**
431      * @throws ClassCastException {@inheritDoc}
432      * @throws NullPointerException if {@code fromElement} or
433      *         {@code toElement} is null
434      * @throws IllegalArgumentException {@inheritDoc}
435      */
436     public NavigableSet<E> subSet(E fromElement, E toElement) {
437         return subSet(fromElement, true, toElement, false);
438     }
439 
440     /**
441      * @throws ClassCastException {@inheritDoc}
442      * @throws NullPointerException if {@code toElement} is null
443      * @throws IllegalArgumentException {@inheritDoc}
444      */
445     public NavigableSet<E> headSet(E toElement) {
446         return headSet(toElement, false);
447     }
448 
449     /**
450      * @throws ClassCastException {@inheritDoc}
451      * @throws NullPointerException if {@code fromElement} is null
452      * @throws IllegalArgumentException {@inheritDoc}
453      */
454     public NavigableSet<E> tailSet(E fromElement) {
455         return tailSet(fromElement, true);
456     }
457 
458     /**
459      * Returns a reverse order view of the elements contained in this set.
460      * The descending set is backed by this set, so changes to the set are
461      * reflected in the descending set, and vice-versa.
462      *
463      * <p>The returned set has an ordering equivalent to
464      * <tt>{@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator())</tt>.
465      * The expression {@code s.descendingSet().descendingSet()} returns a
466      * view of {@code s} essentially equivalent to {@code s}.
467      *
468      * @return a reverse order view of this set
469      */
470     public NavigableSet<E> descendingSet() {
471         return new ConcurrentSkipListSet(m.descendingMap());
472     }
473 
474     // Support for resetting map in clone
475     private void setMap(ConcurrentNavigableMap<E,Object> map) {
476         UNSAFE.putObjectVolatile(this, mapOffset, map);
477     }
478 
479     private static final sun.misc.Unsafe UNSAFE;
480     private static final long mapOffset;
481     static {
482         try {
483             UNSAFE = sun.misc.Unsafe.getUnsafe();
484             Class k = ConcurrentSkipListSet.class;
485             mapOffset = UNSAFE.objectFieldOffset
486                 (k.getDeclaredField("m"));
487         } catch (Exception e) {
488             throw new Error(e);
489         }
490     }
491 }

ConcurrentSkipListSet是通过ConcurrentSkipListMap实现的，它的接口基本上都是通过调用ConcurrentSkipListMap接口来实现的。这里就不再对它的源码进行分析了。

ConcurrentSkipListSet示例

 1 import java.util.*;
 2 import java.util.concurrent.*;
 3 
 4 /*
 5  *   ConcurrentSkipListSet是“线程安全”的集合，而TreeSet是非线程安全的。
 6  *
 7  *   下面是“多个线程同时操作并且遍历集合set”的示例
 8  *   (01) 当set是ConcurrentSkipListSet对象时，程序能正常运行。
 9  *   (02) 当set是TreeSet对象时，程序会产生ConcurrentModificationException异常。
10  *
11  * @author skywang
12  */
13 public class ConcurrentSkipListSetDemo1 {
14 
15     // TODO: set是TreeSet对象时，程序会出错。
16     //private static Set<String> set = new TreeSet<String>();
17     private static Set<String> set = new ConcurrentSkipListSet<String>();
18     public static void main(String[] args) {
19     
20         // 同时启动两个线程对set进行操作！
21         new MyThread("a").start();
22         new MyThread("b").start();
23     }
24 
25     private static void printAll() {
26         String value = null;
27         Iterator iter = set.iterator();
28         while(iter.hasNext()) {
29             value = (String)iter.next();
30             System.out.print(value+", ");
31         }
32         System.out.println();
33     }
34 
35     private static class MyThread extends Thread {
36         MyThread(String name) {
37             super(name);
38         }
39         @Override
40         public void run() {
41                 int i = 0;
42             while (i++ < 10) {
43                 // “线程名” + "序号"
44                 String val = Thread.currentThread().getName() + (i%6);
45                 set.add(val);
46                 // 通过“Iterator”遍历set。
47                 printAll();
48             }
49         }
50     }
51 }

(某一次)运行结果：

a1, b1, 
a1, a1, a2, b1, 
b1, a1, a2, a3, b1,

a1, a2, a3, a1, a4, b1, b2, 
a2, a1, a2, a3, a4, a5, b1, b2, 
a3, a0, a4, a5, a1, b1, a2, b2, 
a3, a0, a4, a1, a5, a2, b1, a3, b2, a4, b3, 
a5, a0, b1, a1, b2, a2, b3, 
a3, a0, a4, a1, a5, a2, b1, a3, b2, a4, b3, a5, b4, 
b1, a0, b2, a1, b3, a2, b4, 
a3, a0, a4, a1, a5, a2, b1, a3, b2, a4, b3, a5, b4, b1, b5, 
b2, a0, a1, a2, a3, a4, a5, b3, b1, b4, b2, b5, 
b3, a0, b4, a1, b5, 
a2, a0, a3, a1, a4, a2, a5, a3, b0, a4, b1, a5, b2, b0, b3, b1, b4, b2, b5, b3, 
b4, a0, b5, 
a1, a2, a3, a4, a5, b0, b1, b2, b3, b4, b5, 
a0, a1, a2, a3, a4, a5, b0, b1, b2, b3, b4, b5, 
a0, a1, a2, a3, a4, a5, b0, b1, b2, b3, b4, b5, 
a0, a1, a2, a3, a4, a5, b0, b1, b2, b3, b4, b5,

结果说明：
示例程序中，启动两个线程(线程a和线程b)分别对ConcurrentSkipListSet进行操作。以线程a而言，它会先获取“线程名”+“序号”，然后将该字符串添加到ConcurrentSkipListSet集合中；接着，遍历并输出集合中的全部元素。线程b的操作和线程a一样，只不过线程b的名字和线程a的名字不同。
当set是ConcurrentSkipListSet对象时，程序能正常运行。如果将set改为TreeSet时，程序会产生ConcurrentModificationException异常。

概要