1.简介
ConcurrentSkipListSet底层是通过ConcurrentNavigableMap来实现的，当然ConcurrentSkipListMap是ConcurrentNavigableMap的子类。
ConcurrentSkipListSet有序的，基于元素的自然排序或者通过比较器确定的顺序；
ConcurrentSkipListSet是线程安全的；
2.源码解释
// 实现了NavigableSet接口，并没有所谓的ConcurrentNavigableSet接口
public class ConcurrentSkipListSet<E>
extends AbstractSet<E>
implements NavigableSet<E>, Cloneable, java.io.Serializable {
private static final long serialVersionUID = -2479143111061671589L;
// 存储使用的map
private final ConcurrentNavigableMap<E,Object> m;
// 初始化
public ConcurrentSkipListSet() {
m = new ConcurrentSkipListMap<E,Object>();
}
// 传入比较器
public ConcurrentSkipListSet(Comparator<? super E> comparator) {
m = new ConcurrentSkipListMap<E,Object>(comparator);
}
// 使用ConcurrentSkipListMap初始化map
// 并将集合c中所有元素放入到map中
public ConcurrentSkipListSet(Collection<? extends E> c) {
m = new ConcurrentSkipListMap<E,Object>();
addAll(c);
}
// 使用ConcurrentSkipListMap初始化map
// 并将有序Set中所有元素放入到map中
public ConcurrentSkipListSet(SortedSet<E> s) {
m = new ConcurrentSkipListMap<E,Object>(s.comparator());
addAll(s);
}
// ConcurrentSkipListSet类内部返回子set时使用的
ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {
this.m = m;
}
// 克隆方法
public ConcurrentSkipListSet<E> clone() {
try {
@SuppressWarnings("unchecked")
ConcurrentSkipListSet<E> clone =
(ConcurrentSkipListSet<E>) super.clone();
clone.setMap(new ConcurrentSkipListMap<E,Object>(m));
return clone;
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
}
/* ---------------- Set operations -------------- */
// 返回元素个数
public int size() {
return m.size();
}
// 检查是否为空
public boolean isEmpty() {
return m.isEmpty();
}
// 检查是否包含某个元素
public boolean contains(Object o) {
return m.containsKey(o);
}
// 添加一个元素
// 调用map的putIfAbsent()方法
public boolean add(E e) {
return m.putIfAbsent(e, Boolean.TRUE) == null;
}
// 移除一个元素
public boolean remove(Object o) {
return m.remove(o, Boolean.TRUE);
}
// 清空所有元素
public void clear() {
m.clear();
}
// 迭代器
public Iterator<E> iterator() {
return m.navigableKeySet().iterator();
}
// 降序迭代器
public Iterator<E> descendingIterator() {
return m.descendingKeySet().iterator();
}
/* ---------------- AbstractSet Overrides -------------- */
// 比较相等方法
public boolean equals(Object o) {
// Override AbstractSet version to avoid calling size()
if (o == this)
return true;
if (!(o instanceof Set))
return false;
Collection<?> c = (Collection<?>) o;
try {
// 这里是通过两次两层for循环来比较
// 这里是有很大优化空间的，参考上篇文章CopyOnWriteArraySet中的彩蛋
return containsAll(c) && c.containsAll(this);
} catch (ClassCastException unused) {
return false;
} catch (NullPointerException unused) {
return false;
}
}
// 移除集合c中所有元素
public boolean removeAll(Collection<?> c) {
// Override AbstractSet version to avoid unnecessary call to size()
boolean modified = false;
for (Object e : c)
if (remove(e))
modified = true;
return modified;
}
/* ---------------- Relational operations -------------- */
// 小于e的最大元素
public E lower(E e) {
return m.lowerKey(e);
}
// 小于等于e的最大元素
public E floor(E e) {
return m.floorKey(e);
}
// 大于等于e的最小元素
public E ceiling(E e) {
return m.ceilingKey(e);
}
// 大于e的最小元素
public E higher(E e) {
return m.higherKey(e);
}
// 弹出最小的元素
public E pollFirst() {
Map.Entry<E,Object> e = m.pollFirstEntry();
return (e == null) ? null : e.getKey();
}
// 弹出最大的元素
public E pollLast() {
Map.Entry<E,Object> e = m.pollLastEntry();
return (e == null) ? null : e.getKey();
}
/* ---------------- SortedSet operations -------------- */
// 取比较器
public Comparator<? super E> comparator() {
return m.comparator();
}
// 最小的元素
public E first() {
return m.firstKey();
}
// 最大的元素
public E last() {
return m.lastKey();
}
// 取两个元素之间的子set
public NavigableSet<E> subSet(E fromElement,
boolean fromInclusive,
E toElement,
boolean toInclusive) {
return new ConcurrentSkipListSet<E>
(m.subMap(fromElement, fromInclusive,
toElement, toInclusive));
}
// 取头子set
public NavigableSet<E> headSet(E toElement, boolean inclusive) {
return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
}
// 取尾子set
public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
}
// 取子set，包含from，不包含to
public NavigableSet<E> subSet(E fromElement, E toElement) {
return subSet(fromElement, true, toElement, false);
}
// 取头子set，不包含to
public NavigableSet<E> headSet(E toElement) {
return headSet(toElement, false);
}
// 取尾子set，包含from
public NavigableSet<E> tailSet(E fromElement) {
return tailSet(fromElement, true);
}
// 降序set
public NavigableSet<E> descendingSet() {
return new ConcurrentSkipListSet<E>(m.descendingMap());
}
// 可分割的迭代器
@SuppressWarnings("unchecked")
public Spliterator<E> spliterator() {
if (m instanceof ConcurrentSkipListMap)
return ((ConcurrentSkipListMap<E,?>)m).keySpliterator();
else
return (Spliterator<E>)((ConcurrentSkipListMap.SubMap<E,?>)m).keyIterator();
}
// 原子更新map，给clone方法使用
private void setMap(ConcurrentNavigableMap<E,Object> map) {
UNSAFE.putObjectVolatile(this, mapOffset, map);
}
// 原子操作相关内容
private static final sun.misc.Unsafe UNSAFE;
private static final long mapOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> k = ConcurrentSkipListSet.class;
mapOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("m"));
} catch (Exception e) {
throw new Error(e);
}
}
}
3.说明
ConcurrentSkipListMap将会在下一篇文章进行介绍