Concurrent Collections
Concurrent sets, queues, and deques are implemented by the classes Concurrent-SkipListSet, ConcurrentLinkedQueue, and ConcurrentLinkedDeque, respectively. These thread-safe classes implement the corresponding NavigableSet, Queue, and Deque interfaces in the java.util package, as shown in Figure 23.4 and summarized in Table 23.5. Their characteristics are summarized in Table 23.6. These concurrent collections are unbounded and non-blocking. They do not allow null values and have a weakly consistent iterator. As they are unbounded, their insert operation will always succeed. Also worth noting is that their bulk operations (addAll(), removeIf(), forEach()) are not guaranteed to perform atomically.
Figure 23.4 Concurrent Collections in the java.util.concurrent Package
Table 23.5 Concurrent Collections in the java.util.concurrent Package
| Concurrent collections | Description |
| ConcurrentSkipListSet<E> implements NavigableSet<E> | An unbounded, non-blocking, concurrent NavigableSet implementation that internally uses a ConcurrentSkipListMap. Elements are sorted according to the natural ordering or by a comparator that is provided in the constructor. |
| ConcurrentLinkedQueue<E> implements Queue<E> | An unbounded, non-blocking, concurrent queue based on linked nodes. |
| ConcurrentLinkedDeque<E> implements Deque<E> | An unbounded, non-blocking, concurrent deque based on linked nodes. |
Table 23.6 Characteristics of Concurrent Collections
| Concurrent collections | null value | Duplicates | Ordering | Kind of iterator |
| ConcurrentSkipListSet<E> implements NavigableSet<E> | No | No | Sort order | Weakly consistent |
| ConcurrentLinkedQueue<E> implements Queue<E> | No | Yes | FIFO order | Weakly consistent |
| ConcurrentLinkedDeque<E> implements Deque<E> | No | Yes | LIFO/FIFO order | Weakly consistent |
The ConcurrentSkipListSet<E> Class
The ConcurrentSkipListSet class implements the NavigableSet interface in the java.util package (§15.5, p. 811). The class implements a scalable, concurrent, sorted set. It provides efficient insertion, removal, and access operations that can be executed safely and concurrently by multiple threads. Its salient properties are summarized in Table 23.6. Selected methods provided by the ConcurrentSkipListSet class are listed below.
// First-last elements
E pollFirst()
E pollLast()
Remove the first and the last elements currently in this concurrent set, respectively.
// Range-view operations
NavigableSet<E> headSet(E toElement, boolean inclusive)
NavigableSet<E> tailSet(E fromElement, boolean inclusive)
NavigableSet<E> subSet(E fromElement, boolean fromInclusive,
E toElement, boolean toInclusive)
Return different views of the underlying concurrent set, depending on the bound elements.
// Closest-matches
E ceiling(E e)
E floor(E e)
E higher(E e)
E lower(E e)
Determine closest-match elements according to various criteria.
// Reverse order
Iterator<E> descendingIterator()
NavigableSet<E> descendingSet()
The first method returns a reverse-order iterator for this concurrent set. The second method returns a reverse-order view of the elements in the set.
