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ConcurrentSkipListSet.javaAPI DocJava SE 6 API16326Tue Jun 10 00:25:56 BST 2008java.util.concurrent

ConcurrentSkipListSet

public class ConcurrentSkipListSet extends AbstractSet implements NavigableSet, Cloneable, Serializable
A scalable concurrent {@link NavigableSet} implementation based on a {@link ConcurrentSkipListMap}. The elements of the set are kept sorted according to their {@linkplain Comparable natural ordering}, or by a {@link Comparator} provided at set creation time, depending on which constructor is used.

This implementation provides expected average log(n) time cost for the contains, add, and remove operations and their variants. Insertion, removal, and access operations safely execute concurrently by multiple threads. Iterators are weakly consistent, returning elements reflecting the state of the set at some point at or since the creation of the iterator. They do not throw {@link ConcurrentModificationException}, and may proceed concurrently with other operations. Ascending ordered views and their iterators are faster than descending ones.

Beware that, unlike in most collections, the size method is not a constant-time operation. Because of the asynchronous nature of these sets, determining the current number of elements requires a traversal of the elements. Additionally, the bulk operations addAll, removeAll, retainAll, and containsAll are not guaranteed to be performed atomically. For example, an iterator operating concurrently with an addAll operation might view only some of the added elements.

This class and its iterators implement all of the optional methods of the {@link Set} and {@link Iterator} interfaces. Like most other concurrent collection implementations, this class does not permit the use of null elements, because null arguments and return values cannot be reliably distinguished from the absence of elements.

This class is a member of the Java Collections Framework.

author
Doug Lea
param
the type of elements maintained by this set
since
1.6

Fields Summary
private static final long
serialVersionUID
private final ConcurrentNavigableMap
m
The underlying map. Uses Boolean.TRUE as value for each element. This field is declared final for the sake of thread safety, which entails some ugliness in clone()
private static final Unsafe
unsafe
private static final long
mapOffset
Constructors Summary
public ConcurrentSkipListSet()
Constructs a new, empty set that orders its elements according to their {@linkplain Comparable natural ordering}.


                         
      
        m = new ConcurrentSkipListMap<E,Object>();
    
public ConcurrentSkipListSet(Comparator comparator)
Constructs a new, empty set that orders its elements according to the specified comparator.

param
comparator the comparator that will be used to order this set. If null, the {@linkplain Comparable natural ordering} of the elements will be used.

        m = new ConcurrentSkipListMap<E,Object>(comparator);
    
public ConcurrentSkipListSet(Collection c)
Constructs a new set containing the elements in the specified collection, that orders its elements according to their {@linkplain Comparable natural ordering}.

param
c The elements that will comprise the new set
throws
ClassCastException if the elements in c are not {@link Comparable}, or are not mutually comparable
throws
NullPointerException if the specified collection or any of its elements are null

        m = new ConcurrentSkipListMap<E,Object>();
        addAll(c);
    
public ConcurrentSkipListSet(SortedSet s)
Constructs a new set containing the same elements and using the same ordering as the specified sorted set.

param
s sorted set whose elements will comprise the new set
throws
NullPointerException if the specified sorted set or any of its elements are null

        m = new ConcurrentSkipListMap<E,Object>(s.comparator());
        addAll(s);
    
ConcurrentSkipListSet(ConcurrentNavigableMap m)
For use by submaps

        this.m = m;
    
Methods Summary
public booleanadd(E e)
Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if the set contains no element e2 such that e.equals(e2). If this set already contains the element, the call leaves the set unchanged and returns false.

param
e element to be added to this set
return
true if this set did not already contain the specified element
throws
ClassCastException if e cannot be compared with the elements currently in this set
throws
NullPointerException if the specified element is null

	return m.putIfAbsent(e, Boolean.TRUE) == null;
    
public Eceiling(E e)

throws
ClassCastException {@inheritDoc}
throws
NullPointerException if the specified element is null

        return m.ceilingKey(e);
    
public voidclear()
Removes all of the elements from this set.

	m.clear();
    
public java.util.concurrent.ConcurrentSkipListSetclone()
Returns a shallow copy of this ConcurrentSkipListSet instance. (The elements themselves are not cloned.)

return
a shallow copy of this set

        ConcurrentSkipListSet<E> clone = null;
	try {
	    clone = (ConcurrentSkipListSet<E>) super.clone();
            clone.setMap(new ConcurrentSkipListMap(m));
	} catch (CloneNotSupportedException e) {
	    throw new InternalError();
	}

        return clone;
    
public java.util.Comparatorcomparator()

        return m.comparator();
    
public booleancontains(java.lang.Object o)
Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that o.equals(e).

param
o object to be checked for containment in this set
return
true if this set contains the specified element
throws
ClassCastException if the specified element cannot be compared with the elements currently in this set
throws
NullPointerException if the specified element is null

	return m.containsKey(o);
    
public java.util.IteratordescendingIterator()
Returns an iterator over the elements in this set in descending order.

return
an iterator over the elements in this set in descending order

	return m.descendingKeySet().iterator();
    
public java.util.NavigableSetdescendingSet()
Returns a reverse order view of the elements contained in this set. The descending set is backed by this set, so changes to the set are reflected in the descending set, and vice-versa.

The returned set has an ordering equivalent to {@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator()). The expression {@code s.descendingSet().descendingSet()} returns a view of {@code s} essentially equivalent to {@code s}.

return
a reverse order view of this set

	return new ConcurrentSkipListSet(m.descendingMap());
    
public booleanequals(java.lang.Object o)
Compares the specified object with this set for equality. Returns true if the specified object is also a set, the two sets have the same size, and every member of the specified set is contained in this set (or equivalently, every member of this set is contained in the specified set). This definition ensures that the equals method works properly across different implementations of the set interface.

param
o the object to be compared for equality with this set
return
true if the specified object is equal to this set

        // Override AbstractSet version to avoid calling size()
	if (o == this)
	    return true;
	if (!(o instanceof Set))
	    return false;
	Collection<?> c = (Collection<?>) o;
        try {
            return containsAll(c) && c.containsAll(this);
        } catch (ClassCastException unused)   {
            return false;
        } catch (NullPointerException unused) {
            return false;
        }
    
public Efirst()

throws
NoSuchElementException {@inheritDoc}

        return m.firstKey();
    
public Efloor(E e)

throws
ClassCastException {@inheritDoc}
throws
NullPointerException if the specified element is null

        return m.floorKey(e);
    
public java.util.NavigableSetheadSet(E toElement, boolean inclusive)

throws
ClassCastException {@inheritDoc}
throws
NullPointerException if {@code toElement} is null
throws
IllegalArgumentException {@inheritDoc}

	return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
    
public java.util.NavigableSetheadSet(E toElement)

throws
ClassCastException {@inheritDoc}
throws
NullPointerException if {@code toElement} is null
throws
IllegalArgumentException {@inheritDoc}

	return headSet(toElement, false);
    
public Ehigher(E e)

throws
ClassCastException {@inheritDoc}
throws
NullPointerException if the specified element is null

        return m.higherKey(e);
    
public booleanisEmpty()
Returns true if this set contains no elements.

return
true if this set contains no elements

	return m.isEmpty();
    
public java.util.Iteratoriterator()
Returns an iterator over the elements in this set in ascending order.

return
an iterator over the elements in this set in ascending order

        return m.navigableKeySet().iterator();
    
public Elast()

throws
NoSuchElementException {@inheritDoc}

        return m.lastKey();
    
public Elower(E e)

throws
ClassCastException {@inheritDoc}
throws
NullPointerException if the specified element is null

        return m.lowerKey(e);
    
public EpollFirst()

        Map.Entry<E,Object> e = m.pollFirstEntry();
        return e == null? null : e.getKey();
    
public EpollLast()

        Map.Entry<E,Object> e = m.pollLastEntry();
        return e == null? null : e.getKey();
    
public booleanremove(java.lang.Object o)
Removes the specified element from this set if it is present. More formally, removes an element e such that o.equals(e), if this set contains such an element. Returns true if this set contained the element (or equivalently, if this set changed as a result of the call). (This set will not contain the element once the call returns.)

param
o object to be removed from this set, if present
return
true if this set contained the specified element
throws
ClassCastException if o cannot be compared with the elements currently in this set
throws
NullPointerException if the specified element is null

	return m.remove(o, Boolean.TRUE);
    
public booleanremoveAll(java.util.Collection c)
Removes from this set all of its elements that are contained in the specified collection. If the specified collection is also a set, this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets.

param
c collection containing elements to be removed from this set
return
true if this set changed as a result of the call
throws
ClassCastException if the types of one or more elements in this set are incompatible with the specified collection
throws
NullPointerException if the specified collection or any of its elements are null

        // Override AbstractSet version to avoid unnecessary call to size()
        boolean modified = false;
        for (Iterator<?> i = c.iterator(); i.hasNext(); )
            if (remove(i.next()))
                modified = true;
        return modified;
    
private voidsetMap(java.util.concurrent.ConcurrentNavigableMap map)

     
        try {
            mapOffset = unsafe.objectFieldOffset
                (ConcurrentSkipListSet.class.getDeclaredField("m"));
        } catch (Exception ex) { throw new Error(ex); }
    
        unsafe.putObjectVolatile(this, mapOffset, map);
    
public intsize()
Returns the number of elements in this set. If this set contains more than Integer.MAX_VALUE elements, it returns Integer.MAX_VALUE.

Beware that, unlike in most collections, this method is NOT a constant-time operation. Because of the asynchronous nature of these sets, determining the current number of elements requires traversing them all to count them. Additionally, it is possible for the size to change during execution of this method, in which case the returned result will be inaccurate. Thus, this method is typically not very useful in concurrent applications.

return
the number of elements in this set

	return m.size();
    
public java.util.NavigableSetsubSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)

throws
ClassCastException {@inheritDoc}
throws
NullPointerException if {@code fromElement} or {@code toElement} is null
throws
IllegalArgumentException {@inheritDoc}

	return new ConcurrentSkipListSet<E>
            (m.subMap(fromElement, fromInclusive,
                      toElement,   toInclusive));
    
public java.util.NavigableSetsubSet(E fromElement, E toElement)

throws
ClassCastException {@inheritDoc}
throws
NullPointerException if {@code fromElement} or {@code toElement} is null
throws
IllegalArgumentException {@inheritDoc}

	return subSet(fromElement, true, toElement, false);
    
public java.util.NavigableSettailSet(E fromElement, boolean inclusive)

throws
ClassCastException {@inheritDoc}
throws
NullPointerException if {@code fromElement} is null
throws
IllegalArgumentException {@inheritDoc}

	return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
    
public java.util.NavigableSettailSet(E fromElement)

throws
ClassCastException {@inheritDoc}
throws
NullPointerException if {@code fromElement} is null
throws
IllegalArgumentException {@inheritDoc}

	return tailSet(fromElement, true);