/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
package jsr166x;
import java.util.*;
import java.util.concurrent.*;
/**
* A scalable concurrent {@link SortedSet} and (priority) {@link
* Queue} implementation based on a {@link ConcurrentSkipListMap}.
* This class maintains a set in ascending order, sorted according to
* the natural order for the element's class (see {@link
* Comparable}), or by the comparator provided at 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 procede concurrently with
* other operations.
*
*
This class provides extended SortedSet methods
* searching for closest matches. Methods lower,
* floor, ceiling, and higher return
* elements respectively less, less than or equal, greater than or
* equal, and greater than a given value, returning null if there is
* no such element. These methods are designed for locating, not
* traversing entries. To traverse, use iterators and/or
* subset.
*
*
The class additionally supports {@link Queue} methods such as
* poll that atomically returns and removes the first
* element, if one exists. Thus, this class can serve as a priority
* queue in which there are no duplicate elements.
*
*
The {@link ConcurrentSkipListSubSet} objects returned by methods
* subset, headSet, and tailSet support the
* same extended set of operations as this class, but operate on their
* designated subrange of elements.
*
*
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.
*
*
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.
*
* @author Doug Lea
* @param the type of elements maintained by this set
*/
public class ConcurrentSkipListSet
extends AbstractSet
implements SortedSet, Queue, Cloneable, java.io.Serializable {
private static final long serialVersionUID = -2479143111061671589L;
/**
* The underlying map. Uses Boolean.TRUE as value for each
* element. Note that this class relies on default serialization,
* which is a little wasteful in saving all of the useless value
* fields of underlying map, but enables this field to be declared
* final, which is necessary for thread safety.
*/
private final ConcurrentSkipListMap m;
/**
* Constructs a new, empty set, sorted according to the elements' natural
* order.
*/
public ConcurrentSkipListSet() {
m = new ConcurrentSkipListMap();
}
/**
* Constructs a new, empty set, sorted according to the specified
* comparator.
*
* @param c the comparator that will be used to sort this set. A
* null value indicates that the elements' natural
* ordering should be used.
*/
public ConcurrentSkipListSet(Comparator c) {
m = new ConcurrentSkipListMap(c);
}
/**
* Constructs a new set containing the elements in the specified
* collection, sorted according to the elements' natural order.
*
* @param c The elements that will comprise the new set.
*
* @throws ClassCastException if the elements in the specified
* collection are not comparable, or are not mutually comparable.
* @throws NullPointerException if the specified collection is null.
*/
public ConcurrentSkipListSet(Collection c) {
m = new ConcurrentSkipListMap();
addAll(c);
}
/**
* Constructs a new set containing the same elements as the specified
* sorted set, sorted according to the same ordering.
*
* @param s sorted set whose elements will comprise the new set.
* @throws NullPointerException if the specified sorted set is null.
*/
public ConcurrentSkipListSet(SortedSet s) {
m = new ConcurrentSkipListMap(s.comparator());
addAll(s);
}
/**
* Returns a shallow copy of this set. (The elements themselves
* are not cloned.)
*
* @return a shallow copy of this set.
*/
public Object clone() {
ConcurrentSkipListSet clone = null;
try {
clone = (ConcurrentSkipListSet) super.clone();
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
clone.m.initialize();
clone.addAll(this);
return clone;
}
/* ---------------- Set operations -------------- */
/**
* 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.
*/
public int size() {
return m.size();
}
/**
* Returns true if this set contains no elements.
* @return true if this set contains no elements.
*/
public boolean isEmpty() {
return m.isEmpty();
}
/**
* Returns true if this set contains the specified element.
*
* @param o the object to be checked for containment in this set.
* @return true if this set contains the specified element.
*
* @throws ClassCastException if the specified object cannot be compared
* with the elements currently in the set.
* @throws NullPointerException if o is null.
*/
public boolean contains(Object o) {
return m.containsKey(o);
}
/**
* Adds the specified element to this set if it is not already present.
*
* @param o element to be added to this set.
* @return true if the set did not already contain the specified
* element.
*
* @throws ClassCastException if the specified object cannot be compared
* with the elements currently in the set.
* @throws NullPointerException if o is null.
*/
public boolean add(E o) {
return m.putIfAbsent(o, Boolean.TRUE) == null;
}
/**
* Removes the specified element from this set if it is present.
*
* @param o object to be removed from this set, if present.
* @return true if the set contained the specified element.
*
* @throws ClassCastException if the specified object cannot be compared
* with the elements currently in the set.
* @throws NullPointerException if o is null.
*/
public boolean remove(Object o) {
return m.removep(o);
}
/**
* Removes all of the elements from this set.
*/
public void clear() {
m.clear();
}
/**
* Returns an iterator over the elements in this set. The elements
* are returned in ascending order.
*
* @return an iterator over the elements in this set.
*/
public Iterator iterator() {
return m.keyIterator();
}
/* ---------------- Queue operations -------------- */
/**
* Adds the specified element, or fails if this element is already
* present.
* @param o the element to insert.
* @return true if it was possible to add the element,
* else false
*/
public boolean offer(E o) {
return add(o);
}
/**
* Retrieves and removes the first (lowest) element.
*
* @return the least element, or null if empty.
*/
public E poll() {
return m.removeFirstKey();
}
/**
* Retrieves and removes the first (lowest) element. This method
* differs from the poll method in that it throws an
* exception if empty.
*
* @return the first (lowest) element.
* @throws NoSuchElementException if empty.
*/
public E remove() {
E x = m.removeFirstKey();
if (x == null)
throw new NoSuchElementException();
return x;
}
/**
* Retrieves, but does not remove, the first (lowest) element,
* returning null if empty.
*
* @return the first (lowest) element, or null if empty.
*/
public E peek() {
return m.lowestKey();
}
/**
* Retrieves, but does not remove, the first (lowest) element.
* This method differs from the peek method only in that
* it throws an exception if empty.
*
* @return the first (lowest) element.
* @throws NoSuchElementException if empty.
*/
public E element() {
return m.firstKey();
}
/* ---------------- Relational operations -------------- */
/**
* Returns an element greater than or equal to the given element, or
* null if there is no such element.
*
* @param o the value to match
* @return an element greater than or equal to given element, or null
* if there is no such element.
* @throws ClassCastException if o cannot be compared with the elements
* currently in the set.
* @throws NullPointerException if o is null
*/
public E ceiling(E o) {
return m.ceilingKey(o);
}
/**
* Returns an element strictly less than the given element, or null if
* there is no such element.
*
* @param o the value to match
* @return the greatest element less than the given element, or
* null if there is no such element.
* @throws ClassCastException if o cannot be compared with the elements
* currently in the set.
* @throws NullPointerException if o is null.
*/
public E lower(E o) {
return m.lowerKey(o);
}
/**
* Returns an element less than or equal to the given element, or null
* if there is no such element.
*
* @param o the value to match
* @return the greatest element less than or equal to given
* element, or null if there is no such element.
* @throws ClassCastException if o cannot be compared with the elements
* currently in the set.
* @throws NullPointerException if o is null.
*/
public E floor(E o) {
return m.floorKey(o);
}
/**
* Returns an element strictly greater than the given element, or null
* if there is no such element.
*
* @param o the value to match
* @return the least element greater than the given element, or
* null if there is no such element.
* @throws ClassCastException if o cannot be compared with the elements
* currently in the set.
* @throws NullPointerException if o is null.
*/
public E higher(E o) {
return m.higherKey(o);
}
/* ---------------- SortedSet operations -------------- */
/**
* Returns the comparator used to order this set, or null
* if this set uses its elements natural ordering.
*
* @return the comparator used to order this set, or null
* if this set uses its elements natural ordering.
*/
public Comparator comparator() {
return m.comparator();
}
/**
* Returns the first (lowest) element currently in this set.
*
* @return the first (lowest) element currently in this set.
* @throws NoSuchElementException sorted set is empty.
*/
public E first() {
return m.firstKey();
}
/**
* Returns the last (highest) element currently in this set.
*
* @return the last (highest) element currently in this set.
* @throws NoSuchElementException sorted set is empty.
*/
public E last() {
return m.lastKey();
}
/**
* Returns a view of the portion of this set whose elements range from
* fromElement, inclusive, to toElement, exclusive. (If
* fromElement and toElement are equal, the returned
* sorted set is empty.) The returned sorted set is backed by this set,
* so changes in the returned sorted set are reflected in this set, and
* vice-versa.
* @param fromElement low endpoint (inclusive) of the subSet.
* @param toElement high endpoint (exclusive) of the subSet.
* @return a view of the portion of this set whose elements range from
* fromElement, inclusive, to toElement,
* exclusive.
* @throws ClassCastException if fromElement and
* toElement cannot be compared to one another using
* this set's comparator (or, if the set has no comparator,
* using natural ordering).
* @throws IllegalArgumentException if fromElement is
* greater than toElement.
* @throws NullPointerException if fromElement or
* toElement is null.
*/
public ConcurrentSkipListSubSet subSet(E fromElement, E toElement) {
return new ConcurrentSkipListSubSet(m, fromElement, toElement);
}
/**
* Returns a view of the portion of this set whose elements are strictly
* less than toElement. The returned sorted set is backed by
* this set, so changes in the returned sorted set are reflected in this
* set, and vice-versa.
* @param toElement high endpoint (exclusive) of the headSet.
* @return a view of the portion of this set whose elements are strictly
* less than toElement.
* @throws ClassCastException if toElement is not compatible
* with this set's comparator (or, if the set has no comparator,
* if toElement does not implement Comparable).
* @throws NullPointerException if toElement is null.
*/
public ConcurrentSkipListSubSet headSet(E toElement) {
return new ConcurrentSkipListSubSet(m, null, toElement);
}
/**
* Returns a view of the portion of this set whose elements are
* greater than or equal to fromElement. The returned
* sorted set is backed by this set, so changes in the returned
* sorted set are reflected in this set, and vice-versa.
* @param fromElement low endpoint (inclusive) of the tailSet.
* @return a view of the portion of this set whose elements are
* greater than or equal to fromElement.
* @throws ClassCastException if fromElement is not
* compatible with this set's comparator (or, if the set has no
* comparator, if fromElement does not implement
* Comparable).
* @throws NullPointerException if fromElement is null.
*/
public ConcurrentSkipListSubSet tailSet(E fromElement) {
return new ConcurrentSkipListSubSet(m, fromElement, null);
}
}