Module java.base
Package java.util.concurrent

# Class ConcurrentSkipListSet<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractSet<E>
java.util.concurrent.ConcurrentSkipListSet<E>

Type Parameters:
  `E` - the type of elements maintained by this set

All Implemented Interfaces:
  `Serializable`, `Cloneable`, `Iterable<E>`, `Collection<E>`, `NavigableSet<E>`, `Set<E>`, `SortedSet<E>`

public class ConcurrentSkipListSet<E> extends AbstractSet<E> implements NavigableSet<E>, Cloneable, Serializable
A scalable concurrent `NavigableSet` implementation based on a `ConcurrentSkipListMap`. The elements of the set are kept sorted according to their natural ordering, or by a `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 and spliterators are weakly consistent.

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, and so may report inaccurate results if this collection is modified during traversal.

Bulk operations that add, remove, or examine multiple elements, such as `AbstractCollection.addAll(java.util.Collection<? extends E>)`, `Collection.removeIf(java.util.function.Predicate<? super E>)` or `Iterable.forEach(java.util.function.Consumer<? super T>)`, are not guaranteed to be performed atomically. For example, a `forEach` traversal concurrent with an `addAll` operation might observe only some of the added elements.

This class and its iterators implement all of the optional methods of the `Set` and `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.

Since:
  1.6
See Also:
  - Serialized Form

## Constructor Summary

Constructors
Constructor
Description
`ConcurrentSkipListSet()`
Constructs a new, empty set that orders its elements according to their natural ordering.
`ConcurrentSkipListSet(Collection<? extends E> c)`
Constructs a new set containing the elements in the specified collection, that orders its elements according to their natural ordering.
`ConcurrentSkipListSet(Comparator<? super E> comparator)`
Constructs a new, empty set that orders its elements according to the specified comparator.
`ConcurrentSkipListSet(SortedSet<E> s)`
Constructs a new set containing the same elements and using the same ordering as the specified sorted set.

## Method Summary

Modifier and Type
Method
Description
`boolean`
`add(E e)`
Adds the specified element to this set if it is not already present.
`E`
`ceiling(E e)`
Returns the least element in this set greater than or equal to the given element, or `null` if there is no such element.
`void`
`clear()`
Removes all of the elements from this set.
`ConcurrentSkipListSet<E>`
`clone()`
Returns a shallow copy of this `ConcurrentSkipListSet` instance.
`Comparator<? super E>`
`comparator()`
Returns the comparator used to order the elements in this set, or `null` if this set uses the natural ordering of its elements.
`boolean`
`contains(Object o)`
Returns `true` if this set contains the specified element.
`Iterator<E>`
`descendingIterator()`
Returns an iterator over the elements in this set in descending order.
`NavigableSet<E>`
`descendingSet()`
Returns a reverse order view of the elements contained in this set.
`boolean`
`equals(Object o)`
Compares the specified object with this set for equality.
`E`
`first()`
Returns the first (lowest) element currently in this set.
`E`
`floor(E e)`
Returns the greatest element in this set less than or equal to the given element, or `null` if there is no such element.
`NavigableSet<E>`
`headSet(E toElement)`
Returns a view of the portion of this set whose elements are strictly less than `toElement`.
`NavigableSet<E>`
`headSet(E toElement, boolean inclusive)`
Returns a view of the portion of this set whose elements are less than (or equal to, if `inclusive` is true) `toElement`.
`E`
`higher(E e)`
Returns the least element in this set strictly greater than the given element, or `null` if there is no such element.
`boolean`
`isEmpty()`
Returns `true` if this set contains no elements.
`Iterator<E>`
`iterator()`
Returns an iterator over the elements in this set in ascending order.
`E`
`last()`
Returns the last (highest) element currently in this set.
`E`
`lower(E e)`
Returns the greatest element in this set strictly less than the given element, or `null` if there is no such element.
`E`
`pollFirst()`
Retrieves and removes the first (lowest) element, or returns `null` if this set is empty.
`E`
`pollLast()`
Retrieves and removes the last (highest) element, or returns `null` if this set is empty.
`boolean`
`remove(Object o)`
Removes the specified element from this set if it is present.
`boolean`
`removeAll(Collection<?> c)`
Removes from this set all of its elements that are contained in the specified collection.
`int`
`size()`
Returns the number of elements in this set.
`Spliterator<E>`
`spliterator()`
Returns a `Spliterator` over the elements in this set.
`NavigableSet<E>`
`subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)`
Returns a view of the portion of this set whose elements range from `fromElement` to `toElement`.
`NavigableSet<E>`
`subSet(E fromElement, E toElement)`
Returns a view of the portion of this set whose elements range from `fromElement`, inclusive, to `toElement`, exclusive.
`NavigableSet<E>`
`tailSet(E fromElement)`
Returns a view of the portion of this set whose elements are greater than or equal to `fromElement`.
`NavigableSet<E>`
`tailSet(E fromElement, boolean inclusive)`
Returns a view of the portion of this set whose elements are greater than (or equal to, if `inclusive` is true) `fromElement`.

### Methods declared in class java.util.AbstractSet

`hashCode`

### Methods declared in class java.util.AbstractCollection

`addAll, containsAll, retainAll, toArray, toArray, toString`

### Methods declared in class java.lang.Object

`finalize, getClass, notify, notifyAll, wait, wait, wait`

### Methods declared in interface java.util.Collection

`parallelStream, removeIf, stream, toArray`

### Methods declared in interface java.lang.Iterable

`forEach`

### Methods declared in interface java.util.Set

`addAll, containsAll, hashCode, retainAll, toArray, toArray`

## Constructor Details

### ConcurrentSkipListSet

public ConcurrentSkipListSet()
Constructs a new, empty set that orders its elements according to their natural ordering.

### ConcurrentSkipListSet

public ConcurrentSkipListSet(Comparator<? super E> comparator)
Constructs a new, empty set that orders its elements according to the specified comparator.

Parameters:
  `comparator` - the comparator that will be used to order this set. If `null`, the natural ordering of the elements will be used.

### ConcurrentSkipListSet

public ConcurrentSkipListSet(Collection<? extends E> c)
Constructs a new set containing the elements in the specified collection, that orders its elements according to their natural ordering.

Parameters:
  `c` - The elements that will comprise the new set
Throws:
  `ClassCastException` - if the elements in `c` are not `Comparable`, or are not mutually comparable
  `NullPointerException` - if the specified collection or any of its elements are null

### ConcurrentSkipListSet

public ConcurrentSkipListSet(SortedSet<E> s)
Constructs a new set containing the same elements and using the same ordering as the specified sorted set.

Parameters:
  `s` - sorted set whose elements will comprise the new set
Throws:
  `NullPointerException` - if the specified sorted set or any of its elements are null

## Method Details

### clone

public ConcurrentSkipListSet<E> clone()
Returns a shallow copy of this `ConcurrentSkipListSet` instance. (The elements themselves are not cloned.)

Overrides:
  `clone` in class `Object`
Returns:
  a shallow copy of this set
See Also:
  - `Cloneable`

### size

public int size()
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.

Specified by:
  `size` in interface `Collection<E>`
Specified by:
  `size` in interface `Set<E>`
Returns:
  the number of elements in this set

### isEmpty

public boolean isEmpty()
Returns `true` if this set contains no elements.

Specified by:
  `isEmpty` in interface `Collection<E>`
Specified by:
  `isEmpty` in interface `Set<E>`
Overrides:
  `isEmpty` in class `AbstractCollection<E>`
Returns:
  `true` if this set contains no elements

### contains

public boolean contains(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)`.

Specified by:
  `contains` in interface `Collection<E>`
Specified by:
  `contains` in interface `Set<E>`
Overrides:
  `contains` in class `AbstractCollection<E>`
Parameters:
  `o` - object to be checked for containment in this set
Returns:
  `true` if this set contains the specified element
Throws:
  `ClassCastException` - if the specified element cannot be compared with the elements currently in this set
  `NullPointerException` - if the specified element is null

### add

public boolean add(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`.

Specified by:
  `add` in interface `Collection<E>`
Specified by:
  `add` in interface `Set<E>`
Overrides:
  `add` in class `AbstractCollection<E>`
Parameters:
  `e` - element to be added to this set
Returns:
  `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
  `NullPointerException` - if the specified element is null

### remove

public boolean remove(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.)

Specified by:
  `remove` in interface `Collection<E>`
Specified by:
  `remove` in interface `Set<E>`
Overrides:
  `remove` in class `AbstractCollection<E>`
Parameters:
  `o` - object to be removed from this set, if present
Returns:
  `true` if this set contained the specified element
Throws:
  `ClassCastException` - if `o` cannot be compared with the elements currently in this set
  `NullPointerException` - if the specified element is null

### clear

public void clear()
Removes all of the elements from this set.

Specified by:
  `clear` in interface `Collection<E>`
Specified by:
  `clear` in interface `Set<E>`
Overrides:
  `clear` in class `AbstractCollection<E>`

### iterator

public Iterator<E> iterator()
Returns an iterator over the elements in this set in ascending order.

Specified by:
  `iterator` in interface `Collection<E>`
Specified by:
  `iterator` in interface `Iterable<E>`
Specified by:
  `iterator` in interface `NavigableSet<E>`
Specified by:
  `iterator` in interface `Set<E>`
Specified by:
  `iterator` in class `AbstractCollection<E>`
Returns:
  an iterator over the elements in this set in ascending order

### descendingIterator

public Iterator<E> descendingIterator()
Returns an iterator over the elements in this set in descending order.

Specified by:
  `descendingIterator` in interface `NavigableSet<E>`
Returns:
  an iterator over the elements in this set in descending order

### equals

public boolean equals(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.

Specified by:
  `equals` in interface `Collection<E>`
Specified by:
  `equals` in interface `Set<E>`
Overrides:
  `equals` in class `AbstractSet<E>`
Parameters:
  `o` - the object to be compared for equality with this set
Returns:
  `true` if the specified object is equal to this set
See Also:
  - `Object.hashCode()`
  - `HashMap`

### removeAll

public boolean removeAll(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.

Specified by:
  `removeAll` in interface `Collection<E>`
Specified by:
  `removeAll` in interface `Set<E>`
Overrides:
  `removeAll` in class `AbstractSet<E>`
Parameters:
  `c` - collection containing elements to be removed from this set
Returns:
  `true` if this set changed as a result of the call
Throws:
  `ClassCastException` - if the class of an element of this set is incompatible with the specified collection (optional)
  `NullPointerException` - if the specified collection or any of its elements are null
See Also:
  - `AbstractCollection.remove(Object)`
  - `AbstractCollection.contains(Object)`

### lower

public E lower(E e)
Description copied from interface: `NavigableSet`
Returns the greatest element in this set strictly less than the given element, or `null` if there is no such element.

Specified by:
  `lower` in interface `NavigableSet<E>`
Parameters:
  `e` - the value to match
Returns:
  the greatest element less than `e`, or `null` if there is no such element
Throws:
  `ClassCastException` - if the specified element cannot be compared with the elements currently in the set
  `NullPointerException` - if the specified element is null

### floor

public E floor(E e)
Description copied from interface: `NavigableSet`
Returns the greatest element in this set less than or equal to the given element, or `null` if there is no such element.

Specified by:
  `floor` in interface `NavigableSet<E>`
Parameters:
  `e` - the value to match
Returns:
  the greatest element less than or equal to `e`, or `null` if there is no such element
Throws:
  `ClassCastException` - if the specified element cannot be compared with the elements currently in the set
  `NullPointerException` - if the specified element is null

### ceiling

public E ceiling(E e)
Description copied from interface: `NavigableSet`
Returns the least element in this set greater than or equal to the given element, or `null` if there is no such element.

Specified by:
  `ceiling` in interface `NavigableSet<E>`
Parameters:
  `e` - the value to match
Returns:
  the least element greater than or equal to `e`, or `null` if there is no such element
Throws:
  `ClassCastException` - if the specified element cannot be compared with the elements currently in the set
  `NullPointerException` - if the specified element is null

### higher

public E higher(E e)
Description copied from interface: `NavigableSet`
Returns the least element in this set strictly greater than the given element, or `null` if there is no such element.

Specified by:
  `higher` in interface `NavigableSet<E>`
Parameters:
  `e` - the value to match
Returns:
  the least element greater than `e`, or `null` if there is no such element
Throws:
  `ClassCastException` - if the specified element cannot be compared with the elements currently in the set
  `NullPointerException` - if the specified element is null

### pollFirst

public E pollFirst()
Description copied from interface: `NavigableSet`
Retrieves and removes the first (lowest) element, or returns `null` if this set is empty.

Specified by:
  `pollFirst` in interface `NavigableSet<E>`
Returns:
  the first element, or `null` if this set is empty

### pollLast

public E pollLast()
Description copied from interface: `NavigableSet`
Retrieves and removes the last (highest) element, or returns `null` if this set is empty.

Specified by:
  `pollLast` in interface `NavigableSet<E>`
Returns:
  the last element, or `null` if this set is empty

### comparator

public Comparator<? super E> comparator()
Description copied from interface: `SortedSet`
Returns the comparator used to order the elements in this set, or `null` if this set uses the natural ordering of its elements.

Specified by:
  `comparator` in interface `SortedSet<E>`
Returns:
  the comparator used to order the elements in this set, or `null` if this set uses the natural ordering of its elements

### first

public E first()
Description copied from interface: `SortedSet`
Returns the first (lowest) element currently in this set.

Specified by:
  `first` in interface `SortedSet<E>`
Returns:
  the first (lowest) element currently in this set
Throws:
  `NoSuchElementException` - if this set is empty

### last

public E last()
Description copied from interface: `SortedSet`
Returns the last (highest) element currently in this set.

Specified by:
  `last` in interface `SortedSet<E>`
Returns:
  the last (highest) element currently in this set
Throws:
  `NoSuchElementException` - if this set is empty

### subSet

public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)
Description copied from interface: `NavigableSet`
Returns a view of the portion of this set whose elements range from `fromElement` to `toElement`. If `fromElement` and `toElement` are equal, the returned set is empty unless `fromInclusive` and `toInclusive` are both true. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. The returned set supports all optional set operations that this set supports.

The returned set will throw an `IllegalArgumentException` on an attempt to insert an element outside its range.

Specified by:
  `subSet` in interface `NavigableSet<E>`
Parameters:
  `fromElement` - low endpoint of the returned set
  `fromInclusive` - `true` if the low endpoint is to be included in the returned view
  `toElement` - high endpoint of the returned set
  `toInclusive` - `true` if the high endpoint is to be included in the returned view
Returns:
  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). Implementations may, but are not required to, throw this exception if `fromElement` or `toElement` cannot be compared to elements currently in the set.
  `NullPointerException` - if `fromElement` or `toElement` is null
  `IllegalArgumentException` - if `fromElement` is greater than `toElement`; or if this set itself has a restricted range, and `fromElement` or `toElement` lies outside the bounds of the range.

### headSet

public NavigableSet<E> headSet(E toElement, boolean inclusive)
Description copied from interface: `NavigableSet`
Returns a view of the portion of this set whose elements are less than (or equal to, if `inclusive` is true) `toElement`. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. The returned set supports all optional set operations that this set supports.

The returned set will throw an `IllegalArgumentException` on an attempt to insert an element outside its range.

Specified by:
  `headSet` in interface `NavigableSet<E>`
Parameters:
  `toElement` - high endpoint of the returned set
  `inclusive` - `true` if the high endpoint is to be included in the returned view
Returns:
  a view of the portion of this set whose elements are less than (or equal to, if `inclusive` is true) `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`). Implementations may, but are not required to, throw this exception if `toElement` cannot be compared to elements currently in the set.
  `NullPointerException` - if `toElement` is null
  `IllegalArgumentException` - if this set itself has a restricted range, and `toElement` lies outside the bounds of the range

### tailSet

public NavigableSet<E> tailSet(E fromElement, boolean inclusive)
Description copied from interface: `NavigableSet`
Returns a view of the portion of this set whose elements are greater than (or equal to, if `inclusive` is true) `fromElement`. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. The returned set supports all optional set operations that this set supports.

The returned set will throw an `IllegalArgumentException` on an attempt to insert an element outside its range.

Specified by:
  `tailSet` in interface `NavigableSet<E>`
Parameters:
  `fromElement` - low endpoint of the returned set
  `inclusive` - `true` if the low endpoint is to be included in the returned view
Returns:
  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`). Implementations may, but are not required to, throw this exception if `fromElement` cannot be compared to elements currently in the set.
  `NullPointerException` - if `fromElement` is null
  `IllegalArgumentException` - if this set itself has a restricted range, and `fromElement` lies outside the bounds of the range

### subSet

public NavigableSet<E> subSet(E fromElement, E toElement)
Description copied from interface: `NavigableSet`
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 set is empty.) The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. The returned set supports all optional set operations that this set supports.

The returned set will throw an `IllegalArgumentException` on an attempt to insert an element outside its range.

Equivalent to `subSet(fromElement, true, toElement, false)`.

Specified by:
  `subSet` in interface `NavigableSet<E>`
Specified by:
  `subSet` in interface `SortedSet<E>`
Parameters:
  `fromElement` - low endpoint (inclusive) of the returned set
  `toElement` - high endpoint (exclusive) of the returned set
Returns:
  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). Implementations may, but are not required to, throw this exception if `fromElement` or `toElement` cannot be compared to elements currently in the set.
  `NullPointerException` - if `fromElement` or `toElement` is null
  `IllegalArgumentException` - if `fromElement` is greater than `toElement`; or if this set itself has a restricted range, and `fromElement` or `toElement` lies outside the bounds of the range

### headSet

public NavigableSet<E> headSet(E toElement)
Description copied from interface: `NavigableSet`
Returns a view of the portion of this set whose elements are strictly less than `toElement`. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. The returned set supports all optional set operations that this set supports.

The returned set will throw an `IllegalArgumentException` on an attempt to insert an element outside its range.

Equivalent to `headSet(toElement, false)`.

Specified by:
  `headSet` in interface `NavigableSet<E>`
Specified by:
  `headSet` in interface `SortedSet<E>`
Parameters:
  `toElement` - high endpoint (exclusive) of the returned set
Returns:
  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`). Implementations may, but are not required to, throw this exception if `toElement` cannot be compared to elements currently in the set.
  `NullPointerException` - if `toElement` is null
  `IllegalArgumentException` - if this set itself has a restricted range, and `toElement` lies outside the bounds of the range

### tailSet

public NavigableSet<E> tailSet(E fromElement)
Description copied from interface: `NavigableSet`
Returns a view of the portion of this set whose elements are greater than or equal to `fromElement`. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. The returned set supports all optional set operations that this set supports.

The returned set will throw an `IllegalArgumentException` on an attempt to insert an element outside its range.

Equivalent to `tailSet(fromElement, true)`.

Specified by:
  `tailSet` in interface `NavigableSet<E>`
Specified by:
  `tailSet` in interface `SortedSet<E>`
Parameters:
  `fromElement` - low endpoint (inclusive) of the returned set
Returns:
  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`). Implementations may, but are not required to, throw this exception if `fromElement` cannot be compared to elements currently in the set.
  `NullPointerException` - if `fromElement` is null
  `IllegalArgumentException` - if this set itself has a restricted range, and `fromElement` lies outside the bounds of the range

### descendingSet

public NavigableSet<E> descendingSet()
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 `Collections.reverseOrder``(comparator())`. The expression `s.descendingSet().descendingSet()` returns a view of `s` essentially equivalent to `s`.

Specified by:
  `descendingSet` in interface `NavigableSet<E>`
Returns:
  a reverse order view of this set

### spliterator

public Spliterator<E> spliterator()
Returns a `Spliterator` over the elements in this set.

The `Spliterator` reports `Spliterator.CONCURRENT`, `Spliterator.NONNULL`, `Spliterator.DISTINCT`, `Spliterator.SORTED` and `Spliterator.ORDERED`, with an encounter order that is ascending order. Overriding implementations should document the reporting of additional characteristic values.

The spliterator's comparator is `null` if the set's comparator is `null`. Otherwise, the spliterator's comparator is the same as or imposes the same total ordering as the set's comparator.

Specified by:
  `spliterator` in interface `Collection<E>`
Specified by:
  `spliterator` in interface `Iterable<E>`
Specified by:
  `spliterator` in interface `Set<E>`
Specified by:
  `spliterator` in interface `SortedSet<E>`
Returns:
  a `Spliterator` over the elements in this set
Since:
  1.8