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# ConcurrentSkipListSet

`public class ConcurrentSkipListSet`
`` `` `extends AbstractSet<E>` `implements NavigableSet<E>, Cloneable, Serializable`

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

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. Additionally, the bulk operations `addAll`, `removeAll`, `retainAll`, `containsAll`, `equals`, and `toArray` 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 `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.

## Summary

### Public constructors
`ConcurrentSkipListSet()` Constructs a new, empty set 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(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(SortedSet<E> s)` Constructs a new set containing the same elements and using the same ordering as the specified sorted set.

### Public methods
`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, boolean inclusive)` Returns a view of the portion of this set whose elements are less than (or equal to, if `inclusive` is true) `toElement`.
`NavigableSet<E>` | `headSet(E toElement)` Returns a view of the portion of this set whose elements are strictly less than `toElement`. Equivalent to `headSet(toElement, false)`.
`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. Equivalent to `subSet(fromElement, true, toElement, false)`.
`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`.
`NavigableSet<E>` | `tailSet(E fromElement)` Returns a view of the portion of this set whose elements are greater than or equal to `fromElement`. Equivalent to `tailSet(fromElement, true)`.

### Inherited methods
From class `java.util.AbstractSet`
`boolean` | `equals(Object o)` Compares the specified object with this set for equality.
`int` | `hashCode()` Returns the hash code value for this set.
`boolean` | `removeAll(Collection<?> c)` Removes from this set all of its elements that are contained in the specified collection (optional operation).
From class `java.util.AbstractCollection`
`boolean` | `add(E e)` Ensures that this collection contains the specified element (optional operation). This implementation always throws an `UnsupportedOperationException`.
`boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation). This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
`void` | `clear()` Removes all of the elements from this collection (optional operation). This implementation iterates over this collection, removing each element using the `Iterator.remove` operation.
`boolean` | `contains(Object o)` Returns `true` if this collection contains the specified element. This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
`boolean` | `containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection. This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection.
`boolean` | `isEmpty()` Returns `true` if this collection contains no elements. This implementation returns `size() == 0`.
`abstract Iterator<E>` | `iterator()` Returns an iterator over the elements contained in this collection.
`boolean` | `remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation). This implementation iterates over the collection looking for the specified element.
`boolean` | `removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
`boolean` | `retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
`abstract int` | `size()` Returns the number of elements in this collection.
`<T> T[]` | `toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index `0`.
`Object[]` | `toArray()` Returns an array containing all of the elements in this collection. This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index `0`.
`String` | `toString()` Returns a string representation of this collection.
From class `java.lang.Object`
`Object` | `clone()` Creates and returns a copy of this object.
`boolean` | `equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void` | `finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>` | `getClass()` Returns the runtime class of this `Object`.
`int` | `hashCode()` Returns a hash code value for the object.
`final void` | `notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void` | `notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String` | `toString()` Returns a string representation of the object.
`final void` | `wait(long timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.
From interface `java.util.Set`
`abstract boolean` | `add(E e)` Adds the specified element to this set if it is not already present (optional operation).
`abstract boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this set if they're not already present (optional operation).
`abstract void` | `clear()` Removes all of the elements from this set (optional operation).
`abstract boolean` | `contains(Object o)` Returns `true` if this set contains the specified element.
`abstract boolean` | `containsAll(Collection<?> c)` Returns `true` if this set contains all of the elements of the specified collection.
`static <E> Set<E>` | `copyOf(Collection<? extends E> coll)` Returns an unmodifiable Set containing the elements of the given Collection.
`abstract boolean` | `equals(Object o)` Compares the specified object with this set for equality.
`abstract int` | `hashCode()` Returns the hash code value for this set.
`abstract boolean` | `isEmpty()` Returns `true` if this set contains no elements.
`abstract Iterator<E>` | `iterator()` Returns an iterator over the elements in this set.
`static <E> Set<E>` | `of(E e1, E e2, E e3)` Returns an immutable set containing three elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9)` Returns an immutable set containing nine elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5)` Returns an immutable set containing five elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6)` Returns an immutable set containing six elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8)` Returns an immutable set containing eight elements.
`static <E> Set<E>` | `of()` Returns an immutable set containing zero elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10)` Returns an immutable set containing ten elements.
`static <E> Set<E>` | `of(E e1, E e2)` Returns an immutable set containing two elements.
`static <E> Set<E>` | `of(E... elements)` Returns an immutable set containing an arbitrary number of elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7)` Returns an immutable set containing seven elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4)` Returns an immutable set containing four elements.
`static <E> Set<E>` | `of(E e1)` Returns an immutable set containing one element.
`abstract boolean` | `remove(Object o)` Removes the specified element from this set if it is present (optional operation).
`abstract boolean` | `removeAll(Collection<?> c)` Removes from this set all of its elements that are contained in the specified collection (optional operation).
`abstract boolean` | `retainAll(Collection<?> c)` Retains only the elements in this set that are contained in the specified collection (optional operation).
`abstract int` | `size()` Returns the number of elements in this set (its cardinality).
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements in this set.
`abstract Object[]` | `toArray()` Returns an array containing all of the elements in this set.
`abstract <T> T[]` | `toArray(T[] a)` Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.
From interface `java.util.Collection`
`abstract boolean` | `add(E e)` Ensures that this collection contains the specified element (optional operation).
`abstract boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`abstract void` | `clear()` Removes all of the elements from this collection (optional operation).
`abstract boolean` | `contains(Object o)` Returns `true` if this collection contains the specified element.
`abstract boolean` | `containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`abstract boolean` | `equals(Object o)` Compares the specified object with this collection for equality.
`abstract int` | `hashCode()` Returns the hash code value for this collection.
`abstract boolean` | `isEmpty()` Returns `true` if this collection contains no elements.
`abstract Iterator<E>` | `iterator()` Returns an iterator over the elements in this collection.
`default Stream<E>` | `parallelStream()` Returns a possibly parallel `Stream` with this collection as its source.
`abstract boolean` | `remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation).
`abstract boolean` | `removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`default boolean` | `removeIf(Predicate<? super E> filter)` Removes all of the elements of this collection that satisfy the given predicate.
`abstract boolean` | `retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`abstract int` | `size()` Returns the number of elements in this collection.
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements in this collection.
`default Stream<E>` | `stream()` Returns a sequential `Stream` with this collection as its source.
`abstract <T> T[]` | `toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
`abstract Object[]` | `toArray()` Returns an array containing all of the elements in this collection.
From interface `java.util.NavigableSet`
`abstract 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.
`abstract Iterator<E>` | `descendingIterator()` Returns an iterator over the elements in this set, in descending order.
`abstract NavigableSet<E>` | `descendingSet()` Returns a reverse order view of the elements contained in this set.
`abstract 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.
`abstract 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`.
`abstract SortedSet<E>` | `headSet(E toElement)` Returns a view of the portion of this set whose elements are strictly less than `toElement`. Equivalent to `headSet(toElement, false)`.
`abstract 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.
`abstract Iterator<E>` | `iterator()` Returns an iterator over the elements in this set, in ascending order.
`abstract 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.
`abstract E` | `pollFirst()` Retrieves and removes the first (lowest) element, or returns `null` if this set is empty.
`abstract E` | `pollLast()` Retrieves and removes the last (highest) element, or returns `null` if this set is empty.
`abstract 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`.
`abstract SortedSet<E>` | `subSet(E fromElement, E toElement)` Returns a view of the portion of this set whose elements range from `fromElement`, inclusive, to `toElement`, exclusive. Equivalent to `subSet(fromElement, true, toElement, false)`.
`abstract SortedSet<E>` | `tailSet(E fromElement)` Returns a view of the portion of this set whose elements are greater than or equal to `fromElement`. Equivalent to `tailSet(fromElement, true)`.
`abstract 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`.
From interface `java.lang.Iterable`
`default void` | `forEach(Consumer<? super T> action)` Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
`abstract Iterator<E>` | `iterator()` Returns an iterator over elements of type `T`.
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements described by this `Iterable`.
From interface `java.util.SortedSet`
`abstract 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.
`abstract E` | `first()` Returns the first (lowest) element currently in this set.
`abstract SortedSet<E>` | `headSet(E toElement)` Returns a view of the portion of this set whose elements are strictly less than `toElement`.
`abstract E` | `last()` Returns the last (highest) element currently in this set.
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements in this sorted set.
`abstract SortedSet<E>` | `subSet(E fromElement, E toElement)` Returns a view of the portion of this set whose elements range from `fromElement`, inclusive, to `toElement`, exclusive.
`abstract SortedSet<E>` | `tailSet(E fromElement)` Returns a view of the portion of this set whose elements are greater than or equal to `fromElement`.

## Public constructors

### 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` | `Comparator`: the comparator that will be used to order this set. If `null`, the 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` | `Collection`: 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` | `SortedSet`: sorted set whose elements will comprise the new set

Throws
`NullPointerException` | if the specified sorted set or any of its elements are null

## Public methods

### 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`.

Parameters
`e` | `E`: element to be added to this set

Returns
`boolean` | `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

### ceiling

    public 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.

Parameters
`e` | `E`: the value to match

Returns
`E` | the least element greater than or equal to `e`, or `null` if there is no such element

Throws
`ClassCastException` |
`NullPointerException` | if the specified element is null

### clear

    public void clear ()

Removes all of the elements from this set.

### clone

    public ConcurrentSkipListSet<E> clone ()

Returns a shallow copy of this `ConcurrentSkipListSet` instance. (The elements themselves are not cloned.)

Returns
`ConcurrentSkipListSet<E>` | a shallow copy of this set

### comparator

    public 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.

Returns
`Comparator<? super E>` | the comparator used to order the elements in this set, or `null` if this set uses the natural ordering of its 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)`.

Parameters
`o` | `Object`: object to be checked for containment in this set

Returns
`boolean` | `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

### descendingIterator

    public Iterator<E> descendingIterator ()

Returns an iterator over the elements in this set in descending order.

Returns
`Iterator<E>` | an iterator over the elements in this set in descending order

### 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`.

Returns
`NavigableSet<E>` | a reverse order view of this set

### 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.

Parameters
`o` | `Object`: the object to be compared for equality with this set

Returns
`boolean` | `true` if the specified object is equal to this set

### first

    public E first ()

Returns the first (lowest) element currently in this set.

Returns
`E` | the first (lowest) element currently in this set

Throws
`NoSuchElementException` |

### floor

    public 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.

Parameters
`e` | `E`: the value to match

Returns
`E` | the greatest element less than or equal to `e`, or `null` if there is no such element

Throws
`ClassCastException` |
`NullPointerException` | if the specified element is null

### headSet

    public 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`. 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.

Parameters
`toElement` | `E`: high endpoint of the returned set
`inclusive` | `boolean`: `true` if the high endpoint is to be included in the returned view

Returns
`NavigableSet<E>` | a view of the portion of this set whose elements are less than (or equal to, if `inclusive` is true) `toElement`

Throws
`ClassCastException` |
`NullPointerException` | if `toElement` is null
`IllegalArgumentException` |

### headSet

    public NavigableSet<E> headSet (E toElement)

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)`.

Parameters
`toElement` | `E`: high endpoint (exclusive) of the returned set

Returns
`NavigableSet<E>` | a view of the portion of this set whose elements are strictly less than `toElement`

Throws
`ClassCastException` |
`NullPointerException` | if `toElement` is null
`IllegalArgumentException` |

### higher

    public 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.

Parameters
`e` | `E`: the value to match

Returns
`E` | the least element greater than `e`, or `null` if there is no such element

Throws
`ClassCastException` |
`NullPointerException` | if the specified element is null

### isEmpty

    public boolean isEmpty ()

Returns `true` if this set contains no elements.

Returns
`boolean` | `true` if this set contains no elements

### iterator

    public Iterator<E> iterator ()

Returns an iterator over the elements in this set in ascending order.

Returns
`Iterator<E>` | an iterator over the elements in this set in ascending order

### last

    public E last ()

Returns the last (highest) element currently in this set.

Returns
`E` | the last (highest) element currently in this set

Throws
`NoSuchElementException` |

### lower

    public 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.

Parameters
`e` | `E`: the value to match

Returns
`E` | the greatest element less than `e`, or `null` if there is no such element

Throws
`ClassCastException` |
`NullPointerException` | if the specified element is null

### pollFirst

    public E pollFirst ()

Retrieves and removes the first (lowest) element, or returns `null` if this set is empty.

Returns
`E` | the first element, or `null` if this set is empty

### pollLast

    public E pollLast ()

Retrieves and removes the last (highest) element, or returns `null` if this set is empty.

Returns
`E` | the last element, or `null` if this set is empty

### 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.)

Parameters
`o` | `Object`: object to be removed from this set, if present

Returns
`boolean` | `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

### 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.

Parameters
`c` | `Collection`: collection containing elements to be removed from this set

Returns
`boolean` | `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

### 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.

Returns
`int` | the number of elements in 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 (see `Spliterator.getComparator()`) is `null` if the set's comparator (see `comparator()`) is `null`. Otherwise, the spliterator's comparator is the same as or imposes the same total ordering as the set's comparator.

Returns
`Spliterator<E>` | a `Spliterator` over the elements in this set

### subSet

    public 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`. 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.

Parameters
`fromElement` | `E`: low endpoint of the returned set
`fromInclusive` | `boolean`: `true` if the low endpoint is to be included in the returned view
`toElement` | `E`: high endpoint of the returned set
`toInclusive` | `boolean`: `true` if the high endpoint is to be included in the returned view

Returns
`NavigableSet<E>` | a view of the portion of this set whose elements range from `fromElement`, inclusive, to `toElement`, exclusive

Throws
`ClassCastException` |
`NullPointerException` | if `fromElement` or `toElement` is null
`IllegalArgumentException` |

### subSet

    public 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. (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)`.

Parameters
`fromElement` | `E`: low endpoint (inclusive) of the returned set
`toElement` | `E`: high endpoint (exclusive) of the returned set

Returns
`NavigableSet<E>` | a view of the portion of this set whose elements range from `fromElement`, inclusive, to `toElement`, exclusive

Throws
`ClassCastException` |
`NullPointerException` | if `fromElement` or `toElement` is null
`IllegalArgumentException` |

### tailSet

    public 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`. 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.

Parameters
`fromElement` | `E`: low endpoint of the returned set
`inclusive` | `boolean`: `true` if the low endpoint is to be included in the returned view

Returns
`NavigableSet<E>` | a view of the portion of this set whose elements are greater than or equal to `fromElement`

Throws
`ClassCastException` |
`NullPointerException` | if `fromElement` is null
`IllegalArgumentException` |

### tailSet

    public NavigableSet<E> tailSet (E fromElement)

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)`.

Parameters
`fromElement` | `E`: low endpoint (inclusive) of the returned set

Returns
`NavigableSet<E>` | a view of the portion of this set whose elements are greater than or equal to `fromElement`

Throws
`ClassCastException` |
`NullPointerException` | if `fromElement` is null
`IllegalArgumentException` |

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