Package it.unimi.dsi.fastutil.chars

Class CharLinkedOpenHashSet

java.lang.Object
java.util.AbstractCollection<java.lang.Character>
it.unimi.dsi.fastutil.chars.AbstractCharCollection
it.unimi.dsi.fastutil.chars.AbstractCharSet
it.unimi.dsi.fastutil.chars.AbstractCharSortedSet
it.unimi.dsi.fastutil.chars.CharLinkedOpenHashSet
All Implemented Interfaces:
CharBidirectionalIterable, CharCollection, it.unimi.dsi.fastutil.chars.CharIterable, CharSet, CharSortedSet, it.unimi.dsi.fastutil.Hash, java.io.Serializable, java.lang.Cloneable, java.lang.Iterable<java.lang.Character>, java.util.Collection<java.lang.Character>, java.util.Set<java.lang.Character>, java.util.SortedSet<java.lang.Character>
public class CharLinkedOpenHashSet
extends AbstractCharSortedSet
implements java.io.Serializable, java.lang.Cloneable, it.unimi.dsi.fastutil.Hash
A type-specific linked hash set with with a fast, small-footprint implementation.

Instances of this class use a hash table to represent a set. The table is filled up to a specified load factor, and then doubled in size to accommodate new entries. If the table is emptied below one fourth of the load factor, it is halved in size; however, the table is never reduced to a size smaller than that at creation time: this approach makes it possible to create sets with a large capacity in which insertions and deletions do not cause immediately rehashing. Moreover, halving is not performed when deleting entries from an iterator, as it would interfere with the iteration process.

Note that clear() does not modify the hash table size. Rather, a family of trimming methods lets you control the size of the table; this is particularly useful if you reuse instances of this class.

Iterators generated by this set will enumerate elements in the same order in which they have been added to the set (addition of elements already present in the set does not change the iteration order). Note that this order has nothing in common with the natural order of the keys. The order is kept by means of a doubly linked list, represented via an array of longs parallel to the table.

This class implements the interface of a sorted set, so to allow easy access of the iteration order: for instance, you can get the first element in iteration order with first() without having to create an iterator; however, this class partially violates the SortedSet contract because all subset methods throw an exception and comparator() returns always null.

Additional methods, such as addAndMoveToFirst(), make it easy to use instances of this class as a cache (e.g., with LRU policy).

The iterators provided by this class are type-specific list iterators, and can be started at any element which is in the set (if the provided element is not in the set, a NoSuchElementException exception will be thrown). If, however, the provided element is not the first or last element in the set, the first access to the list index will require linear time, as in the worst case the entire set must be scanned in iteration order to retrieve the positional index of the starting element. If you use just the methods of a type-specific BidirectionalIterator, however, all operations will be performed in constant time.

See Also:
Hash, HashCommon, Serialized Form

  • Nested Class Summary

    Nested classes/interfaces inherited from interface it.unimi.dsi.fastutil.Hash

    it.unimi.dsi.fastutil.Hash.Strategy<K extends java.lang.Object>

  • Field Summary

    Fields inherited from interface it.unimi.dsi.fastutil.Hash

    DEFAULT_GROWTH_FACTOR, DEFAULT_INITIAL_SIZE, DEFAULT_LOAD_FACTOR, FAST_LOAD_FACTOR, FREE, OCCUPIED, PRIMES, REMOVED, VERY_FAST_LOAD_FACTOR

  • Constructor Summary

    Constructors
    Constructor Description
    CharLinkedOpenHashSet()
    Creates a new hash set with initial expected Hash.DEFAULT_INITIAL_SIZE elements and Hash.DEFAULT_LOAD_FACTOR as load factor.
    CharLinkedOpenHashSet​(char[] a)
    Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor copying the elements of an array.
    CharLinkedOpenHashSet​(char[] a, float f)
    Creates a new hash set copying the elements of an array.
    CharLinkedOpenHashSet​(char[] a, int offset, int length)
    Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor and fills it with the elements of a given array.
    CharLinkedOpenHashSet​(char[] a, int offset, int length, float f)
    Creates a new hash set and fills it with the elements of a given array.
    CharLinkedOpenHashSet​(int expected)
    Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor.
    CharLinkedOpenHashSet​(int expected, float f)
    Creates a new hash set.
    CharLinkedOpenHashSet​(CharCollection c)
    Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor copying a given type-specific collection.
    CharLinkedOpenHashSet​(CharCollection c, float f)
    Creates a new hash set copying a given type-specific collection.
    CharLinkedOpenHashSet​(it.unimi.dsi.fastutil.chars.CharIterator i)
    Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor using elements provided by a type-specific iterator.
    CharLinkedOpenHashSet​(it.unimi.dsi.fastutil.chars.CharIterator i, float f)
    Creates a new hash set using elements provided by a type-specific iterator.
    CharLinkedOpenHashSet​(java.util.Collection<? extends java.lang.Character> c)
    Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor copying a given collection.
    CharLinkedOpenHashSet​(java.util.Collection<? extends java.lang.Character> c, float f)
    Creates a new hash set copying a given collection.
    CharLinkedOpenHashSet​(java.util.Iterator<?> i)
    Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor using elements provided by an iterator.
    CharLinkedOpenHashSet​(java.util.Iterator<?> i, float f)
    Creates a new hash set using elements provided by an iterator.

  • Method Summary

    Modifier and Type Method Description
    boolean add​(char k)
    Ensures that this collection contains the specified element (optional operation).
    boolean addAll​(CharCollection c)
    Adds all elements of the given type-specific collection to this collection.
    boolean addAll​(java.util.Collection<? extends java.lang.Character> c)
    boolean addAndMoveToFirst​(char k)
    Adds a key to the set; if the key is already present, it is moved to the first position of the iteration order.
    boolean addAndMoveToLast​(char k)
    Adds a key to the set; if the key is already present, it is moved to the last position of the iteration order.
    void clear()  
    CharLinkedOpenHashSet clone()
    Returns a deep copy of this set.
    CharComparator comparator()
    boolean contains​(char k)
    Returns true if this collection contains the specified element.
    char firstChar()
    Returns the first element of this set in iteration order.
    void forEach​(CharConsumer action)  
    int hashCode()
    Returns a hash code for this set.
    CharSortedSet headSet​(char to)
    Returns a view of the portion of this sorted set whose elements are strictly less than toElement.
    boolean isEmpty()  
    CharListIterator iterator()
    Returns a type-specific list iterator on the elements in this set, starting from the first element.
    CharListIterator iterator​(char from)
    Returns a type-specific list iterator on the elements in this set, starting from a given element of the set.
    char lastChar()
    Returns the last element of this set in iteration order.
    static CharLinkedOpenHashSet of()
    Creates a new empty hash set.
    static CharLinkedOpenHashSet of​(char e)
    Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor using the given element.
    static CharLinkedOpenHashSet of​(char... a)
    Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor using a list of elements.
    static CharLinkedOpenHashSet of​(char e0, char e1)
    Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor using the elements given.
    static CharLinkedOpenHashSet of​(char e0, char e1, char e2)
    Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor using the elements given.
    boolean remove​(char k)
    Removes an element from this set.
    char removeFirstChar()
    Removes the first key in iteration order.
    char removeLastChar()
    Removes the the last key in iteration order.
    int size()  
    it.unimi.dsi.fastutil.chars.CharSpliterator spliterator()
    Returns a type-specific spliterator on the elements of this sorted-set.
    CharSortedSet subSet​(char from, char to)
    Returns a view of the portion of this sorted set whose elements range from fromElement, inclusive, to toElement, exclusive.
    CharSortedSet tailSet​(char from)
    Returns a view of the portion of this sorted set whose elements are greater than or equal to fromElement.
    boolean trim()
    Rehashes this set, making the table as small as possible.
    boolean trim​(int n)
    Rehashes this set if the table is too large.

    Methods inherited from class it.unimi.dsi.fastutil.chars.AbstractCharSet

    equals, rem

    Methods inherited from class it.unimi.dsi.fastutil.chars.AbstractCharCollection

    add, contains, containsAll, containsAll, remove, removeAll, removeAll, retainAll, retainAll, toArray, toCharArray, toCharArray, toString

    Methods inherited from class java.util.AbstractCollection

    toArray, toArray

    Methods inherited from class java.lang.Object

    getClass, notify, notifyAll, wait, wait, wait

    Methods inherited from interface it.unimi.dsi.fastutil.chars.CharCollection

    containsAll, intIterator, intParallelStream, intSpliterator, intStream, parallelStream, removeAll, removeIf, removeIf, removeIf, retainAll, stream, toArray, toCharArray, toCharArray

    Methods inherited from interface it.unimi.dsi.fastutil.chars.CharIterable

    forEach, forEach

    Methods inherited from interface it.unimi.dsi.fastutil.chars.CharSet

    add, contains, rem, remove

    Methods inherited from interface it.unimi.dsi.fastutil.chars.CharSortedSet

    first, headSet, last, subSet, tailSet

    Methods inherited from interface java.util.Collection

    toArray

    Methods inherited from interface java.util.Set

    add, contains, containsAll, equals, remove, removeAll, retainAll, toArray, toArray

  • Constructor Details

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(int expected, float f)
      Creates a new hash set.

      The actual table size will be the least power of two greater than expected/f.

      Parameters:
      expected - the expected number of elements in the hash set.
      f - the load factor.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(int expected)
      Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor.
      Parameters:
      expected - the expected number of elements in the hash set.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet()
      Creates a new hash set with initial expected Hash.DEFAULT_INITIAL_SIZE elements and Hash.DEFAULT_LOAD_FACTOR as load factor.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(java.util.Collection<? extends java.lang.Character> c, float f)
      Creates a new hash set copying a given collection.
      Parameters:
      c - a Collection to be copied into the new hash set.
      f - the load factor.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(java.util.Collection<? extends java.lang.Character> c)
      Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor copying a given collection.
      Parameters:
      c - a Collection to be copied into the new hash set.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(CharCollection c, float f)
      Creates a new hash set copying a given type-specific collection.
      Parameters:
      c - a type-specific collection to be copied into the new hash set.
      f - the load factor.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(CharCollection c)
      Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor copying a given type-specific collection.
      Parameters:
      c - a type-specific collection to be copied into the new hash set.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(it.unimi.dsi.fastutil.chars.CharIterator i, float f)
      Creates a new hash set using elements provided by a type-specific iterator.
      Parameters:
      i - a type-specific iterator whose elements will fill the set.
      f - the load factor.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(it.unimi.dsi.fastutil.chars.CharIterator i)
      Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor using elements provided by a type-specific iterator.
      Parameters:
      i - a type-specific iterator whose elements will fill the set.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(java.util.Iterator<?> i, float f)
      Creates a new hash set using elements provided by an iterator.
      Parameters:
      i - an iterator whose elements will fill the set.
      f - the load factor.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(java.util.Iterator<?> i)
      Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor using elements provided by an iterator.
      Parameters:
      i - an iterator whose elements will fill the set.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(char[] a, int offset, int length, float f)
      Creates a new hash set and fills it with the elements of a given array.
      Parameters:
      a - an array whose elements will be used to fill the set.
      offset - the first element to use.
      length - the number of elements to use.
      f - the load factor.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(char[] a, int offset, int length)
      Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor and fills it with the elements of a given array.
      Parameters:
      a - an array whose elements will be used to fill the set.
      offset - the first element to use.
      length - the number of elements to use.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(char[] a, float f)
      Creates a new hash set copying the elements of an array.
      Parameters:
      a - an array to be copied into the new hash set.
      f - the load factor.

    • CharLinkedOpenHashSet

      public CharLinkedOpenHashSet​(char[] a)
      Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor copying the elements of an array.
      Parameters:
      a - an array to be copied into the new hash set.

  • Method Details

    • of

      public static CharLinkedOpenHashSet of()
      Creates a new empty hash set.
      Returns:
      a new empty hash set.

    • of

      public static CharLinkedOpenHashSet of​(char e)
      Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor using the given element.
      Parameters:
      e - the element that the returned set will contain.
      Returns:
      a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor containing e.

    • of

      public static CharLinkedOpenHashSet of​(char e0, char e1)
      Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor using the elements given.
      Parameters:
      e0 - the first element.
      e1 - the second element.
      Returns:
      a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor containing e0 and e1.
      Throws:
      java.lang.IllegalArgumentException - if there were duplicate entries.

    • of

      public static CharLinkedOpenHashSet of​(char e0, char e1, char e2)
      Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor using the elements given.
      Parameters:
      e0 - the first element.
      e1 - the second element.
      e2 - the third element.
      Returns:
      a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor containing e0, e1, and e2.
      Throws:
      java.lang.IllegalArgumentException - if there were duplicate entries.

    • of

      public static CharLinkedOpenHashSet of​(char... a)
      Creates a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor using a list of elements.
      Parameters:
      a - a list of elements that will be used to initialize the new hash set.
      Returns:
      a new hash set with Hash.DEFAULT_LOAD_FACTOR as load factor containing the elements of a.
      Throws:
      java.lang.IllegalArgumentException - if a duplicate entry was encountered.

    • addAll

      public boolean addAll​(CharCollection c)
      Description copied from interface: CharCollection
      Adds all elements of the given type-specific collection to this collection.
      Specified by:
      addAll in interface CharCollection
      Overrides:
      addAll in class AbstractCharCollection
      Parameters:
      c - a type-specific collection.
      Returns:
      true if this collection changed as a result of the call.
      See Also:
      Collection.addAll(Collection)

    • addAll

      public boolean addAll​(java.util.Collection<? extends java.lang.Character> c)
      Description copied from class: AbstractCharCollection
      Specified by:
      addAll in interface java.util.Collection<java.lang.Character>
      Specified by:
      addAll in interface java.util.Set<java.lang.Character>
      Overrides:
      addAll in class AbstractCharCollection

    • add

      public boolean add​(char k)
      Description copied from class: AbstractCharCollection
      Ensures that this collection contains the specified element (optional operation).
      Specified by:
      add in interface CharCollection
      Overrides:
      add in class AbstractCharCollection
      See Also:
      Collection.add(Object)

    • remove

      public boolean remove​(char k)
      Description copied from class: AbstractCharSet
      Removes an element from this set. Delegates to the type-specific rem() method implemented by type-specific abstract Collection superclass.
      Specified by:
      remove in interface CharSet
      Overrides:
      remove in class AbstractCharSet
      See Also:
      Collection.remove(Object)

    • contains

      public boolean contains​(char k)
      Description copied from class: AbstractCharCollection
      Returns true if this collection contains the specified element.
      Specified by:
      contains in interface CharCollection
      Overrides:
      contains in class AbstractCharCollection
      See Also:
      Collection.contains(Object)

    • removeFirstChar

      public char removeFirstChar()
      Removes the first key in iteration order.
      Returns:
      the first key.
      Throws:
      java.util.NoSuchElementException - is this set is empty.

    • removeLastChar

      public char removeLastChar()
      Removes the the last key in iteration order.
      Returns:
      the last key.
      Throws:
      java.util.NoSuchElementException - is this set is empty.

    • addAndMoveToFirst

      public boolean addAndMoveToFirst​(char k)
      Adds a key to the set; if the key is already present, it is moved to the first position of the iteration order.
      Parameters:
      k - the key.
      Returns:
      true if the key was not present.

    • addAndMoveToLast

      public boolean addAndMoveToLast​(char k)
      Adds a key to the set; if the key is already present, it is moved to the last position of the iteration order.
      Parameters:
      k - the key.
      Returns:
      true if the key was not present.

    • clear

      public void clear()
      Specified by:
      clear in interface java.util.Collection<java.lang.Character>
      Specified by:
      clear in interface java.util.Set<java.lang.Character>
      Overrides:
      clear in class java.util.AbstractCollection<java.lang.Character>

    • size

      public int size()
      Specified by:
      size in interface java.util.Collection<java.lang.Character>
      Specified by:
      size in interface java.util.Set<java.lang.Character>
      Specified by:
      size in class java.util.AbstractCollection<java.lang.Character>

    • isEmpty

      public boolean isEmpty()
      Specified by:
      isEmpty in interface java.util.Collection<java.lang.Character>
      Specified by:
      isEmpty in interface java.util.Set<java.lang.Character>
      Overrides:
      isEmpty in class java.util.AbstractCollection<java.lang.Character>

    • firstChar

      public char firstChar()
      Returns the first element of this set in iteration order.
      Specified by:
      firstChar in interface CharSortedSet
      Returns:
      the first element in iteration order.
      See Also:
      SortedSet.first()

    • lastChar

      public char lastChar()
      Returns the last element of this set in iteration order.
      Specified by:
      lastChar in interface CharSortedSet
      Returns:
      the last element in iteration order.
      See Also:
      SortedSet.last()

    • tailSet

      public CharSortedSet tailSet​(char from)
      Returns a view of the portion of this sorted set whose elements are greater than or equal to fromElement.
      Specified by:
      tailSet in interface CharSortedSet
      See Also:
      SortedSet.tailSet(Object)
      Implementation Specification:
      This implementation just throws an UnsupportedOperationException.

    • headSet

      public CharSortedSet headSet​(char to)
      Returns a view of the portion of this sorted set whose elements are strictly less than toElement.
      Specified by:
      headSet in interface CharSortedSet
      See Also:
      SortedSet.headSet(Object)
      Implementation Specification:
      This implementation just throws an UnsupportedOperationException.

    • subSet

      public CharSortedSet subSet​(char from, char to)
      Returns a view of the portion of this sorted set whose elements range from fromElement, inclusive, to toElement, exclusive.
      Specified by:
      subSet in interface CharSortedSet
      See Also:
      SortedSet.subSet(Object,Object)
      Implementation Specification:
      This implementation just throws an UnsupportedOperationException.

    • comparator

      public CharComparator comparator()
      Specified by:
      comparator in interface CharSortedSet
      Specified by:
      comparator in interface java.util.SortedSet<java.lang.Character>
      Implementation Specification:
      This implementation just returns null.

    • iterator

      public CharListIterator iterator​(char from)
      Returns a type-specific list iterator on the elements in this set, starting from a given element of the set. Please see the class documentation for implementation details.
      Specified by:
      iterator in interface CharSortedSet
      Parameters:
      from - an element to start from.
      Returns:
      a type-specific list iterator starting at the given element.
      Throws:
      java.lang.IllegalArgumentException - if from does not belong to the set.

    • iterator

      public CharListIterator iterator()
      Returns a type-specific list iterator on the elements in this set, starting from the first element. Please see the class documentation for implementation details.
      Specified by:
      iterator in interface CharBidirectionalIterable
      Specified by:
      iterator in interface CharCollection
      Specified by:
      iterator in interface it.unimi.dsi.fastutil.chars.CharIterable
      Specified by:
      iterator in interface CharSet
      Specified by:
      iterator in interface CharSortedSet
      Specified by:
      iterator in interface java.util.Collection<java.lang.Character>
      Specified by:
      iterator in interface java.lang.Iterable<java.lang.Character>
      Specified by:
      iterator in interface java.util.Set<java.lang.Character>
      Specified by:
      iterator in class AbstractCharSortedSet
      Returns:
      a type-specific list iterator starting at the first element.

    • spliterator

      public it.unimi.dsi.fastutil.chars.CharSpliterator spliterator()
      Returns a type-specific spliterator on the elements of this sorted-set.

      SortedSet spliterators must report at least Spliterator.DISTINCT, Spliterator.ORDERED, and Spliterator.SORTED. The returned spliterator's getComparator() must be the same (or at the very least, consistent with) this instance's CharSortedSet.comparator().

      See SortedSet.spliterator() for more documentation on the requirements of the returned spliterator.

      There isn't a way to split efficiently while still preserving order for a linked data structure, so this implementation is just backed by the iterator. Thus, this spliterator is not well optimized for parallel streams.

      Note, contrary to the specification of SortedSet, this spliterator does not, report Spliterator.SORTED. This is because iteration order is based on insertion order, not natural ordering.

      Specified by:
      spliterator in interface CharCollection
      Specified by:
      spliterator in interface it.unimi.dsi.fastutil.chars.CharIterable
      Specified by:
      spliterator in interface CharSet
      Specified by:
      spliterator in interface CharSortedSet
      Specified by:
      spliterator in interface java.util.Collection<java.lang.Character>
      Specified by:
      spliterator in interface java.lang.Iterable<java.lang.Character>
      Specified by:
      spliterator in interface java.util.Set<java.lang.Character>
      Specified by:
      spliterator in interface java.util.SortedSet<java.lang.Character>
      Returns:
      a type-specific spliterator on the elements of this collection.

    • forEach

      public void forEach​(CharConsumer action)
      Specified by:
      forEach in interface it.unimi.dsi.fastutil.chars.CharIterable

    • trim

      public boolean trim()
      Rehashes this set, making the table as small as possible.

      This method rehashes the table to the smallest size satisfying the load factor. It can be used when the set will not be changed anymore, so to optimize access speed and size.

      If the table size is already the minimum possible, this method does nothing.

      Returns:
      true if there was enough memory to trim the set.
      See Also:
      trim(int)

    • trim

      public boolean trim​(int n)
      Rehashes this set if the table is too large.

      Let N be the smallest table size that can hold max(n,size()) entries, still satisfying the load factor. If the current table size is smaller than or equal to N, this method does nothing. Otherwise, it rehashes this set in a table of size N.

      This method is useful when reusing sets. Clearing a set leaves the table size untouched. If you are reusing a set many times, you can call this method with a typical size to avoid keeping around a very large table just because of a few large transient sets.

      Parameters:
      n - the threshold for the trimming.
      Returns:
      true if there was enough memory to trim the set.
      See Also:
      trim()

    • clone

      public CharLinkedOpenHashSet clone()
      Returns a deep copy of this set.

      This method performs a deep copy of this hash set; the data stored in the set, however, is not cloned. Note that this makes a difference only for object keys.

      Returns:
      a deep copy of this set.

    • hashCode

      public int hashCode()
      Returns a hash code for this set. This method overrides the generic method provided by the superclass. Since equals() is not overriden, it is important that the value returned by this method is the same value as the one returned by the overriden method.
      Specified by:
      hashCode in interface java.util.Collection<java.lang.Character>
      Specified by:
      hashCode in interface java.util.Set<java.lang.Character>
      Overrides:
      hashCode in class AbstractCharSet
      Returns:
      a hash code for this set.