Module java.base
Package java.util

Class TreeSet<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 TreeSet<E>
extends AbstractSet<E>
implements NavigableSet<E>, Cloneable, Serializable
    A NavigableSet implementation based on a TreeMap. The elements are ordered using their natural ordering, or by a Comparator provided at set creation time, depending on which constructor is used.

    This implementation provides guaranteed log(n) time cost for the basic operations (add, remove and contains).

    Note that the ordering maintained by a set (whether or not an explicit comparator is provided) must be consistent with equals if it is to correctly implement the Set interface. (See Comparable or Comparator for a precise definition of consistent with equals.) This is so because the Set interface is defined in terms of the equals operation, but a TreeSet instance performs all element comparisons using its compareTo (or compare) method, so two elements that are deemed equal by this method are, from the standpoint of the set, equal. The behavior of a set is well-defined even if its ordering is inconsistent with equals; it just fails to obey the general contract of the Set interface.

    Note that this implementation is not synchronized. If multiple threads access a tree set concurrently, and at least one of the threads modifies the set, it must be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the set. If no such object exists, the set should be "wrapped" using the Collections.synchronizedSortedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the set: 

       SortedSet s = Collections.synchronizedSortedSet(new TreeSet(...));

    The iterators returned by this class's iterator method are fail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

    Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

    This class is a member of the Java Collections Framework.

    Since:
    1.2
    See Also:
    Collection, Set, HashSet, Comparable, Comparator, TreeMap, Serialized Form

    • Constructor Summary

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

    • Method Summary

      All Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      boolean add​(E e)
      Adds the specified element to this set if it is not already present.
      boolean addAll​(Collection<? extends E> c)
      Adds all of the elements in the specified collection to this set.
      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.
      Object clone()
      Returns a shallow copy of this TreeSet instance.
      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.
      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.
      SortedSet<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.
      int size()
      Returns the number of elements in this set (its cardinality).
      Spliterator<E> spliterator()
      Creates a late-binding and fail-fast 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.
      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.
      SortedSet<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.

    • Constructor Detail

      • TreeSet

        public TreeSet()
        Constructs a new, empty tree set, sorted according to the natural ordering of its elements. All elements inserted into the set must implement the Comparable interface. Furthermore, all such elements must be mutually comparable: e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the set. If the user attempts to add an element to the set that violates this constraint (for example, the user attempts to add a string element to a set whose elements are integers), the add call will throw a ClassCastException.

      • TreeSet

        public TreeSet​(Comparator<? super E> comparator)
        Constructs a new, empty tree set, sorted according to the specified comparator. All elements inserted into the set must be mutually comparable by the specified comparator: comparator.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the set. If the user attempts to add an element to the set that violates this constraint, the add call will throw a ClassCastException.
        Parameters:
        comparator - the comparator that will be used to order this set. If null, the natural ordering of the elements will be used.

      • TreeSet

        public TreeSet​(Collection<? extends E> c)
        Constructs a new tree set containing the elements in the specified collection, sorted according to the natural ordering of its elements. All elements inserted into the set must implement the Comparable interface. Furthermore, all such elements must be mutually comparable: e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the set.
        Parameters:
        c - collection whose elements 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 is null

      • TreeSet

        public TreeSet​(SortedSet<E> s)
        Constructs a new tree 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 is null

    • Method Detail

      • 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
        Since:
        1.6

      • descendingSet

        public NavigableSet<E> descendingSet()
        Description copied from interface: NavigableSet
        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. If either set is modified while an iteration over either set is in progress (except through the iterator's own remove operation), the results of the iteration are undefined.

        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
        Since:
        1.6

      • size

        public int size()
        Returns the number of elements in this set (its cardinality).
        Specified by:
        size in interface Collection<E>
        Specified by:
        size in interface Set<E>
        Returns:
        the number of elements in this set (its cardinality)

      • 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 Objects.equals(o, 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 object cannot be compared with the elements currently in the set
        NullPointerException - if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

      • 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 Objects.equals(e, 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 the specified object cannot be compared with the elements currently in this set
        NullPointerException - if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

      • 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 Objects.equals(o, 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 the specified object cannot be compared with the elements currently in this set
        NullPointerException - if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

      • clear

        public void clear()
        Removes all of the elements from this set. The set will be empty after this call returns.
        Specified by:
        clear in interface Collection<E>
        Specified by:
        clear in interface Set<E>
        Overrides:
        clear in class AbstractCollection<E>

      • addAll

        public boolean addAll​(Collection<? extends E> c)
        Adds all of the elements in the specified collection to this set.
        Specified by:
        addAll in interface Collection<E>
        Specified by:
        addAll in interface Set<E>
        Overrides:
        addAll in class AbstractCollection<E>
        Parameters:
        c - collection containing elements to be added to this set
        Returns:
        true if this set changed as a result of the call
        Throws:
        ClassCastException - if the elements provided cannot be compared with the elements currently in the set
        NullPointerException - if the specified collection is null or if any element is null and this set uses natural ordering, or its comparator does not permit null elements
        See Also:
        AbstractCollection.add(Object)

      • 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 and this set uses natural ordering, or its comparator does not permit null elements
        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.
        Since:
        1.6

      • 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 and this set uses natural ordering, or its comparator does not permit null elements
        IllegalArgumentException - if this set itself has a restricted range, and toElement lies outside the bounds of the range
        Since:
        1.6

      • 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 and this set uses natural ordering, or its comparator does not permit null elements
        IllegalArgumentException - if this set itself has a restricted range, and fromElement lies outside the bounds of the range
        Since:
        1.6

      • subSet

        public SortedSet<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 and this set uses natural ordering, or its comparator does not permit null elements
        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 SortedSet<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 and this set uses natural ordering, or its comparator does not permit null elements
        IllegalArgumentException - if this set itself has a restricted range, and toElement lies outside the bounds of the range

      • tailSet

        public SortedSet<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 and this set uses natural ordering, or its comparator does not permit null elements
        IllegalArgumentException - if this set itself has a restricted range, and fromElement lies outside the bounds of the range

      • 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

      • 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 and this set uses natural ordering, or its comparator does not permit null elements
        Since:
        1.6

      • 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 and this set uses natural ordering, or its comparator does not permit null elements
        Since:
        1.6

      • 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 and this set uses natural ordering, or its comparator does not permit null elements
        Since:
        1.6

      • 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 and this set uses natural ordering, or its comparator does not permit null elements
        Since:
        1.6

      • 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
        Since:
        1.6

      • 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
        Since:
        1.6

      • clone

        public Object clone()
        Returns a shallow copy of this TreeSet instance. (The elements themselves are not cloned.)
        Overrides:
        clone in class Object
        Returns:
        a shallow copy of this set
        See Also:
        Cloneable