Module java.base
Package java.util

Class HashSet<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractSet<E>
java.util.HashSet<E>
Type Parameters:
E - the type of elements maintained by this set
All Implemented Interfaces:
Serializable, Cloneable, Iterable<E>, Collection<E>, Set<E>
Direct Known Subclasses:
JobStateReasons, LinkedHashSet
public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, Serializable
This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.

This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets. Iterating over this set requires time proportional to the sum of the HashSet instance's size (the number of elements) plus the "capacity" of the backing HashMap instance (the number of buckets). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

Note that this implementation is not synchronized. If multiple threads access a hash 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.synchronizedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the set:

   Set s = Collections.synchronizedSet(new HashSet(...));

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 throws 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:

  • Constructor Summary

    Constructors
    Constructor
    Description
    HashSet()
    Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).
    HashSet(int initialCapacity)
    Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75).
    HashSet(int initialCapacity, float loadFactor)
    Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor.
    HashSet(Collection<? extends E> c)
    Constructs a new set containing the elements in the specified collection.

  • Method Summary

    Modifier and Type
    Method
    Description
    boolean
    add(E e)
    Adds the specified element to this set if it is not already present.
    void
    clear()
    Removes all of the elements from this set.
    Object
    clone()
    Returns a shallow copy of this HashSet instance: the elements themselves are not cloned.
    boolean
    contains(Object o)
    Returns true if this set contains the specified element.
    boolean
    isEmpty()
    Returns true if this set contains no elements.
    Iterator<E>
    iterator()
    Returns an iterator over the elements in this set.
    static <T> HashSet<T>
    newHashSet(int numElements)
    Creates a new, empty HashSet suitable for the expected number of elements.
    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.
    Object[]
    toArray()
    Returns an array containing all of the 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.

    Methods declared in class java.util.AbstractSet

    equals, hashCode, removeAll

    Methods declared in class java.util.AbstractCollection

    addAll, containsAll, retainAll, toArray, toArray, toString

    Methods declared in class java.lang.Object

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

    Methods declared in interface java.util.Collection

    parallelStream, removeIf, stream, toArray

    Methods declared in interface java.lang.Iterable

    forEach

    Methods declared in interface java.util.Set

    addAll, containsAll, equals, hashCode, removeAll, retainAll

  • Constructor Details

    • HashSet

      public HashSet()
      Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).

    • HashSet

      public HashSet(Collection<? extends E> c)
      Constructs a new set containing the elements in the specified collection. The HashMap is created with default load factor (0.75) and an initial capacity sufficient to contain the elements in the specified collection.
      Parameters:
      c - the collection whose elements are to be placed into this set
      Throws:
      NullPointerException - if the specified collection is null

    • HashSet

      public HashSet(int initialCapacity, float loadFactor)
      Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor.
      API Note:
      To create a HashSet with an initial capacity that accommodates an expected number of elements, use newHashSet.
      Parameters:
      initialCapacity - the initial capacity of the hash map
      loadFactor - the load factor of the hash map
      Throws:
      IllegalArgumentException - if the initial capacity is less than zero, or if the load factor is nonpositive

    • HashSet

      public HashSet(int initialCapacity)
      Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75).
      API Note:
      To create a HashSet with an initial capacity that accommodates an expected number of elements, use newHashSet.
      Parameters:
      initialCapacity - the initial capacity of the hash table
      Throws:
      IllegalArgumentException - if the initial capacity is less than zero

  • Method Details

    • iterator

      public Iterator<E> iterator()
      Returns an iterator over the elements in this set. The elements are returned in no particular order.
      Specified by:
      iterator in interface Collection<E>
      Specified by:
      iterator in interface Iterable<E>
      Specified by:
      iterator in interface Set<E>
      Specified by:
      iterator in class AbstractCollection<E>
      Returns:
      an Iterator over the elements in this set
      See Also:

    • 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 - element whose presence in this set is to be tested
      Returns:
      true if this set contains the specified element

    • 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 this 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

    • 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 the set contained the specified element

    • 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>

    • clone

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

    • spliterator

      public Spliterator<E> spliterator()
      Creates a late-binding and fail-fast Spliterator over the elements in this set.

      The Spliterator reports Spliterator.SIZED and Spliterator.DISTINCT. Overriding implementations should document the reporting of additional characteristic values.

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

    • toArray

      public Object[] toArray()
      Description copied from class: AbstractCollection
      Returns an array containing all of the elements in this collection. If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. The returned array's runtime component type is Object.

      The returned array will be "safe" in that no references to it are maintained by this collection. (In other words, this method must allocate a new array even if this collection is backed by an array). The caller is thus free to modify the returned array.

      Specified by:
      toArray in interface Collection<E>
      Specified by:
      toArray in interface Set<E>
      Overrides:
      toArray in class AbstractCollection<E>
      Returns:
      an array, whose runtime component type is Object, containing all of the elements in this collection

    • toArray

      public <T> T[] toArray(T[] a)
      Description copied from class: AbstractCollection
      Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection.

      If this collection fits in the specified array with room to spare (i.e., the array has more elements than this collection), the element in the array immediately following the end of the collection is set to null. (This is useful in determining the length of this collection only if the caller knows that this collection does not contain any null elements.)

      If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

      Specified by:
      toArray in interface Collection<E>
      Specified by:
      toArray in interface Set<E>
      Overrides:
      toArray in class AbstractCollection<E>
      Type Parameters:
      T - the component type of the array to contain the collection
      Parameters:
      a - the array into which the elements of this collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
      Returns:
      an array containing all of the elements in this collection

    • newHashSet

      public static <T> HashSet<T> newHashSet(int numElements)
      Creates a new, empty HashSet suitable for the expected number of elements. The returned set uses the default load factor of 0.75, and its initial capacity is generally large enough so that the expected number of elements can be added without resizing the set.
      Type Parameters:
      T - the type of elements maintained by the new set
      Parameters:
      numElements - the expected number of elements
      Returns:
      the newly created set
      Throws:
      IllegalArgumentException - if numElements is negative
      Since:
      19