public class java.util
HashSet<E>


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Java SE 6
  
Extends: AbstractCollection > AbstractSet
Implements: Set, Cloneable, Serializable
Extended by: JobStateReasons, LinkedHashSet
Details
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.
Ethe type of elements maintained by this set
version1.37, 04/21/06
since1.2
See also java.util.Collection, java.util.Set, java.util.TreeSet, java.util.HashMap


Constructors
public HashSet()
Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).
public HashSet(Collection c) Details
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.
cthe collection whose elements are to be placed into this set
ThrowsNullPointerException: if the specified collection is null
public HashSet(int initialCapacity, float loadFactor) Details
Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor.
initialCapacitythe initial capacity of the hash map
loadFactorthe load factor of the hash map
ThrowsIllegalArgumentException: if the initial capacity is less than zero, or if the load factor is nonpositive
public HashSet(int initialCapacity) Details
Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75).
initialCapacitythe initial capacity of the hash table
ThrowsIllegalArgumentException: if the initial capacity is less than zero

Methods
public boolean add(Object e) Details
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 (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.
eelement to be added to this set
returntrue if this set did not already contain the specified element
public void clear()
Removes all of the elements from this set. The set will be empty after this call returns.
public Object clone() Details
Returns a shallow copy of this HashSet instance: the elements themselves are not cloned.
returna shallow copy of this set
public boolean contains(Object o) Details
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==null ? e==null : o.equals(e)).
oelement whose presence in this set is to be tested
returntrue if this set contains the specified element
public Iterator iterator() Details
Returns an iterator over the elements in this set. The elements are returned in no particular order.
returnan Iterator over the elements in this set
See also java.util.ConcurrentModificationException
public boolean remove(Object o) Details
Removes the specified element from this set if it is present. More formally, removes an element e such that (o==null ? e==null : 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.)
oobject to be removed from this set, if present
returntrue if the set contained the specified element
public int size() Details
Returns the number of elements in this set (its cardinality).
returnthe number of elements in this set (its cardinality)

Properties
public boolean isEmpty() Details
Returns true if this set contains no elements.
returntrue if this set contains no elements
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Java SE 6
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