Java™ Platform
Standard Ed. 6

java.util
Class HashSet<E>

java.lang.Object
  extended by java.util.AbstractCollection<E>
      extended by java.util.AbstractSet<E>
          extended by 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:
Collection, Set, TreeSet, HashMap, Serialized Form

Constructor Summary
HashSet()
          Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).
HashSet(Collection<? extends E> c)
          Constructs a new set containing the elements in the specified collection.
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.
 
Method Summary
 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.
 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).
 
Methods inherited from class java.util.AbstractSet
equals, hashCode, removeAll
 
Methods inherited from class java.util.AbstractCollection
addAll, containsAll, retainAll, toArray, toArray, toString
 
Methods inherited from class java.lang.Object
finalize, getClass, notify, notifyAll, wait, wait, wait
 
Methods inherited from interface java.util.Set
addAll, containsAll, equals, hashCode, removeAll, retainAll, toArray, toArray
 

Constructor Detail

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.

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).

Parameters:
initialCapacity - the initial capacity of the hash table
Throws:
IllegalArgumentException - if the initial capacity is less than zero
Method Detail

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 Iterable<E>
Specified by:
iterator in interface Collection<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:
ConcurrentModificationException

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>
Specified by:
size in class AbstractCollection<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 (o==null ? e==null : o.equals(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 (e==null ? e2==null : e.equals(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 (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.)

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:
Cloneable

Java™ Platform
Standard Ed. 6

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