JavaTM 2 Platform
Std. Ed. v1.3.1

java.util
Class HashSet

java.lang.Object
  |
  +--java.util.AbstractCollection
        |
        +--java.util.AbstractSet
              |
              +--java.util.HashSet
All Implemented Interfaces:
Cloneable, Collection, Serializable, Set

public class HashSet
extends AbstractSet
implements Set, 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 intial 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 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 HashSet instance:

     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.

Since:
1.2
See Also:
Collection, Set, TreeSet, Collections.synchronizedSet(Set), HashMap, Serialized Form

Constructor Summary
HashSet()
          Constructs a new, empty set; the backing HashMap instance has default capacity and load factor, which is 0.75.
HashSet(Collection 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, which is 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(Object o)
          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 iterator()
          Returns an iterator over the elements in this set.
 boolean remove(Object o)
          Removes the given 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 capacity and load factor, which is 0.75.

HashSet

public HashSet(Collection c)
Constructs a new set containing the elements in the specified collection. The capacity of the backing HashMap instance is twice the size of the specified collection or eleven (whichever is greater), and the default load factor (which is 0.75) is used.
Parameters:
c - the collection whose elements are to be placed into this set.

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, which is 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 iterator()
Returns an iterator over the elements in this set. The elements are returned in no particular order.
Specified by:
iterator in interface Set
Overrides:
iterator in class AbstractCollection
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 Set
Overrides:
size in class AbstractCollection
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 Set
Overrides:
isEmpty in class AbstractCollection
Returns:
true if this set contains no elements.

contains

public boolean contains(Object o)
Returns true if this set contains the specified element.
Specified by:
contains in interface Set
Overrides:
contains in class AbstractCollection
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(Object o)
Adds the specified element to this set if it is not already present.
Specified by:
add in interface Set
Overrides:
add in class AbstractCollection
Parameters:
o - element to be added to this set.
Returns:
true if the set did not already contain the specified element.

remove

public boolean remove(Object o)
Removes the given element from this set if it is present.
Specified by:
remove in interface Set
Overrides:
remove in class AbstractCollection
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.
Specified by:
clear in interface Set
Overrides:
clear in class AbstractCollection
Following copied from interface: java.util.Set
Throws:
UnsupportedOperationException - if the clear method is not supported by this set.

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.

JavaTM 2 Platform
Std. Ed. v1.3.1

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For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

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