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Class com.sun.java.util.collections.HashSet

java.lang.Object
   |
   +----com.sun.java.util.collections.AbstractCollection
           |
           +----com.sun.java.util.collections.AbstractSet
                   |
                   +----com.sun.java.util.collections.HashSet

public class HashSet
extends AbstractSet
implements Set, Cloneable, Serializable
This class implements the Set interface, backed by a hash table (actually a HashMap). 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 the hash function disperses the elements properly among the buckets. Iterating over the Set requires time proportional to the sum of the HashSet's size (the number of elements) plus the "capacity" of the backing HashMap (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 HashSet concurrently, and at least one of the threads modifies the HashSet, it must be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the HashSet. If no such object exists, the HashSet should be "wrapped" using the Collections.synchronizedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the HashSet:

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

The Iterators returned by HashSet's iterator method are fail-fast: if the HashSet 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.

See Also:
Collection, Set, TreeSet, synchronizedSet, HashMap

Constructor Index

 o HashSet()
Constructs a new, empty HashSet; the backing HashMap has default capacity and load factor, which is 0.75.
 o HashSet(Collection)
Constructs a new HashSet containing the elements in the specified Collection.
 o HashSet(int)
Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and default load factor, which is 0.75.
 o HashSet(int, float)
Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and the specified load factor.

Method Index

 o add(Object)
Adds the specified element to this HashSet if it is not already present.
 o clear()
Removes all of the elements from this HashSet.
 o clone()
Returns a shallow copy of this HashSet.
 o contains(Object)
Returns true if this HashSet contains the specified element.
 o isEmpty()
Returns true if this HashSet contains no elements.
 o iterator()
Returns an Iterator over the elements in this HashSet.
 o remove(Object)
Removes the given element from this HashSet if it is present.
 o size()
Returns the number of elements in this HashSet (its cardinality).

Constructors

 o HashSet
 public HashSet()
Constructs a new, empty HashSet; the backing HashMap has default capacity and load factor, which is 0.75.

 o HashSet
 public HashSet(Collection c)
Constructs a new HashSet containing the elements in the specified Collection. The capacity of the backing HashMap is twice the size of the specified Collection or eleven (whichever is greater), and the default load factor (which is 0.75) is used.

Throws: NullPointerException
if the specified collection or one of its elements is null.
 o HashSet
 public HashSet(int initialCapacity,
                float loadFactor)
Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and the specified load factor.

Parameters:
initialCapacity - the initial capacity of the HashMap.
loadFactor - the load factor of the HashMap.
Throws: IllegalArgumentException
if the initial capacity is less than zero, or if the load factor is nonpositive.
 o HashSet
 public HashSet(int initialCapacity)
Constructs a new, empty HashSet; the backing HashMap has the specified initial capacity and default load factor, which is 0.75.

Parameters:
initialCapacity - the initial capacity of the hashtable.
Throws: IllegalArgumentException
if the initial capacity is less than zero.

Methods

 o iterator
 public Iterator iterator()
Returns an Iterator over the elements in this HashSet. The elements are returned in no particular order.

Returns:
an Iterator over the elements in this HashSet.
Overrides:
iterator in class AbstractCollection
See Also:
ConcurrentModificationException
 o size
 public int size()
Returns the number of elements in this HashSet (its cardinality).

Returns:
the number of elements in this HashSet (its cardinality).
Overrides:
size in class AbstractCollection
 o isEmpty
 public boolean isEmpty()
Returns true if this HashSet contains no elements.

Returns:
true if this HashSet contains no elements.
Overrides:
isEmpty in class AbstractCollection
 o contains
 public boolean contains(Object o)
Returns true if this HashSet contains the specified element.

Returns:
true if this HashSet contains the specified element.
Throws: NullPointerException
if the specified element is null.
Overrides:
contains in class AbstractCollection
 o add
 public boolean add(Object o)
Adds the specified element to this HashSet if it is not already present.

Parameters:
o - element to be added to this HashSet.
Returns:
true if the HashSet did not already contain the specified element.
Throws: NullPointerException
if the specified element is null.
Overrides:
add in class AbstractCollection
 o remove
 public boolean remove(Object o)
Removes the given element from this HashSet if it is present.

Parameters:
o - object to be removed from this HashSet, if present.
Returns:
true if the HashSet contained the specified element.
Throws: NullPointerException
if the specified element is null.
Overrides:
remove in class AbstractCollection
 o clear
 public void clear()
Removes all of the elements from this HashSet.

Overrides:
clear in class AbstractCollection
 o clone
 public Object clone()
Returns a shallow copy of this HashSet. (The elements themselves are not cloned.)

Returns:
a shallow copy of this HashSet.
Overrides:
clone in class Object

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