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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 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.

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

Constructor Index

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

Method Index

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

Constructors

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

 o 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.

 o 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.
 o 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.

Methods

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

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

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

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

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

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

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

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

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

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