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Java Source Code / Java Documentation » 6.0 JDK Modules » j2me » java.util 
Source Cross Reference  Class Diagram Java Document (Java Doc) 


java.lang.Object
   java.util.AbstractCollection
      java.util.AbstractSet
         java.util.HashSet

All known Subclasses:   java.util.LinkedHashSet,
HashSet
public class HashSet extends AbstractSet implements Set,Cloneable,java.io.Serializable(Code)
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 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.

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.
author:
   Josh Bloch
version:
   1.19, 02/02/00
See Also:   Collection
See Also:   Set
See Also:   TreeSet
See Also:   Collections.synchronizedSet(Set)
See Also:   HashMap
since:
   1.2



Field Summary
final static  longserialVersionUID
    

Constructor Summary
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)
     Constructs a new set containing the elements in the specified collection.
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.
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.
 HashSet(int initialCapacity, float loadFactor, boolean dummy)
     Constructs a new, empty linked hash set.

Method Summary
public  booleanadd(Object o)
     Adds the specified element to this set if it is not already present.
Parameters:
  o - element to be added to this set.
public  voidclear()
     Removes all of the elements from this set.
public  Objectclone()
     Returns a shallow copy of this HashSet instance: the elements themselves are not cloned.
public  booleancontains(Object o)
     Returns true if this set contains the specified element.
Parameters:
  o - element whose presence in this set is to be tested.
public  booleanisEmpty()
     Returns true if this set contains no elements.
public  Iteratoriterator()
     Returns an iterator over the elements in this set.
public  booleanremove(Object o)
     Removes the specified element from this set if it is present.
Parameters:
  o - object to be removed from this set, if present.
public  intsize()
     Returns the number of elements in this set (its cardinality).

Field Detail
serialVersionUID
final static long serialVersionUID(Code)




Constructor Detail
HashSet
public HashSet()(Code)
Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).



HashSet
public HashSet(Collection c)(Code)
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)(Code)
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.
Parameters:
  loadFactor - the load factor of the hash map.
throws:
  IllegalArgumentException - if the initial capacity is lessthan zero, or if the load factor is nonpositive.



HashSet
public HashSet(int initialCapacity)(Code)
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 lessthan zero.



HashSet
HashSet(int initialCapacity, float loadFactor, boolean dummy)(Code)
Constructs a new, empty linked hash set. (This package private constructor is only used by LinkedHashSet.) The backing HashMap instance is a LinkedHashMap with the specified initial capacity and the specified load factor.
Parameters:
  initialCapacity - the initial capacity of the hash map.
Parameters:
  loadFactor - the load factor of the hash map.
Parameters:
  dummy - ignored (distinguishes thisconstructor from other int, float constructor.)
throws:
  IllegalArgumentException - if the initial capacity is lessthan zero, or if the load factor is nonpositive.




Method Detail
add
public boolean add(Object o)(Code)
Adds the specified element to this set if it is not already present.
Parameters:
  o - element to be added to this set. true if the set did not already contain the specifiedelement.



clear
public void clear()(Code)
Removes all of the elements from this set.



clone
public Object clone()(Code)
Returns a shallow copy of this HashSet instance: the elements themselves are not cloned. a shallow copy of this set.



contains
public boolean contains(Object o)(Code)
Returns true if this set contains the specified element.
Parameters:
  o - element whose presence in this set is to be tested. true if this set contains the specified element.



isEmpty
public boolean isEmpty()(Code)
Returns true if this set contains no elements. true if this set contains no elements.



iterator
public Iterator iterator()(Code)
Returns an iterator over the elements in this set. The elements are returned in no particular order. an Iterator over the elements in this set.
See Also:   ConcurrentModificationException



remove
public boolean remove(Object o)(Code)
Removes the specified element from this set if it is present.
Parameters:
  o - object to be removed from this set, if present. true if the set contained the specified element.



size
public int size()(Code)
Returns the number of elements in this set (its cardinality). the number of elements in this set (its cardinality).



Methods inherited from java.util.AbstractSet
public boolean equals(Object o)(Code)(Java Doc)
public int hashCode()(Code)(Java Doc)
public boolean removeAll(Collection c)(Code)(Java Doc)

Methods inherited from java.util.AbstractCollection
public boolean add(Object o)(Code)(Java Doc)
public boolean addAll(Collection c)(Code)(Java Doc)
public void clear()(Code)(Java Doc)
public boolean contains(Object o)(Code)(Java Doc)
public boolean containsAll(Collection c)(Code)(Java Doc)
public boolean isEmpty()(Code)(Java Doc)
abstract public Iterator iterator()(Code)(Java Doc)
public boolean remove(Object o)(Code)(Java Doc)
public boolean removeAll(Collection c)(Code)(Java Doc)
public boolean retainAll(Collection c)(Code)(Java Doc)
abstract public int size()(Code)(Java Doc)
public Object[] toArray()(Code)(Java Doc)
public Object[] toArray(Object a)(Code)(Java Doc)
public String toString()(Code)(Java Doc)

Methods inherited from java.lang.Object
public boolean equals(Object obj)(Code)(Java Doc)
final native public Class getClass()(Code)(Java Doc)
native public int hashCode()(Code)(Java Doc)
final native public void notify()(Code)(Java Doc)
final native public void notifyAll()(Code)(Java Doc)
public String toString()(Code)(Java Doc)
final native public void wait(long timeout) throws InterruptedException(Code)(Java Doc)
final public void wait(long timeout, int nanos) throws InterruptedException(Code)(Java Doc)
final public void wait() throws InterruptedException(Code)(Java Doc)

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