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HashSet.javaAPI DocJava SE 6 API11083Tue Jun 10 00:25:54 BST 2008java.util

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 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 {@link Collections#synchronizedSet 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 {@link 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.

param
the type of elements maintained by this set
author
Josh Bloch
author
Neal Gafter
version
1.37, 04/21/06
see
Collection
see
Set
see
TreeSet
see
HashMap
since
1.2

Fields Summary
static final long
serialVersionUID
private transient HashMap
map
private static final Object
PRESENT
Constructors Summary
public HashSet()
Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).


                           
      
	map = new HashMap<E,Object>();
    
public HashSet(Collection 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.

param
c the collection whose elements are to be placed into this set
throws
NullPointerException if the specified collection is null

	map = new HashMap<E,Object>(Math.max((int) (c.size()/.75f) + 1, 16));
	addAll(c);
    
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.

param
initialCapacity the initial capacity of the hash map
param
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

	map = new HashMap<E,Object>(initialCapacity, loadFactor);
    
public HashSet(int initialCapacity)
Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75).

param
initialCapacity the initial capacity of the hash table
throws
IllegalArgumentException if the initial capacity is less than zero

	map = new HashMap<E,Object>(initialCapacity);
    
HashSet(int initialCapacity, float loadFactor, boolean dummy)
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.

param
initialCapacity the initial capacity of the hash map
param
loadFactor the load factor of the hash map
param
dummy ignored (distinguishes this constructor from other int, float constructor.)
throws
IllegalArgumentException if the initial capacity is less than zero, or if the load factor is nonpositive

	map = new LinkedHashMap<E,Object>(initialCapacity, loadFactor);
    
Methods Summary
public booleanadd(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.

param
e element to be added to this set
return
true if this set did not already contain the specified element

	return map.put(e, PRESENT)==null;
    
public voidclear()
Removes all of the elements from this set. The set will be empty after this call returns.

	map.clear();
    
public java.lang.Objectclone()
Returns a shallow copy of this HashSet instance: the elements themselves are not cloned.

return
a shallow copy of this set

	try {
	    HashSet<E> newSet = (HashSet<E>) super.clone();
	    newSet.map = (HashMap<E, Object>) map.clone();
	    return newSet;
	} catch (CloneNotSupportedException e) {
	    throw new InternalError();
	}
    
public booleancontains(java.lang.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)).

param
o element whose presence in this set is to be tested
return
true if this set contains the specified element

	return map.containsKey(o);
    
public booleanisEmpty()
Returns true if this set contains no elements.

return
true if this set contains no elements

	return map.isEmpty();
    
public java.util.Iteratoriterator()
Returns an iterator over the elements in this set. The elements are returned in no particular order.

return
an Iterator over the elements in this set
see
ConcurrentModificationException

	return map.keySet().iterator();
    
private voidreadObject(java.io.ObjectInputStream s)
Reconstitute the HashSet instance from a stream (that is, deserialize it).

	// Read in any hidden serialization magic
	s.defaultReadObject();

        // Read in HashMap capacity and load factor and create backing HashMap
        int capacity = s.readInt();
        float loadFactor = s.readFloat();
        map = (((HashSet)this) instanceof LinkedHashSet ?
               new LinkedHashMap<E,Object>(capacity, loadFactor) :
               new HashMap<E,Object>(capacity, loadFactor));

        // Read in size
        int size = s.readInt();

	// Read in all elements in the proper order.
	for (int i=0; i<size; i++) {
            E e = (E) s.readObject();
            map.put(e, PRESENT);
        }
    
public booleanremove(java.lang.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.)

param
o object to be removed from this set, if present
return
true if the set contained the specified element

	return map.remove(o)==PRESENT;
    
public intsize()
Returns the number of elements in this set (its cardinality).

return
the number of elements in this set (its cardinality)

	return map.size();
    
private voidwriteObject(java.io.ObjectOutputStream s)
Save the state of this HashSet instance to a stream (that is, serialize it).

serialData
The capacity of the backing HashMap instance (int), and its load factor (float) are emitted, followed by the size of the set (the number of elements it contains) (int), followed by all of its elements (each an Object) in no particular order.

	// Write out any hidden serialization magic
	s.defaultWriteObject();

        // Write out HashMap capacity and load factor
        s.writeInt(map.capacity());
        s.writeFloat(map.loadFactor());

        // Write out size
        s.writeInt(map.size());

	// Write out all elements in the proper order.
	for (Iterator i=map.keySet().iterator(); i.hasNext(); )
            s.writeObject(i.next());