HashSetpublic 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 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. |
| 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.
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.
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, which is
0.75.
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.
map = new LinkedHashMap<E,Object>(initialCapacity, loadFactor);
|
| Methods Summary |
|---|
| public boolean | add(E o)Adds the specified element to this set if it is not already
present.
return map.put(o, PRESENT)==null;
| | public void | clear()Removes all of the elements from this set.
map.clear();
| | public java.lang.Object | clone()Returns a shallow copy of this HashSet instance: the elements
themselves are not cloned.
try {
HashSet<E> newSet = (HashSet<E>) super.clone();
newSet.map = (HashMap<E, Object>) map.clone();
return newSet;
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
| | public boolean | contains(java.lang.Object o)Returns true if this set contains the specified element.
return map.containsKey(o);
| | public boolean | isEmpty()Returns true if this set contains no elements.
return map.isEmpty();
| | public java.util.Iterator | iterator()Returns an iterator over the elements in this set. The elements
are returned in no particular order.
return map.keySet().iterator();
| | private void | readObject(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 boolean | remove(java.lang.Object o)Removes the specified element from this set if it is present.
return map.remove(o)==PRESENT;
| | public int | size()Returns the number of elements in this set (its cardinality).
return map.size();
| | private void | writeObject(java.io.ObjectOutputStream s)Save the state of this HashSet instance to a stream (that is,
serialize this set).
// 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());
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