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 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. |
| 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 (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 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.
return map.put(e, PRESENT)==null;
| | public void | clear()Removes all of the elements from this set.
The set will be empty after this call returns.
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.
More formally, returns true if and only if this set
contains an element e such that
(o==null ? e==null : o.equals(e)).
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.
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.)
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 it).
// 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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