- Type Parameters:
E
- the type of elements maintained by this set
- All Implemented Interfaces:
Serializable
,Cloneable
,Iterable<E>
,Collection<E>
,Set<E>
- Direct Known Subclasses:
JobStateReasons
,LinkedHashSet
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
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 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.
- Since:
- 1.2
- See Also:
-
Constructor Summary
ConstructorDescriptionHashSet()
Constructs a new, empty set; the backingHashMap
instance has default initial capacity (16) and load factor (0.75).HashSet
(int initialCapacity) Constructs a new, empty set; the backingHashMap
instance has the specified initial capacity and default load factor (0.75).HashSet
(int initialCapacity, float loadFactor) Constructs a new, empty set; the backingHashMap
instance has the specified initial capacity and the specified load factor.HashSet
(Collection<? extends E> c) Constructs a new set containing the elements in the specified collection. -
Method Summary
Modifier and TypeMethodDescriptionboolean
Adds the specified element to this set if it is not already present.void
clear()
Removes all of the elements from this set.clone()
Returns a shallow copy of thisHashSet
instance: the elements themselves are not cloned.boolean
Returnstrue
if this set contains the specified element.boolean
isEmpty()
Returnstrue
if this set contains no elements.iterator()
Returns an iterator over the elements in this set.static <T> HashSet
<T> newHashSet
(int numElements) Creates a new, empty HashSet suitable for the expected number of elements.boolean
Removes the specified element from this set if it is present.int
size()
Returns the number of elements in this set (its cardinality).Creates a late-binding and fail-fastSpliterator
over the elements in this set.Object[]
toArray()
Returns an array containing all of the elements in this collection.<T> T[]
toArray
(T[] a) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.Methods declared in class java.util.AbstractSet
equals, hashCode, removeAll
Methods declared in class java.util.AbstractCollection
addAll, containsAll, retainAll, toArray, toArray, toString
Methods declared in interface java.util.Collection
parallelStream, removeIf, stream, toArray
-
Constructor Details
-
HashSet
public HashSet()Constructs a new, empty set; the backingHashMap
instance has default initial capacity (16) and load factor (0.75). -
HashSet
Constructs a new set containing the elements in the specified collection. TheHashMap
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) Constructs a new, empty set; the backingHashMap
instance has the specified initial capacity and the specified load factor.- API Note:
- To create a
HashSet
with an initial capacity that accommodates an expected number of elements, usenewHashSet
. - Parameters:
initialCapacity
- the initial capacity of the hash maploadFactor
- the load factor of the hash map- Throws:
IllegalArgumentException
- if the initial capacity is less than zero, or if the load factor is nonpositive
-
HashSet
public HashSet(int initialCapacity) Constructs a new, empty set; the backingHashMap
instance has the specified initial capacity and default load factor (0.75).- API Note:
- To create a
HashSet
with an initial capacity that accommodates an expected number of elements, usenewHashSet
. - Parameters:
initialCapacity
- the initial capacity of the hash table- Throws:
IllegalArgumentException
- if the initial capacity is less than zero
-
-
Method Details
-
iterator
Returns an iterator over the elements in this set. The elements are returned in no particular order. -
size
-
isEmpty
public boolean isEmpty()Returnstrue
if this set contains no elements.- Specified by:
isEmpty
in interfaceCollection<E>
- Specified by:
isEmpty
in interfaceSet<E>
- Overrides:
isEmpty
in classAbstractCollection<E>
- Returns:
true
if this set contains no elements
-
contains
Returnstrue
if this set contains the specified element. More formally, returnstrue
if and only if this set contains an elemente
such thatObjects.equals(o, e)
.- Specified by:
contains
in interfaceCollection<E>
- Specified by:
contains
in interfaceSet<E>
- Overrides:
contains
in classAbstractCollection<E>
- Parameters:
o
- element whose presence in this set is to be tested- Returns:
true
if this set contains the specified element
-
add
Adds the specified element to this set if it is not already present. More formally, adds the specified elemente
to this set if this set contains no elemente2
such thatObjects.equals(e, e2)
. If this set already contains the element, the call leaves the set unchanged and returnsfalse
.- Specified by:
add
in interfaceCollection<E>
- Specified by:
add
in interfaceSet<E>
- Overrides:
add
in classAbstractCollection<E>
- Parameters:
e
- element to be added to this set- Returns:
true
if this set did not already contain the specified element
-
remove
Removes the specified element from this set if it is present. More formally, removes an elemente
such thatObjects.equals(o, e)
, if this set contains such an element. Returnstrue
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.)- Specified by:
remove
in interfaceCollection<E>
- Specified by:
remove
in interfaceSet<E>
- Overrides:
remove
in classAbstractCollection<E>
- Parameters:
o
- object to be removed from this set, if present- Returns:
true
if the set contained the specified element
-
clear
public void clear()Removes all of the elements from this set. The set will be empty after this call returns.- Specified by:
clear
in interfaceCollection<E>
- Specified by:
clear
in interfaceSet<E>
- Overrides:
clear
in classAbstractCollection<E>
-
clone
-
spliterator
Creates a late-binding and fail-fastSpliterator
over the elements in this set.The
Spliterator
reportsSpliterator.SIZED
andSpliterator.DISTINCT
. Overriding implementations should document the reporting of additional characteristic values.- Specified by:
spliterator
in interfaceCollection<E>
- Specified by:
spliterator
in interfaceIterable<E>
- Specified by:
spliterator
in interfaceSet<E>
- Returns:
- a
Spliterator
over the elements in this set - Since:
- 1.8
-
toArray
Description copied from class:AbstractCollection
Returns an array containing all of the elements in this collection. If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. The returned array's runtime component type isObject
.The returned array will be "safe" in that no references to it are maintained by this collection. (In other words, this method must allocate a new array even if this collection is backed by an array). The caller is thus free to modify the returned array.
- Specified by:
toArray
in interfaceCollection<E>
- Specified by:
toArray
in interfaceSet<E>
- Overrides:
toArray
in classAbstractCollection<E>
- Returns:
- an array, whose runtime component
type is
Object
, containing all of the elements in this collection
-
toArray
public <T> T[] toArray(T[] a) Description copied from class:AbstractCollection
Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection.If this collection fits in the specified array with room to spare (i.e., the array has more elements than this collection), the element in the array immediately following the end of the collection is set to
null
. (This is useful in determining the length of this collection only if the caller knows that this collection does not contain anynull
elements.)If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.
- Specified by:
toArray
in interfaceCollection<E>
- Specified by:
toArray
in interfaceSet<E>
- Overrides:
toArray
in classAbstractCollection<E>
- Type Parameters:
T
- the component type of the array to contain the collection- Parameters:
a
- the array into which the elements of this collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.- Returns:
- an array containing all of the elements in this collection
-
newHashSet
Creates a new, empty HashSet suitable for the expected number of elements. The returned set uses the default load factor of 0.75, and its initial capacity is generally large enough so that the expected number of elements can be added without resizing the set.- Type Parameters:
T
- the type of elements maintained by the new set- Parameters:
numElements
- the expected number of elements- Returns:
- the newly created set
- Throws:
IllegalArgumentException
- if numElements is negative- Since:
- 19
-