Class HashSet<E>
- java.lang.Object
-
- java.util.AbstractCollection<E>
-
- java.util.AbstractSet<E>
-
- java.util.HashSet<E>
- 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
public class HashSet<E> extends AbstractSet<E> implements Set<E>, 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 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:
-
Collection
,Set
,TreeSet
,HashMap
, Serialized Form
Constructor Summary
Constructor | Description |
---|---|
HashSet() | Constructs a new, empty set; the backing |
HashSet(int initialCapacity) | Constructs a new, empty set; the backing |
HashSet(int initialCapacity,
float loadFactor) | Constructs a new, empty set; the backing |
HashSet(Collection<? extends E> c) | Constructs a new set containing the elements in the specified collection. |
Method Summary
Modifier and Type | Method | Description |
---|---|---|
boolean | add(E e) | Adds the specified element to this set if it is not already present. |
void | clear() | Removes all of the elements from this set. |
Object | clone() | Returns a shallow copy of this |
boolean | contains(Object o) | Returns |
boolean | isEmpty() | Returns |
Iterator<E> | iterator() | Returns an iterator over the elements in this set. |
boolean | remove(Object o) | Removes the specified element from this set if it is present. |
int | size() | Returns the number of elements in this set (its cardinality). |
Spliterator<E> | spliterator() | Creates a late-binding and fail-fast |
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 class java.lang.Object
finalize, getClass, notify, notifyAll, wait, wait, wait
Methods declared in interface java.util.Collection
parallelStream, removeIf, stream, toArray
Methods declared in interface java.lang.Iterable
forEach
Methods declared in interface java.util.Set
addAll, containsAll, equals, hashCode, removeAll, retainAll, toArray, toArray
Constructor Detail
HashSet
public HashSet()
Constructs a new, empty set; the backing HashMap
instance has default initial capacity (16) and load factor (0.75).
HashSet
public HashSet(Collection<? extends E> 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.
- 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 backing HashMap
instance has the specified initial capacity and the specified load factor.
- Parameters:
-
initialCapacity
- the initial capacity of the hash map -
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
HashSet
public HashSet(int initialCapacity)
Constructs a new, empty set; the backing HashMap
instance has the specified initial capacity and default load factor (0.75).
- Parameters:
-
initialCapacity
- the initial capacity of the hash table - Throws:
-
IllegalArgumentException
- if the initial capacity is less than zero
Method Detail
iterator
public Iterator<E> iterator()
Returns an iterator over the elements in this set. The elements are returned in no particular order.
- Specified by:
-
iterator
in interfaceCollection<E>
- Specified by:
-
iterator
in interfaceIterable<E>
- Specified by:
-
iterator
in interfaceSet<E>
- Specified by:
-
iterator
in classAbstractCollection<E>
- Returns:
- an Iterator over the elements in this set
- See Also:
ConcurrentModificationException
size
public int size()
Returns the number of elements in this set (its cardinality).
- Specified by:
-
size
in interfaceCollection<E>
- Specified by:
-
size
in interfaceSet<E>
- Returns:
- the number of elements in this set (its cardinality)
isEmpty
public boolean isEmpty()
Returns true
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
public boolean contains(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 Objects.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
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 Objects.equals(e, e2)
. If this set already contains the element, the call leaves the set unchanged and returns false
.
- 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
public boolean remove(Object o)
Removes the specified element from this set if it is present. More formally, removes an element e
such that Objects.equals(o, 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.)
- 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
public Object clone()
Returns a shallow copy of this HashSet
instance: the elements themselves are not cloned.
spliterator
public Spliterator<E> spliterator()
Creates a late-binding and fail-fast Spliterator
over the elements in this set.
The Spliterator
reports Spliterator.SIZED
and Spliterator.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