E - the type of elements maintained by this set
public class HashSet<E> extends AbstractSet<E> implements Set <E>, Cloneable , Serializable
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.
Collection,
Set,
TreeSet,
HashMap,
Serialized Form
| Constructor and Description |
|---|
HashSet()
Constructs a new, empty set; the backing
HashMap instance has default initial capacity (16) and load factor (0.75).
|
HashSet(Collection
Constructs a new set containing the elements in the specified collection.
|
HashSet(int initialCapacity)
Constructs a new, empty set; the backing
HashMap instance has the specified initial capacity and default load factor (0.75).
|
HashSet(int initialCapacity, float loadFactor)
Constructs a new, empty set; the backing
HashMap instance has the specified initial capacity and the specified load factor.
|
| Modifier and Type | Method and 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
HashSet instance: the elements themselves are not cloned.
|
boolean |
contains(Object
Returns
true if this set contains the specified element.
|
boolean |
isEmpty()
Returns
true if this set contains no elements.
|
Iterator |
iterator()
Returns an iterator over the elements in this set.
|
boolean |
remove(Object
Removes the specified element from this set if it is present.
|
int |
size()
Returns the number of elements in this set (its cardinality).
|
Spliterator |
spliterator()
Creates a
late-binding and
fail-fast
Spliterator over the elements in this set.
|
equals, hashCode, removeAlladdAll, containsAll, retainAll, toArray, toArray, toStringfinalize, getClass, notify, notifyAll, wait, wait, waitaddAll, containsAll, equals, hashCode, removeAll, retainAll, toArray, toArrayparallelStream, removeIf, streampublic HashSet()
public HashSet(Collection<? extends E> c)
c - the collection whose elements are to be placed into this set
NullPointerException - if the specified collection is null
public HashSet(int initialCapacity,
float loadFactor)
initialCapacity - the initial capacity of the hash map
loadFactor - the load factor of the hash map
IllegalArgumentException - if the initial capacity is less than zero, or if the load factor is nonpositive
public HashSet(int initialCapacity)
initialCapacity - the initial capacity of the hash table
IllegalArgumentException - if the initial capacity is less than zero
public Iterator<E> iterator()
iterator in interface
Iterable<E>
iterator in interface
Collection<E>
iterator in interface
Set<E>
iterator in class
AbstractCollection<E>
ConcurrentModificationException
public int size()
size in interface
Collection<E>
size in interface
Set<E>
size in class
AbstractCollection<E>
public boolean isEmpty()
isEmpty in interface
Collection<E>
isEmpty in interface
Set<E>
isEmpty in class
AbstractCollection<E>
public boolean contains(Objecto)
contains in interface
Collection<E>
contains in interface
Set<E>
contains in class
AbstractCollection<E>
o - element whose presence in this set is to be tested
public boolean add(E e)
add in interface
Collection<E>
add in interface
Set<E>
add in class
AbstractCollection<E>
e - element to be added to this set
public boolean remove(Objecto)
remove in interface
Collection<E>
remove in interface
Set<E>
remove in class
AbstractCollection<E>
o - object to be removed from this set, if present
public void clear()
public Objectclone()
public Spliterator<E> spliterator()
Spliterator over the elements in this set.
The Spliterator reports Spliterator and Spliterator. Overriding implementations should document the reporting of additional characteristic values.
spliterator in interface
Iterable<E>
spliterator in interface
Collection<E>
spliterator in interface
Set<E>
Spliterator over the elements in this set