E - the type of elements maintained by this setpublic 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<? extends E> c)
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 o)
Returns true if this set contains the specified element.
|
boolean |
isEmpty()
Returns true if this set contains no elements.
|
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).
|
equals, hashCode, removeAlladdAll, containsAll, retainAll, toArray, toArray, toStringfinalize, getClass, notify, notifyAll, wait, wait, waitaddAll, containsAll, equals, hashCode, removeAll, retainAll, toArray, toArrayparallelStream, removeIf, spliterator, streampublic HashSet()
public HashSet(Collection<? extends E> c)
c - the collection whose elements are to be placed into this setNullPointerException - if the specified collection is nullpublic HashSet(int initialCapacity,
float loadFactor)
initialCapacity - the initial capacity of the hash maploadFactor - the load factor of the hash mapIllegalArgumentException - if the initial capacity is less
than zero, or if the load factor is nonpositivepublic HashSet(int initialCapacity)
initialCapacity - the initial capacity of the hash tableIllegalArgumentException - if the initial capacity is less
than zeropublic Iterator<E> iterator()
iterator in interface Iterable<E>iterator in interface Collection<E>iterator in interface Set<E>iterator in class AbstractCollection<E>ConcurrentModificationExceptionpublic 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(Object o)
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 testedpublic 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 setpublic boolean remove(Object o)
remove in interface Collection<E>remove in interface Set<E>remove in class AbstractCollection<E>o - object to be removed from this set, if presentpublic void clear()
clear in interface Collection<E>clear in interface Set<E>clear in class AbstractCollection<E>Copyright © 2025 API Design. All Rights Reserved.