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 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,
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, toStringpublic 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)
initialCapacity - the initial capacity of the hash tableIllegalArgumentException - if the initial capacity is less than zeropublic 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 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 void clear()
clear in interface Collection<E>clear in interface Set<E>clear in class AbstractCollection<E>public Object clone()
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 isEmpty()
isEmpty in interface Collection<E>isEmpty in interface Set<E>isEmpty in class AbstractCollection<E>public Iterator<E> iterator()
iterator in interface Iterable<E>iterator in interface Collection<E>iterator in interface Set<E>iterator in class AbstractCollection<E>ConcurrentModificationExceptionpublic 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 int size()
size in interface Collection<E>size in interface Set<E>size in class AbstractCollection<E>