E - the type of elements maintained by this setSerializable, Cloneable, Iterable<E>, Collection<E>, Set<E>
JobStateReasons, LinkedHashSet
public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, Serializable
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.
| Constructor | Description |
|---|---|
HashSet() |
Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75). |
HashSet |
Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75). |
HashSet |
Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor. |
HashSet |
Constructs a new set containing the elements in the specified collection. |
| Modifier and Type | Method | Description |
|---|---|---|
boolean |
add |
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 |
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. |
static <T> HashSet |
newHashSet |
Creates a new, empty HashSet suitable for the expected number of elements. |
boolean |
remove |
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. |
Object[] |
toArray() |
Returns an array containing all of the elements in this collection. |
<T> T[] |
toArray |
Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. |
equals, hashCode, removeAll
addAll, containsAll, retainAll, toArray, toArray, toString
parallelStream, removeIf, stream, toArray
public HashSet()
HashMap instance has default initial capacity (16) and load factor (0.75).public HashSet(Collection<? extends E> c)
HashMap is created with default load factor (0.75) and an initial capacity sufficient to contain the elements in the specified collection.c - the collection whose elements are to be placed into this setNullPointerException - if the specified collection is nullpublic HashSet(int initialCapacity, float loadFactor)
HashMap instance has the specified initial capacity and the specified load factor.HashSet with an initial capacity that accommodates an expected number of elements, use newHashSet.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)
HashMap instance has the specified initial capacity and default load factor (0.75).HashSet with an initial capacity that accommodates an expected number of elements, use newHashSet.initialCapacity - the initial capacity of the hash tableIllegalArgumentException - if the initial capacity is less than zeropublic Iterator<E> iterator()
public int size()
public boolean isEmpty()
true if this set contains no elements.isEmpty in interface Collection<E>
isEmpty in interface Set<E>
isEmpty in class AbstractCollection<E>
true if this set contains no elementspublic boolean contains(Object o)
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).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 testedtrue if this set contains the specified elementpublic boolean add(E e)
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.add in interface Collection<E>
add in interface Set<E>
add in class AbstractCollection<E>
e - element to be added to this settrue if this set did not already contain the specified elementpublic boolean remove(Object o)
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.)remove in interface Collection<E>
remove in interface Set<E>
remove in class AbstractCollection<E>
o - object to be removed from this set, if presenttrue if the set contained the specified elementpublic void clear()
clear in interface Collection<E>
clear in interface Set<E>
clear in class AbstractCollection<E>
public Object clone()
HashSet instance: the elements themselves are not cloned.public Spliterator<E> spliterator()
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.
spliterator in interface Collection<E>
spliterator in interface Iterable<E>
spliterator in interface Set<E>
Spliterator over the elements in this setpublic Object[] toArray()
AbstractCollectionObject. 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.
toArray in interface Collection<E>
toArray in interface Set<E>
toArray in class AbstractCollection<E>
Object, containing all of the elements in this collectionpublic <T> T[] toArray(T[] a)
AbstractCollectionIf 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 any null 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.
toArray in interface Collection<E>
toArray in interface Set<E>
toArray in class AbstractCollection<E>
T - the component type of the array to contain the collectiona - 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.public static <T> HashSet<T> newHashSet(int numElements)
T - the type of elements maintained by the new setnumElements - the expected number of elementsIllegalArgumentException - if numElements is negative
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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/HashSet.html