- java.lang.Object
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- java.util.AbstractCollection<E>
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- java.util.AbstractSet<E>
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- java.util.HashSet<E>
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- 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 theSetinterface, backed by a hash table (actually aHashMapinstance). 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 thenullelement.This class offers constant time performance for the basic operations (
add,remove,containsandsize), assuming the hash function disperses the elements properly among the buckets. Iterating over this set requires time proportional to the sum of theHashSetinstance's size (the number of elements) plus the "capacity" of the backingHashMapinstance (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.synchronizedSetmethod. 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
iteratormethod are fail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator's ownremovemethod, the Iterator throws aConcurrentModificationException. 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
ConcurrentModificationExceptionon 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
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Constructor Summary
Constructors Constructor Description HashSet()Constructs a new, empty set; the backingHashMapinstance has default initial capacity (16) and load factor (0.75).HashSet(int initialCapacity)Constructs a new, empty set; the backingHashMapinstance has the specified initial capacity and default load factor (0.75).HashSet(int initialCapacity, float loadFactor)Constructs a new, empty set; the backingHashMapinstance has the specified initial capacity and the specified load factor.HashSet(Collection<? extends E> c)Constructs a new set containing the elements in the specified collection.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description booleanadd(E e)Adds the specified element to this set if it is not already present.voidclear()Removes all of the elements from this set.Objectclone()Returns a shallow copy of thisHashSetinstance: the elements themselves are not cloned.booleancontains(Object o)Returnstrueif this set contains the specified element.booleanisEmpty()Returnstrueif this set contains no elements.Iterator<E>iterator()Returns an iterator over the elements in this set.booleanremove(Object o)Removes the specified element from this set if it is present.intsize()Returns the number of elements in this set (its cardinality).Spliterator<E>spliterator()Creates a late-binding and fail-fastSpliteratorover the elements in this set.-
Methods declared in class java.util.AbstractSet
equals, hashCode, removeAll
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Methods declared in class java.util.AbstractCollection
addAll, containsAll, retainAll, toArray, toArray, toString
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Methods declared in interface java.util.Collection
parallelStream, removeIf, stream, toArray
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Constructor Detail
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HashSet
public HashSet()
Constructs a new, empty set; the backingHashMapinstance has default initial capacity (16) and load factor (0.75).
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HashSet
public HashSet(Collection<? extends E> c)
Constructs a new set containing the elements in the specified collection. TheHashMapis 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
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HashSet
public HashSet(int initialCapacity, float loadFactor)Constructs a new, empty set; the backingHashMapinstance has the specified initial capacity and the specified load factor.- Parameters:
initialCapacity- the initial capacity of the hash maploadFactor- the load factor of the hash map- Throws:
IllegalArgumentException- if the initial capacity is less than zero, or if the load factor is nonpositive
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HashSet
public HashSet(int initialCapacity)
Constructs a new, empty set; the backingHashMapinstance 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
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Method Detail
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iterator
public Iterator<E> iterator()
Returns an iterator over the elements in this set. The elements are returned in no particular order.- Specified by:
iteratorin interfaceCollection<E>- Specified by:
iteratorin interfaceIterable<E>- Specified by:
iteratorin interfaceSet<E>- Specified by:
iteratorin classAbstractCollection<E>- Returns:
- an Iterator over the elements in this set
- See Also:
ConcurrentModificationException
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size
public int size()
Returns the number of elements in this set (its cardinality).
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isEmpty
public boolean isEmpty()
Returnstrueif this set contains no elements.- Specified by:
isEmptyin interfaceCollection<E>- Specified by:
isEmptyin interfaceSet<E>- Overrides:
isEmptyin classAbstractCollection<E>- Returns:
trueif this set contains no elements
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contains
public boolean contains(Object o)
Returnstrueif this set contains the specified element. More formally, returnstrueif and only if this set contains an elementesuch thatObjects.equals(o, e).- Specified by:
containsin interfaceCollection<E>- Specified by:
containsin interfaceSet<E>- Overrides:
containsin classAbstractCollection<E>- Parameters:
o- element whose presence in this set is to be tested- Returns:
trueif this set contains the specified element
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add
public boolean add(E e)
Adds the specified element to this set if it is not already present. More formally, adds the specified elementeto this set if this set contains no elemente2such thatObjects.equals(e, e2). If this set already contains the element, the call leaves the set unchanged and returnsfalse.- Specified by:
addin interfaceCollection<E>- Specified by:
addin interfaceSet<E>- Overrides:
addin classAbstractCollection<E>- Parameters:
e- element to be added to this set- Returns:
trueif this set did not already contain the specified element
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remove
public boolean remove(Object o)
Removes the specified element from this set if it is present. More formally, removes an elementesuch thatObjects.equals(o, e), if this set contains such an element. Returnstrueif 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:
removein interfaceCollection<E>- Specified by:
removein interfaceSet<E>- Overrides:
removein classAbstractCollection<E>- Parameters:
o- object to be removed from this set, if present- Returns:
trueif the set contained the specified element
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clear
public void clear()
Removes all of the elements from this set. The set will be empty after this call returns.- Specified by:
clearin interfaceCollection<E>- Specified by:
clearin interfaceSet<E>- Overrides:
clearin classAbstractCollection<E>
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clone
public Object clone()
Returns a shallow copy of thisHashSetinstance: the elements themselves are not cloned.
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spliterator
public Spliterator<E> spliterator()
Creates a late-binding and fail-fastSpliteratorover the elements in this set.The
SpliteratorreportsSpliterator.SIZEDandSpliterator.DISTINCT. Overriding implementations should document the reporting of additional characteristic values.- Specified by:
spliteratorin interfaceCollection<E>- Specified by:
spliteratorin interfaceIterable<E>- Specified by:
spliteratorin interfaceSet<E>- Returns:
- a
Spliteratorover the elements in this set - Since:
- 1.8
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