Class HashSet<E>
- 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
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.
- Since:
- 1.2
- See Also:
- 
Constructor SummaryConstructorsConstructorDescriptionHashSet()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 SummaryModifier and TypeMethodDescriptionbooleanAdds the specified element to this set if it is not already present.voidclear()Removes all of the elements from this set.clone()Returns a shallow copy of thisHashSetinstance: the elements themselves are not cloned.booleanReturnstrueif this set contains the specified element.booleanisEmpty()Returnstrueif this set contains no elements.iterator()Returns an iterator over the elements in this set.static <T> HashSet<T> newHashSet(int numElements) Creates a new, empty HashSet suitable for the expected number of elements.booleanRemoves the specified element from this set if it is present.intsize()Returns the number of elements in this set (its cardinality).Creates a late-binding and fail-fastSpliteratorover the elements in this set.Object[]toArray()Returns an array containing all of the elements in this collection.<T> T[]toArray(T[] a) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.Methods declared in class java.util.AbstractSetequals, hashCode, removeAllMethods declared in class java.util.AbstractCollectionaddAll, containsAll, retainAll, toArray, toArray, toStringMethods declared in interface java.util.CollectionparallelStream, removeIf, stream, toArray
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Constructor Details- 
HashSetpublic HashSet()Constructs a new, empty set; the backingHashMapinstance has default initial capacity (16) and load factor (0.75).
- 
HashSetConstructs 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
 
- 
HashSetpublic HashSet(int initialCapacity, float loadFactor) Constructs a new, empty set; the backingHashMapinstance has the specified initial capacity and the specified load factor.- API Note:
- To create a HashSetwith an initial capacity that accommodates an expected number of elements, usenewHashSet.
- Parameters:
- initialCapacity- the initial capacity of the hash map
- loadFactor- the load factor of the hash map
- Throws:
- IllegalArgumentException- if the initial capacity is less than zero, or if the load factor is nonpositive
 
- 
HashSetpublic HashSet(int initialCapacity) Constructs a new, empty set; the backingHashMapinstance has the specified initial capacity and default load factor (0.75).- API Note:
- To create a HashSetwith an initial capacity that accommodates an expected number of elements, usenewHashSet.
- Parameters:
- initialCapacity- the initial capacity of the hash table
- Throws:
- IllegalArgumentException- if the initial capacity is less than zero
 
 
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Method Details- 
iteratorReturns an iterator over the elements in this set. The elements are returned in no particular order.
- 
size
- 
isEmptypublic boolean isEmpty()Returnstrueif this set contains no elements.- Specified by:
- isEmptyin interface- Collection<E>
- Specified by:
- isEmptyin interface- Set<E>
- Overrides:
- isEmptyin class- AbstractCollection<E>
- Returns:
- trueif this set contains no elements
 
- 
containsReturnstrueif this set contains the specified element. More formally, returnstrueif and only if this set contains an elementesuch thatObjects.equals(o, e).- Specified by:
- containsin interface- Collection<E>
- Specified by:
- containsin interface- Set<E>
- Overrides:
- containsin class- AbstractCollection<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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addAdds 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 interface- Collection<E>
- Specified by:
- addin interface- Set<E>
- Overrides:
- addin class- AbstractCollection<E>
- Parameters:
- e- element to be added to this set
- Returns:
- trueif this set did not already contain the specified element
 
- 
removeRemoves 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 interface- Collection<E>
- Specified by:
- removein interface- Set<E>
- Overrides:
- removein class- AbstractCollection<E>
- Parameters:
- o- object to be removed from this set, if present
- Returns:
- trueif the set contained the specified element
 
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clearpublic void clear()Removes all of the elements from this set. The set will be empty after this call returns.- Specified by:
- clearin interface- Collection<E>
- Specified by:
- clearin interface- Set<E>
- Overrides:
- clearin class- AbstractCollection<E>
 
- 
clone
- 
spliteratorCreates 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 interface- Collection<E>
- Specified by:
- spliteratorin interface- Iterable<E>
- Specified by:
- spliteratorin interface- Set<E>
- Returns:
- a Spliteratorover the elements in this set
- Since:
- 1.8
 
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toArrayDescription copied from class:AbstractCollectionReturns an array containing all of the elements in this collection. 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. The returned array's runtime component type isObject.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. - Specified by:
- toArrayin interface- Collection<E>
- Specified by:
- toArrayin interface- Set<E>
- Overrides:
- toArrayin class- AbstractCollection<E>
- Returns:
- an array, whose runtime component
 type is Object, containing all of the elements in this collection
 
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toArraypublic <T> T[] toArray(T[] a) Description copied from class:AbstractCollectionReturns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection.If 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 anynullelements.)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. - Specified by:
- toArrayin interface- Collection<E>
- Specified by:
- toArrayin interface- Set<E>
- Overrides:
- toArrayin class- AbstractCollection<E>
- Type Parameters:
- T- the component type of the array to contain the collection
- Parameters:
- a- 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.
- Returns:
- an array containing all of the elements in this collection
 
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newHashSetCreates a new, empty HashSet suitable for the expected number of elements. The returned set uses the default load factor of 0.75, and its initial capacity is generally large enough so that the expected number of elements can be added without resizing the set.- Type Parameters:
- T- the type of elements maintained by the new set
- Parameters:
- numElements- the expected number of elements
- Returns:
- the newly created set
- Throws:
- IllegalArgumentException- if numElements is negative
- Since:
- 19
 
 
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