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- Type Parameters:
E- the type of elements in this collection
- All Superinterfaces:
Iterable<E>
- All Known Subinterfaces:
BeanContext,BeanContextServices,BlockingDeque<E>,BlockingQueue<E>,Deque<E>,EventSet,List<E>,NavigableSet<E>,Queue<E>,Set<E>,SortedSet<E>,TransferQueue<E>
- All Known Implementing Classes:
AbstractCollection,AbstractList,AbstractQueue,AbstractSequentialList,AbstractSet,ArrayBlockingQueue,ArrayDeque,ArrayList,AttributeList,BeanContextServicesSupport,BeanContextSupport,ConcurrentHashMap.KeySetView,ConcurrentLinkedDeque,ConcurrentLinkedQueue,ConcurrentSkipListSet,CopyOnWriteArrayList,CopyOnWriteArraySet,DelayQueue,EnumSet,HashSet,JobStateReasons,LinkedBlockingDeque,LinkedBlockingQueue,LinkedHashSet,LinkedList,LinkedTransferQueue,PriorityBlockingQueue,PriorityQueue,RoleList,RoleUnresolvedList,Stack,SynchronousQueue,TreeSet,Vector
public interface Collection<E> extends Iterable<E>
The root interface in the collection hierarchy. A collection represents a group of objects, known as its elements. Some collections allow duplicate elements and others do not. Some are ordered and others unordered. The JDK does not provide any direct implementations of this interface: it provides implementations of more specific subinterfaces likeSetandList. This interface is typically used to pass collections around and manipulate them where maximum generality is desired.Bags or multisets (unordered collections that may contain duplicate elements) should implement this interface directly.
All general-purpose
Collectionimplementation classes (which typically implementCollectionindirectly through one of its subinterfaces) should provide two "standard" constructors: a void (no arguments) constructor, which creates an empty collection, and a constructor with a single argument of typeCollection, which creates a new collection with the same elements as its argument. In effect, the latter constructor allows the user to copy any collection, producing an equivalent collection of the desired implementation type. There is no way to enforce this convention (as interfaces cannot contain constructors) but all of the general-purposeCollectionimplementations in the Java platform libraries comply.Certain methods are specified to be optional. If a collection implementation doesn't implement a particular operation, it should define the corresponding method to throw
UnsupportedOperationException. Such methods are marked "optional operation" in method specifications of the collections interfaces.Some collection implementations have restrictions on the elements that they may contain. For example, some implementations prohibit null elements, and some have restrictions on the types of their elements. Attempting to add an ineligible element throws an unchecked exception, typically
NullPointerExceptionorClassCastException. Attempting to query the presence of an ineligible element may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible element whose completion would not result in the insertion of an ineligible element into the collection may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface.It is up to each collection to determine its own synchronization policy. In the absence of a stronger guarantee by the implementation, undefined behavior may result from the invocation of any method on a collection that is being mutated by another thread; this includes direct invocations, passing the collection to a method that might perform invocations, and using an existing iterator to examine the collection.
Many methods in Collections Framework interfaces are defined in terms of the
equalsmethod. For example, the specification for thecontains(Object o)method says: "returnstrueif and only if this collection contains at least one elementesuch that(o==null ? e==null : o.equals(e))." This specification should not be construed to imply that invokingCollection.containswith a non-null argumentowill causeo.equals(e)to be invoked for any elemente. Implementations are free to implement optimizations whereby theequalsinvocation is avoided, for example, by first comparing the hash codes of the two elements. (TheObject.hashCode()specification guarantees that two objects with unequal hash codes cannot be equal.) More generally, implementations of the various Collections Framework interfaces are free to take advantage of the specified behavior of underlyingObjectmethods wherever the implementor deems it appropriate.Some collection operations which perform recursive traversal of the collection may fail with an exception for self-referential instances where the collection directly or indirectly contains itself. This includes the
clone(),equals(),hashCode()andtoString()methods. Implementations may optionally handle the self-referential scenario, however most current implementations do not do so.View Collections
Most collections manage storage for elements they contain. By contrast, view collections themselves do not store elements, but instead they rely on a backing collection to store the actual elements. Operations that are not handled by the view collection itself are delegated to the backing collection. Examples of view collections include the wrapper collections returned by methods such as
Collections.checkedCollection,Collections.synchronizedCollection, andCollections.unmodifiableCollection. Other examples of view collections include collections that provide a different representation of the same elements, for example, as provided byList.subList,NavigableSet.subSet, orMap.entrySet. Any changes made to the backing collection are visible in the view collection. Correspondingly, any changes made to the view collection — if changes are permitted — are written through to the backing collection. Although they technically aren't collections, instances ofIteratorandListIteratorcan also allow modifications to be written through to the backing collection, and in some cases, modifications to the backing collection will be visible to the Iterator during iteration.Unmodifiable Collections
Certain methods of this interface are considered "destructive" and are called "mutator" methods in that they modify the group of objects contained within the collection on which they operate. They can be specified to throw
UnsupportedOperationExceptionif this collection implementation does not support the operation. Such methods should (but are not required to) throw anUnsupportedOperationExceptionif the invocation would have no effect on the collection. For example, consider a collection that does not support theaddoperation. What will happen if theaddAllmethod is invoked on this collection, with an empty collection as the argument? The addition of zero elements has no effect, so it is permissible for this collection simply to do nothing and not to throw an exception. However, it is recommended that such cases throw an exception unconditionally, as throwing only in certain cases can lead to programming errors.An unmodifiable collection is a collection, all of whose mutator methods (as defined above) are specified to throw
UnsupportedOperationException. Such a collection thus cannot be modified by calling any methods on it. For a collection to be properly unmodifiable, any view collections derived from it must also be unmodifiable. For example, if a List is unmodifiable, the List returned byList.subListis also unmodifiable.An unmodifiable collection is not necessarily immutable. If the contained elements are mutable, the entire collection is clearly mutable, even though it might be unmodifiable. For example, consider two unmodifiable lists containing mutable elements. The result of calling
list1.equals(list2)might differ from one call to the next if the elements had been mutated, even though both lists are unmodifiable. However, if an unmodifiable collection contains all immutable elements, it can be considered effectively immutable.Unmodifiable View Collections
An unmodifiable view collection is a collection that is unmodifiable and that is also a view onto a backing collection. Its mutator methods throw
UnsupportedOperationException, as described above, while reading and querying methods are delegated to the backing collection. The effect is to provide read-only access to the backing collection. This is useful for a component to provide users with read access to an internal collection, while preventing them from modifying such collections unexpectedly. Examples of unmodifiable view collections are those returned by theCollections.unmodifiableCollection,Collections.unmodifiableList, and related methods.Note that changes to the backing collection might still be possible, and if they occur, they are visible through the unmodifiable view. Thus, an unmodifiable view collection is not necessarily immutable. However, if the backing collection of an unmodifiable view is effectively immutable, or if the only reference to the backing collection is through an unmodifiable view, the view can be considered effectively immutable.
This interface is a member of the Java Collections Framework.
- Implementation Requirements:
- The default method implementations (inherited or otherwise) do not apply any
synchronization protocol. If a
Collectionimplementation has a specific synchronization protocol, then it must override default implementations to apply that protocol. - Since:
- 1.2
- See Also:
Set,List,Map,SortedSet,SortedMap,HashSet,TreeSet,ArrayList,LinkedList,Vector,Collections,Arrays,AbstractCollection
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Method Summary
All Methods Instance Methods Abstract Methods Default Methods Modifier and Type Method Description booleanadd(E e)Ensures that this collection contains the specified element (optional operation).booleanaddAll(Collection<? extends E> c)Adds all of the elements in the specified collection to this collection (optional operation).voidclear()Removes all of the elements from this collection (optional operation).booleancontains(Object o)Returnstrueif this collection contains the specified element.booleancontainsAll(Collection<?> c)Returnstrueif this collection contains all of the elements in the specified collection.booleanequals(Object o)Compares the specified object with this collection for equality.inthashCode()Returns the hash code value for this collection.booleanisEmpty()Returnstrueif this collection contains no elements.Iterator<E>iterator()Returns an iterator over the elements in this collection.default Stream<E>parallelStream()Returns a possibly parallelStreamwith this collection as its source.booleanremove(Object o)Removes a single instance of the specified element from this collection, if it is present (optional operation).booleanremoveAll(Collection<?> c)Removes all of this collection's elements that are also contained in the specified collection (optional operation).default booleanremoveIf(Predicate<? super E> filter)Removes all of the elements of this collection that satisfy the given predicate.booleanretainAll(Collection<?> c)Retains only the elements in this collection that are contained in the specified collection (optional operation).intsize()Returns the number of elements in this collection.default Spliterator<E>spliterator()Creates aSpliteratorover the elements in this collection.default Stream<E>stream()Returns a sequentialStreamwith this collection as its source.Object[]toArray()Returns an array containing all of the elements in this collection.default <T> T[]toArray(IntFunction<T[]> generator)Returns an array containing all of the elements in this collection, using the providedgeneratorfunction to allocate the returned array.<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.
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Method Detail
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size
int size()
Returns the number of elements in this collection. If this collection contains more thanInteger.MAX_VALUEelements, returnsInteger.MAX_VALUE.- Returns:
- the number of elements in this collection
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isEmpty
boolean isEmpty()
Returnstrueif this collection contains no elements.- Returns:
trueif this collection contains no elements
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contains
boolean contains(Object o)
Returnstrueif this collection contains the specified element. More formally, returnstrueif and only if this collection contains at least one elementesuch thatObjects.equals(o, e).- Parameters:
o- element whose presence in this collection is to be tested- Returns:
trueif this collection contains the specified element- Throws:
ClassCastException- if the type of the specified element is incompatible with this collection (optional)NullPointerException- if the specified element is null and this collection does not permit null elements (optional)
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iterator
Iterator<E> iterator()
Returns an iterator over the elements in this collection. There are no guarantees concerning the order in which the elements are returned (unless this collection is an instance of some class that provides a guarantee).
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toArray
Object[] toArray()
Returns 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.
- API Note:
- This method acts as a bridge between array-based and collection-based APIs.
It returns an array whose runtime type is
Object[]. UsetoArray(T[])to reuse an existing array, or usetoArray(IntFunction)to control the runtime type of the array. - Returns:
- an array, whose runtime component
type is
Object, containing all of the elements in this collection
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toArray
<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. 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.
- API Note:
- This method acts as a bridge between array-based and collection-based APIs.
It allows an existing array to be reused under certain circumstances.
Use
toArray()to create an array whose runtime type isObject[], or usetoArray(IntFunction)to control the runtime type of the array.Suppose
xis a collection known to contain only strings. The following code can be used to dump the collection into a previously allocatedStringarray:String[] y = new String[SIZE]; ... y = x.toArray(y);The return value is reassigned to the variable
y, because a new array will be allocated and returned if the collectionxhas too many elements to fit into the existing arrayy.Note that
toArray(new Object[0])is identical in function totoArray(). - 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
- Throws:
ArrayStoreException- if the runtime type of any element in this collection is not assignable to the runtime component type of the specified arrayNullPointerException- if the specified array is null
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toArray
default <T> T[] toArray(IntFunction<T[]> generator)
Returns an array containing all of the elements in this collection, using the providedgeneratorfunction to allocate the returned array.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.
- API Note:
- This method acts as a bridge between array-based and collection-based APIs.
It allows creation of an array of a particular runtime type. Use
toArray()to create an array whose runtime type isObject[], or usetoArray(T[])to reuse an existing array.Suppose
xis a collection known to contain only strings. The following code can be used to dump the collection into a newly allocated array ofString:String[] y = x.toArray(String[]::new); - Implementation Requirements:
- The default implementation calls the generator function with zero
and then passes the resulting array to
toArray(T[]). - Type Parameters:
T- the component type of the array to contain the collection- Parameters:
generator- a function which produces a new array of the desired type and the provided length- Returns:
- an array containing all of the elements in this collection
- Throws:
ArrayStoreException- if the runtime type of any element in this collection is not assignable to the runtime component type of the generated arrayNullPointerException- if the generator function is null- Since:
- 11
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add
boolean add(E e)
Ensures that this collection contains the specified element (optional operation). Returnstrueif this collection changed as a result of the call. (Returnsfalseif this collection does not permit duplicates and already contains the specified element.)Collections that support this operation may place limitations on what elements may be added to this collection. In particular, some collections will refuse to add
nullelements, and others will impose restrictions on the type of elements that may be added. Collection classes should clearly specify in their documentation any restrictions on what elements may be added.If a collection refuses to add a particular element for any reason other than that it already contains the element, it must throw an exception (rather than returning
false). This preserves the invariant that a collection always contains the specified element after this call returns.- Parameters:
e- element whose presence in this collection is to be ensured- Returns:
trueif this collection changed as a result of the call- Throws:
UnsupportedOperationException- if theaddoperation is not supported by this collectionClassCastException- if the class of the specified element prevents it from being added to this collectionNullPointerException- if the specified element is null and this collection does not permit null elementsIllegalArgumentException- if some property of the element prevents it from being added to this collectionIllegalStateException- if the element cannot be added at this time due to insertion restrictions
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remove
boolean remove(Object o)
Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an elementesuch thatObjects.equals(o, e), if this collection contains one or more such elements. Returnstrueif this collection contained the specified element (or equivalently, if this collection changed as a result of the call).- Parameters:
o- element to be removed from this collection, if present- Returns:
trueif an element was removed as a result of this call- Throws:
ClassCastException- if the type of the specified element is incompatible with this collection (optional)NullPointerException- if the specified element is null and this collection does not permit null elements (optional)UnsupportedOperationException- if theremoveoperation is not supported by this collection
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containsAll
boolean containsAll(Collection<?> c)
Returnstrueif this collection contains all of the elements in the specified collection.- Parameters:
c- collection to be checked for containment in this collection- Returns:
trueif this collection contains all of the elements in the specified collection- Throws:
ClassCastException- if the types of one or more elements in the specified collection are incompatible with this collection (optional)NullPointerException- if the specified collection contains one or more null elements and this collection does not permit null elements (optional), or if the specified collection is null.- See Also:
contains(Object)
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addAll
boolean addAll(Collection<? extends E> c)
Adds all of the elements in the specified collection to this collection (optional operation). The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. (This implies that the behavior of this call is undefined if the specified collection is this collection, and this collection is nonempty.)- Parameters:
c- collection containing elements to be added to this collection- Returns:
trueif this collection changed as a result of the call- Throws:
UnsupportedOperationException- if theaddAlloperation is not supported by this collectionClassCastException- if the class of an element of the specified collection prevents it from being added to this collectionNullPointerException- if the specified collection contains a null element and this collection does not permit null elements, or if the specified collection is nullIllegalArgumentException- if some property of an element of the specified collection prevents it from being added to this collectionIllegalStateException- if not all the elements can be added at this time due to insertion restrictions- See Also:
add(Object)
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removeAll
boolean removeAll(Collection<?> c)
Removes all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.- Parameters:
c- collection containing elements to be removed from this collection- Returns:
trueif this collection changed as a result of the call- Throws:
UnsupportedOperationException- if theremoveAllmethod is not supported by this collectionClassCastException- if the types of one or more elements in this collection are incompatible with the specified collection (optional)NullPointerException- if this collection contains one or more null elements and the specified collection does not support null elements (optional), or if the specified collection is null- See Also:
remove(Object),contains(Object)
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removeIf
default boolean removeIf(Predicate<? super E> filter)
Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.- Implementation Requirements:
- The default implementation traverses all elements of the collection using
its
iterator(). Each matching element is removed usingIterator.remove(). If the collection's iterator does not support removal then anUnsupportedOperationExceptionwill be thrown on the first matching element. - Parameters:
filter- a predicate which returnstruefor elements to be removed- Returns:
trueif any elements were removed- Throws:
NullPointerException- if the specified filter is nullUnsupportedOperationException- if elements cannot be removed from this collection. Implementations may throw this exception if a matching element cannot be removed or if, in general, removal is not supported.- Since:
- 1.8
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retainAll
boolean retainAll(Collection<?> c)
Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.- Parameters:
c- collection containing elements to be retained in this collection- Returns:
trueif this collection changed as a result of the call- Throws:
UnsupportedOperationException- if theretainAlloperation is not supported by this collectionClassCastException- if the types of one or more elements in this collection are incompatible with the specified collection (optional)NullPointerException- if this collection contains one or more null elements and the specified collection does not permit null elements (optional), or if the specified collection is null- See Also:
remove(Object),contains(Object)
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clear
void clear()
Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns.- Throws:
UnsupportedOperationException- if theclearoperation is not supported by this collection
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equals
boolean equals(Object o)
Compares the specified object with this collection for equality.While the
Collectioninterface adds no stipulations to the general contract for theObject.equals, programmers who implement theCollectioninterface "directly" (in other words, create a class that is aCollectionbut is not aSetor aList) must exercise care if they choose to override theObject.equals. It is not necessary to do so, and the simplest course of action is to rely onObject's implementation, but the implementor may wish to implement a "value comparison" in place of the default "reference comparison." (TheListandSetinterfaces mandate such value comparisons.)The general contract for the
Object.equalsmethod states that equals must be symmetric (in other words,a.equals(b)if and only ifb.equals(a)). The contracts forList.equalsandSet.equalsstate that lists are only equal to other lists, and sets to other sets. Thus, a customequalsmethod for a collection class that implements neither theListnorSetinterface must returnfalsewhen this collection is compared to any list or set. (By the same logic, it is not possible to write a class that correctly implements both theSetandListinterfaces.)- Overrides:
equalsin classObject- Parameters:
o- object to be compared for equality with this collection- Returns:
trueif the specified object is equal to this collection- See Also:
Object.equals(Object),Set.equals(Object),List.equals(Object)
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hashCode
int hashCode()
Returns the hash code value for this collection. While theCollectioninterface adds no stipulations to the general contract for theObject.hashCodemethod, programmers should take note that any class that overrides theObject.equalsmethod must also override theObject.hashCodemethod in order to satisfy the general contract for theObject.hashCodemethod. In particular,c1.equals(c2)implies thatc1.hashCode()==c2.hashCode().- Overrides:
hashCodein classObject- Returns:
- the hash code value for this collection
- See Also:
Object.hashCode(),Object.equals(Object)
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spliterator
default Spliterator<E> spliterator()
Creates aSpliteratorover the elements in this collection. Implementations should document characteristic values reported by the spliterator. Such characteristic values are not required to be reported if the spliterator reportsSpliterator.SIZEDand this collection contains no elements.The default implementation should be overridden by subclasses that can return a more efficient spliterator. In order to preserve expected laziness behavior for the
stream()andparallelStream()methods, spliterators should either have the characteristic ofIMMUTABLEorCONCURRENT, or be late-binding. If none of these is practical, the overriding class should describe the spliterator's documented policy of binding and structural interference, and should override thestream()andparallelStream()methods to create streams using aSupplierof the spliterator, as in:Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)These requirements ensure that streams produced by the
stream()andparallelStream()methods will reflect the contents of the collection as of initiation of the terminal stream operation.- Specified by:
spliteratorin interfaceIterable<E>- Implementation Requirements:
- The default implementation creates a
late-binding spliterator
from the collection's
Iterator. The spliterator inherits the fail-fast properties of the collection's iterator.The created
SpliteratorreportsSpliterator.SIZED. - Implementation Note:
- The created
Spliteratoradditionally reportsSpliterator.SUBSIZED.If a spliterator covers no elements then the reporting of additional characteristic values, beyond that of
SIZEDandSUBSIZED, does not aid clients to control, specialize or simplify computation. However, this does enable shared use of an immutable and empty spliterator instance (seeSpliterators.emptySpliterator()) for empty collections, and enables clients to determine if such a spliterator covers no elements. - Returns:
- a
Spliteratorover the elements in this collection - Since:
- 1.8
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stream
default Stream<E> stream()
Returns a sequentialStreamwith this collection as its source.This method should be overridden when the
spliterator()method cannot return a spliterator that isIMMUTABLE,CONCURRENT, or late-binding. (Seespliterator()for details.)- Implementation Requirements:
- The default implementation creates a sequential
Streamfrom the collection'sSpliterator. - Returns:
- a sequential
Streamover the elements in this collection - Since:
- 1.8
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parallelStream
default Stream<E> parallelStream()
Returns a possibly parallelStreamwith this collection as its source. It is allowable for this method to return a sequential stream.This method should be overridden when the
spliterator()method cannot return a spliterator that isIMMUTABLE,CONCURRENT, or late-binding. (Seespliterator()for details.)- Implementation Requirements:
- The default implementation creates a parallel
Streamfrom the collection'sSpliterator. - Returns:
- a possibly parallel
Streamover the elements in this collection - Since:
- 1.8
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