- 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>,- SequencedCollection<E>,- SequencedSet<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
SequencedCollection interface.
 The JDK does not provide any direct
 implementations of this interface: it provides implementations of more
 specific subinterfaces like Set and List.  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 Collection implementation classes (which
 typically implement Collection indirectly 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 type Collection, 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-purpose Collection
 implementations 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
 NullPointerException or ClassCastException.  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 equals method.  For example,
 the specification for the contains(Object o)
 method says: "returns true if and only if this collection
 contains at least one element e such that
 (o==null ? e==null : o.equals(e))."  This specification should
 not be construed to imply that invoking Collection.contains
 with a non-null argument o will cause o.equals(e) to be
 invoked for any element e.  Implementations are free to implement
 optimizations whereby the equals invocation is avoided, for
 example, by first comparing the hash codes of the two elements.  (The
 Object.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 underlying Object methods 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() and toString()
 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, and
 Collections.unmodifiableCollection.
 Other examples of view collections include collections that provide a
 different representation of the same elements, for example, as
 provided by List.subList,
 NavigableSet.subSet,
 Map.entrySet, or
 SequencedCollection.reversed.
 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 of
 Iterator and ListIterator can 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
 UnsupportedOperationException if this collection implementation
 does not support the operation. Such methods should (but are not required
 to) throw an UnsupportedOperationException if the invocation would
 have no effect on the collection. For example, consider a collection that
 does not support the add operation. What will happen if the
 addAll method 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 by
 List.subList is 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 the
 Collections.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.
Serializability of Collections
Serializability of collections is optional. As such, none of the collections
 interfaces are declared to implement the Serializable interface.
 However, serializability is regarded as being generally useful, so most collection
 implementations are serializable.
 
The collection implementations that are public classes (such as ArrayList
 or HashMap) are declared to implement the Serializable interface if they
 are in fact serializable. Some collections implementations are not public classes,
 such as the unmodifiable collections. In such cases, the
 serializability of such collections is described in the specification of the method
 that creates them, or in some other suitable place. In cases where the serializability
 of a collection is not specified, there is no guarantee about the serializability of such
 collections. In particular, many view collections are not serializable,
 even if the original collection is serializable.
 
A collection implementation that implements the Serializable interface cannot
 be guaranteed to be serializable. The reason is that in general, collections
 contain elements of other types, and it is not possible to determine statically
 whether instances of some element type are actually serializable. For example, consider
 a serializable Collection<E>, where E does not implement the
 Serializable interface. The collection may be serializable, if it contains only
 elements of some serializable subtype of E, or if it is empty. Collections are
 thus said to be conditionally serializable, as the serializability of the collection
 as a whole depends on whether the collection itself is serializable and on whether all
 contained elements are also serializable.
 
An additional case occurs with instances of SortedSet and SortedMap.
 These collections can be created with a Comparator that imposes an ordering on
 the set elements or map keys. Such a collection is serializable only if the provided
 Comparator is also serializable.
 
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:
- 
Method SummaryModifier and TypeMethodDescriptionbooleanEnsures 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).booleanReturnstrueif this collection contains the specified element.booleancontainsAll(Collection<?> c) Returnstrueif this collection contains all of the elements in the specified collection.booleanCompares 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()Returns an iterator over the elements in this collection.Returns a possibly parallelStreamwith this collection as its source.booleanRemoves 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 booleanRemoves 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> Creates aSpliteratorover the elements in this collection.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.
- 
Method Details- 
sizeint 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
 
- 
isEmptyboolean isEmpty()Returnstrueif this collection contains no elements.- Returns:
- trueif this collection contains no elements
 
- 
containsReturnstrueif 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)
 
- 
iteratorReturns 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).
- 
toArrayObject[] 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
 
- 
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 array
- NullPointerException- if the specified array is null
 
- 
toArrayReturns 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 array
- NullPointerException- if the generator function is null
- Since:
- 11
 
- 
addEnsures 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 the- addoperation is not supported by this collection
- ClassCastException- if the class of the specified element prevents it from being added to this collection
- NullPointerException- if the specified element is null and this collection does not permit null elements
- IllegalArgumentException- if some property of the element prevents it from being added to this collection
- IllegalStateException- if the element cannot be added at this time due to insertion restrictions
 
- 
removeRemoves 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 the- removeoperation is not supported by this collection
 
- 
containsAllReturnstrueif 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:
 
- 
addAllAdds 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.) If the specified collection has a defined encounter order, processing of its elements generally occurs in that order.- 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 the- addAlloperation is not supported by this collection
- ClassCastException- if the class of an element of the specified collection prevents it from being added to this collection
- NullPointerException- if the specified collection contains a null element and this collection does not permit null elements, or if the specified collection is null
- IllegalArgumentException- if some property of an element of the specified collection prevents it from being added to this collection
- IllegalStateException- if not all the elements can be added at this time due to insertion restrictions
- See Also:
 
- 
removeAllRemoves 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 the- removeAllmethod is not supported by this collection
- ClassCastException- 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:
 
- 
removeIfRemoves 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 returns- truefor elements to be removed
- Returns:
- trueif any elements were removed
- Throws:
- NullPointerException- if the specified filter is null
- UnsupportedOperationException- 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
 
- 
retainAllRetains 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 the- retainAlloperation is not supported by this collection
- ClassCastException- 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:
 
- 
clearvoid clear()Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns.- Throws:
- UnsupportedOperationException- if the- clearoperation is not supported by this collection
 
- 
equalsCompares 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.)
- 
hashCodeint 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().
- 
spliteratorCreates 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 interface- Iterable<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
 
- 
streamReturns 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
 
- 
parallelStreamReturns 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
 
 
-