- 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>
,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
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
, or
Map.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 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.
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
Collection
implementation has a specific synchronization protocol, then it must override default implementations to apply that protocol. - Since:
- 1.2
- See Also:
-
Method Summary
Modifier and TypeMethodDescriptionboolean
Ensures that this collection contains the specified element (optional operation).boolean
addAll
(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation).void
clear()
Removes all of the elements from this collection (optional operation).boolean
Returnstrue
if this collection contains the specified element.boolean
containsAll
(Collection<?> c) Returnstrue
if this collection contains all of the elements in the specified collection.boolean
Compares the specified object with this collection for equality.int
hashCode()
Returns the hash code value for this collection.boolean
isEmpty()
Returnstrue
if this collection contains no elements.iterator()
Returns an iterator over the elements in this collection.Returns a possibly parallelStream
with this collection as its source.boolean
Removes a single instance of the specified element from this collection, if it is present (optional operation).boolean
removeAll
(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).default boolean
Removes all of the elements of this collection that satisfy the given predicate.boolean
retainAll
(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).int
size()
Returns the number of elements in this collection.default Spliterator<E>
Creates aSpliterator
over the elements in this collection.stream()
Returns a sequentialStream
with 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 providedgenerator
function 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
-
size
int size()Returns the number of elements in this collection. If this collection contains more thanInteger.MAX_VALUE
elements, returnsInteger.MAX_VALUE
.- Returns:
- the number of elements in this collection
-
isEmpty
boolean isEmpty()Returnstrue
if this collection contains no elements.- Returns:
true
if this collection contains no elements
-
contains
Returnstrue
if this collection contains the specified element. More formally, returnstrue
if and only if this collection contains at least one elemente
such thatObjects.equals(o, e)
.- Parameters:
o
- element whose presence in this collection is to be tested- Returns:
true
if 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)
-
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). -
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
-
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 anynull
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.
- 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
x
is a collection known to contain only strings. The following code can be used to dump the collection into a previously allocatedString
array: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 collectionx
has 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
-
toArray
Returns an array containing all of the elements in this collection, using the providedgenerator
function 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
x
is 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
-
add
Ensures that this collection contains the specified element (optional operation). Returnstrue
if this collection changed as a result of the call. (Returnsfalse
if 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
null
elements, 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:
true
if this collection changed as a result of the call- Throws:
UnsupportedOperationException
- if theadd
operation 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
-
remove
Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an elemente
such thatObjects.equals(o, e)
, if this collection contains one or more such elements. Returnstrue
if 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:
true
if 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 theremove
operation is not supported by this collection
-
containsAll
Returnstrue
if this collection contains all of the elements in the specified collection.- Parameters:
c
- collection to be checked for containment in this collection- Returns:
true
if 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:
-
addAll
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:
true
if this collection changed as a result of the call- Throws:
UnsupportedOperationException
- if theaddAll
operation 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:
-
removeAll
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:
true
if this collection changed as a result of the call- Throws:
UnsupportedOperationException
- if theremoveAll
method 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:
-
removeIf
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 anUnsupportedOperationException
will be thrown on the first matching element. - Parameters:
filter
- a predicate which returnstrue
for elements to be removed- Returns:
true
if 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
-
retainAll
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:
true
if this collection changed as a result of the call- Throws:
UnsupportedOperationException
- if theretainAll
operation 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:
-
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 theclear
operation is not supported by this collection
-
equals
Compares the specified object with this collection for equality.While the
Collection
interface adds no stipulations to the general contract for theObject.equals
, programmers who implement theCollection
interface "directly" (in other words, create a class that is aCollection
but is not aSet
or 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." (TheList
andSet
interfaces mandate such value comparisons.)The general contract for the
Object.equals
method states that equals must be symmetric (in other words,a.equals(b)
if and only ifb.equals(a)
). The contracts forList.equals
andSet.equals
state that lists are only equal to other lists, and sets to other sets. Thus, a customequals
method for a collection class that implements neither theList
norSet
interface must returnfalse
when 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 theSet
andList
interfaces.) -
hashCode
int hashCode()Returns the hash code value for this collection. While theCollection
interface adds no stipulations to the general contract for theObject.hashCode
method, programmers should take note that any class that overrides theObject.equals
method must also override theObject.hashCode
method in order to satisfy the general contract for theObject.hashCode
method. In particular,c1.equals(c2)
implies thatc1.hashCode()==c2.hashCode()
. -
spliterator
Creates aSpliterator
over 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.SIZED
and 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 ofIMMUTABLE
orCONCURRENT
, 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 aSupplier
of 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:
spliterator
in 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
Spliterator
reportsSpliterator.SIZED
. - Implementation Note:
- The created
Spliterator
additionally reportsSpliterator.SUBSIZED
.If a spliterator covers no elements then the reporting of additional characteristic values, beyond that of
SIZED
andSUBSIZED
, 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
Spliterator
over the elements in this collection - Since:
- 1.8
-
stream
Returns a sequentialStream
with 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
Stream
from the collection'sSpliterator
. - Returns:
- a sequential
Stream
over the elements in this collection - Since:
- 1.8
-
parallelStream
Returns a possibly parallelStream
with 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
Stream
from the collection'sSpliterator
. - Returns:
- a possibly parallel
Stream
over the elements in this collection - Since:
- 1.8
-