E - the type of elements in this collectionpublic interface Collection<E> extends Iterable<E>
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.
The "destructive" methods contained in this interface, that is, the
methods that modify the collection on which they operate, are specified to
throw UnsupportedOperationException if this collection does not
support the operation. If this is the case, these methods may, but are not
required to, throw an UnsupportedOperationException if the
invocation would have no effect on the collection. For example, invoking
the addAll(Collection) method on an unmodifiable collection may,
but is not required to, throw the exception if the collection to be added
is empty.
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.
This interface is a member of the Java Collections Framework.
Collection implementation has a
specific synchronization protocol, then it must override default
implementations to apply that protocol.Set,
List,
Map,
SortedSet,
SortedMap,
HashSet,
TreeSet,
ArrayList,
LinkedList,
Vector,
Collections,
Arrays,
AbstractCollection| Modifier and Type | Method and Description |
|---|---|
boolean |
add(E e)
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 |
contains(Object o)
Returns true if this collection contains the specified element.
|
boolean |
containsAll(Collection<?> c)
Returns true if this collection contains all of the elements
in the specified collection.
|
boolean |
equals(Object o)
Compares the specified object with this collection for equality.
|
int |
hashCode()
Returns the hash code value for this collection.
|
boolean |
isEmpty()
Returns true if this collection contains no elements.
|
Iterator<E> |
iterator()
Returns an iterator over the elements in this collection.
|
default Stream<E> |
parallelStream()
Returns a possibly parallel
Stream with this collection as its
source. |
boolean |
remove(Object o)
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 |
removeIf(Predicate<? super E> filter)
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> |
spliterator()
Creates a
Spliterator over the elements in this collection. |
default Stream<E> |
stream()
Returns a sequential
Stream with this collection as its source. |
Object[] |
toArray()
Returns an array containing all of the elements in this collection.
|
<T> T[] |
toArray(T[] a)
Returns an array containing all of the elements in this collection;
the runtime type of the returned array is that of the specified array.
|
int size()
boolean isEmpty()
boolean contains(Object o)
o - element whose presence in this collection is to be testedClassCastException - 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<E> iterator()
Object[] toArray()
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.
This method acts as bridge between array-based and collection-based APIs.
<T> T[] toArray(T[] a)
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 any null 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.
Like the toArray() method, this method acts as bridge between
array-based and collection-based APIs. Further, this method allows
precise control over the runtime type of the output array, and may,
under certain circumstances, be used to save allocation costs.
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 of String:
String[] y = x.toArray(new String[0]);
Note that toArray(new Object[0]) is identical in function to
toArray().T - the runtime type of the array to contain the collectiona - 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.ArrayStoreException - if the runtime type of the specified array
is not a supertype of the runtime type of every element in
this collectionNullPointerException - if the specified array is nullboolean add(E e)
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.
e - element whose presence in this collection is to be ensuredUnsupportedOperationException - if the add 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 restrictionsboolean remove(Object o)
o - element to be removed from this collection, if presentClassCastException - 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 remove operation
is not supported by this collectionboolean containsAll(Collection<?> c)
c - collection to be checked for containment in this collectionClassCastException - 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.contains(Object)boolean addAll(Collection<? extends E> c)
c - collection containing elements to be added to this collectionUnsupportedOperationException - if the addAll 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 restrictionsadd(Object)boolean removeAll(Collection<?> c)
c - collection containing elements to be removed from this collectionUnsupportedOperationException - if the removeAll 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 nullremove(Object),
contains(Object)default boolean removeIf(Predicate<? super E> filter)
iterator(). Each matching element is removed using
Iterator.remove(). If the collection's iterator does not
support removal then an UnsupportedOperationException will be
thrown on the first matching element.filter - a predicate which returns true for elements to be
removedtrue if any elements were removedNullPointerException - 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.boolean retainAll(Collection<?> c)
c - collection containing elements to be retained in this collectionUnsupportedOperationException - if the retainAll 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 nullremove(Object),
contains(Object)void clear()
UnsupportedOperationException - if the clear operation
is not supported by this collectionboolean equals(Object o)
While the Collection interface adds no stipulations to the general contract for the Object.equals, programmers who implement the Collection interface "directly" (in other words, create a class that is a Collection but is not a Set or a List) must exercise care if they choose to override the Object.equals. It is not necessary to do so, and the simplest course of action is to rely on Object's implementation, but the implementor may wish to implement a "value comparison" in place of the default "reference comparison." (The List and Set 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 if b.equals(a)). The contracts for List.equals and Set.equals state that lists are only equal to other lists, and sets to other sets. Thus, a custom equals method for a collection class that implements neither the List nor Set interface must return false 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 the Set and List interfaces.)
equals in class Objecto - object to be compared for equality with this collectionObject.equals(Object),
Set.equals(Object),
List.equals(Object)int hashCode()
hashCode in class ObjectObject.hashCode(),
Object.equals(Object)default Spliterator<E> spliterator()
Spliterator 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 reports Spliterator.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() and
parallelStream()} methods, spliterators should either have the
characteristic of IMMUTABLE or CONCURRENT, 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 the stream() and parallelStream()
methods to create streams using a Supplier of the spliterator,
as in:
Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
These requirements ensure that streams produced by the
stream() and parallelStream() methods will reflect the
contents of the collection as of initiation of the terminal stream
operation.
spliterator in interface Iterable<E>Iterator. The spliterator inherits the
fail-fast properties of the collection's iterator.
The created Spliterator reports Spliterator.SIZED.
Spliterator additionally reports
Spliterator.SUBSIZED.
If a spliterator covers no elements then the reporting of additional
characteristic values, beyond that of SIZED and SUBSIZED,
does not aid clients to control, specialize or simplify computation.
However, this does enable shared use of an immutable and empty
spliterator instance (see Spliterators.emptySpliterator()) for
empty collections, and enables clients to determine if such a spliterator
covers no elements.
Spliterator over the elements in this collectiondefault Stream<E> stream()
Stream with this collection as its source.
This method should be overridden when the spliterator()
method cannot return a spliterator that is IMMUTABLE,
CONCURRENT, or late-binding. (See spliterator()
for details.)
Stream from the
collection's Spliterator.Stream over the elements in this collectiondefault Stream<E> parallelStream()
Stream 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 is IMMUTABLE,
CONCURRENT, or late-binding. (See spliterator()
for details.)
Stream from the
collection's Spliterator.Stream over the elements in this
collection Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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