BlockingDeque
<E>,
BlockingQueue
<E>,
DataSet
<T>,
Deque
<E>,
List
<E>,
NavigableSet
<E>,
Queue
<E>,
Set
<E>,
SortedSet
<E>
is new.
BlockingDeque
<E>,
BlockingQueue
<E>,
DataSet
<T>,
Deque
<E>,
List
<E>,
NavigableSet
<E>,
Queue
<E>,
Set
<E>,
SortedSet
<E>
ArrayDeque
ArrayList
,
ConcurrentSkipListSet
,
CopyOnWriteArrayList
,
CopyOnWriteArraySet
,
DelayQueue
,
EnumSet
,
HashSet
,
JobStateReasons
,
LinkedBlockingDeque
,
LinkedBlockingQueue
,
LinkedHashSet
,
LinkedList
,
PriorityBlockingQueue
,
PriorityQueue
public interface Collection<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 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.
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.
This interface is a member of the
Java Collections Framework
.
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.
This interface is a member of the
Java Collections Framework
.
| Method Summary | ||
|---|---|---|
| boolean |
add
(
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. |
|
| boolean |
remove
(
Object
o) Removes a single instance of the specified element from this collection, if it is present (optional operation). |
|
| boolean |
removeAll
(
Collection
Removes all
of
this collection's elements that are also contained in the specified collection (optional operation). |
|
| 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. |
|
| Object [] |
toArray
() Returns an array containing all of the elements in this collection. |
|
|
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 Detail |
|---|
int size()
boolean isEmpty()
boolean contains(Object o)
(o==null ? e==null : o.equals(e))
tested
(optional)
permit
(optional)
Iterator<E> iterator()
Object[] toArray()
this
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
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()
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.
costs
Suppose
x
l
is a collection known to contain only strings. The following code can be used to dump the collection into a newly allocated array of
is a
List
known to contain only strings. The following code can be used to dump the list into a newly allocated array of
String
:
String[]x=(String[]) v.toArray(newString[0]);
Note that toArray(new Object[0]) is identical in function to toArray() .
all of
the elements
in
if
the runtime type of the specified array is not a supertype of the runtime type of every element in this
collection
boolean add(Eo)
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
ensured
if the
add
operation
is not supported by this
collection
if the
class of the specified element prevents it from being added to this
collection
permit
elements
- 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
boolean remove(Object o)
(o==null ? e==null : o.equals(e))
present
an element was removed
this
(optional)
permit
(optional)
if the
remove
operation
is not supported by this
collection
boolean containsAll(Collection<?> c)
collection
(optional)
permit
(optional), or
boolean addAll(Collection<? extends E> c)
collection containing
elements to be
added to
collection
operation is not supported by this collection
collection
a
element
permit
if
some
property
collection
IllegalStateException
- if not all the elements can be added at this time due to insertion restrictions
boolean removeAll(Collection<?> c)
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.
collection containing
elements to be removed from this
collection
collection
(optional)
(optional), or
boolean retainAll(Collection<?> c)
collection containing
elements to be retained in this
collection
operation
collection
(optional)
permit
(optional), or
void clear()
The
returns.
operation
collection
boolean 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
implementer
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.)
object
collection
int hashCode()