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java.util
Interface Collection<E>
-
All Superinterfaces:
-
Iterable
<E>
-
All Known Subinterfaces:
-
BeanContext
,
BeanContextServices
,
BlockingQueue
<E>,
List
<E>,
Queue
<E>,
Set
<E>,
SortedSet
<E>
-
All Known Implementing Classes:
-
AbstractCollection
,
AbstractList
,
AbstractQueue
,
AbstractSequentialList
,
AbstractSet
,
ArrayBlockingQueue
,
ArrayList
,
AttributeList
,
BeanContextServicesSupport
,
BeanContextSupport
,
ConcurrentLinkedQueue
,
CopyOnWriteArrayList
,
CopyOnWriteArraySet
,
DelayQueue
,
EnumSet
,
HashSet
,
JobStateReasons
,
LinkedBlockingQueue
,
LinkedHashSet
,
LinkedList
,
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
SDK
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.
This interface is a member of the
Java Collections Framework
.
Many methods in Collections Framework interfaces are defined in terms of the
equals
@link{Object.equals(Object) 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.
-
Since:
-
1.2
-
See Also:
-
Set
,
List
,
Map
,
SortedSet
,
SortedMap
,
HashSet
,
TreeSet
,
ArrayList
,
LinkedList
,
Vector
,
Collections
,
Arrays
,
AbstractCollection
Method Summary
|
boolean |
add
(
E
o)
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
<?> c)
Removes all 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. |
size
int size()
-
Returns the number of elements in this collection. If this collection contains more than
Integer.MAX_VALUE
elements, returns
Integer.MAX_VALUE
.
-
-
-
Returns:
-
the number of elements in this collection
isEmpty
boolean isEmpty()
-
Returns
true
if this collection contains no elements.
-
-
-
Returns:
-
true
if this collection contains no elements
contains
boolean contains(Object o)
-
Returns
true
if this collection contains the specified element. More formally, returns
true
if and only if this collection contains at least one element
e
such that
(o==null ? e==null : o.equals(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 support null elements (optional).
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).
-
-
Specified by:
-
iterator
in interface
Iterable
<
E
>
-
-
Returns:
-
an
Iterator
over the elements in this collection
toArray
Object[] toArray()
-
Returns an array containing all of the elements in this collection. If the 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 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.
-
-
-
Returns:
-
an array 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 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
l
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(new String[0]);
Note that
toArray(new Object[0])
is identical in function to
toArray()
.
-
-
-
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 the elements of this collection
-
Throws:
-
ArrayStoreException
- the runtime type of the specified array is not a supertype of the runtime type of every element in this collection.
-
NullPointerException
- if the specified array is
null
.
add
boolean add(E o)
-
Ensures that this collection contains the specified element (optional operation). Returns
true
if this collection changed as a result of the call. (Returns
false
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:
-
o - element whose presence in this collection is to be ensured.
-
Returns:
-
true
if this collection changed as a result of the call
-
Throws:
-
UnsupportedOperationException
-
add
is not supported by this collection.
-
ClassCastException
- 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 support null elements.
-
IllegalArgumentException
- some aspect of this element prevents it from being added to this collection.
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 element
e
such that
(o==null ? e==null : o.equals(e))
, if this collection contains one or more such elements. Returns true 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 this collection changed as a result of the 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 support null elements (optional).
-
UnsupportedOperationException
- remove is not supported by this collection.
containsAll
boolean containsAll(Collection<?> c)
-
Returns
true
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 support null elements (optional).
-
NullPointerException
- if the specified collection is
null
.
-
See Also:
-
contains(Object)
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 - elements to be inserted into this collection.
-
Returns:
-
true
if this collection changed as a result of the call
-
Throws:
-
UnsupportedOperationException
- if this collection does not support the
addAll
method.
-
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 one or more null elements and this collection does not support null elements, or if the specified collection is
null
.
-
IllegalArgumentException
- some aspect of an element of the specified collection prevents it from being added to this collection.
-
See Also:
-
add(Object)
removeAll
boolean removeAll(Collection<?> c)
-
Removes all 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 - elements to be removed from this collection.
-
Returns:
-
true
if this collection changed as a result of the call
-
Throws:
-
UnsupportedOperationException
- if the
removeAll
method 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).
-
NullPointerException
- if the specified collection is
null
.
-
See Also:
-
remove(Object)
,
contains(Object)
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 - elements to be retained in this collection.
-
Returns:
-
true
if this collection changed as a result of the call
-
Throws:
-
UnsupportedOperationException
- if the
retainAll
method 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).
-
NullPointerException
- if the specified collection is
null
.
-
See Also:
-
remove(Object)
,
contains(Object)
clear
void clear()
-
Removes all of the elements from this collection (optional operation). This collection will be empty after this method returns unless it throws an exception.
-
-
-
Throws:
-
UnsupportedOperationException
- if the
clear
method is not supported by this collection.
equals
boolean equals(Object o)
-
Compares the specified object with this collection for equality.
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 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.)
-
-
Overrides:
-
equals
in class
Object
-
-
Parameters:
-
o - Object to be compared for equality with this collection.
-
Returns:
-
true
if the specified object is equal to this collection
-
See Also:
-
Object.equals(Object)
,
Set.equals(Object)
,
List.equals(Object)
hashCode
int hashCode()
-
Returns the hash code value for this collection. While the
Collection
interface adds no stipulations to the general contract for the
Object.hashCode
method, programmers should take note that any class that overrides the
Object.equals
method must also override the
Object.hashCode
method in order to satisfy the general contract for the
Object.hashCode
method. In particular,
c1.equals(c2)
implies that
c1.hashCode()==c2.hashCode()
.
-
-
Overrides:
-
hashCode
in class
Object
-
-
Returns:
-
the hash code value for this collection
-
See Also:
-
Object.hashCode()
,
Object.equals(Object)