Collection (Java Platform SE 6) - JDK 5 Documentation v6.0, Java 2 SDK 英文文档
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Java™ Platform Standard Ed. 6 |
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java.util
Interface Collection<E>
- 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>
- All Known Implementing Classes:
- AbstractCollection, AbstractList, AbstractQueue, AbstractSequentialList, AbstractSet, ArrayBlockingQueue, ArrayDeque, ArrayList, AttributeList, BeanContextServicesSupport, BeanContextSupport, ConcurrentLinkedQueue, ConcurrentSkipListSet, CopyOnWriteArrayList, CopyOnWriteArraySet, DelayQueue, EnumSet, HashSet, JobStateReasons, LinkedBlockingDeque, 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 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.
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.
- Since:
- 1.2
- See Also:
Set,List,Map,SortedSet,SortedMap,HashSet,TreeSet,ArrayList,LinkedList,Vector,Collections,Arrays,AbstractCollection
| Method Summary | ||
|---|---|---|
boolean |
add(E e)
Ensures that this collection contains the specified element (optional operation). |
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boolean |
addAll(Collection<? extends E> c)
Adds all of the elements in the specified collection to this collection (optional operation). |
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void |
clear()
Removes all of the elements from this collection (optional operation). |
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boolean |
contains(Object o)
Returns true if this collection contains the specified element. |
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boolean |
containsAll(Collection<?> c)
Returns true if this collection contains all of the elements in the specified collection. |
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boolean |
equals(Object o)
Compares the specified object with this collection for equality. |
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int |
hashCode()
Returns the hash code value for this collection. |
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boolean |
isEmpty()
Returns true if this collection contains no elements. |
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Iterator<E> |
iterator()
Returns an iterator over the elements in this collection. |
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boolean |
remove(Object o)
Removes a single instance of the specified element from this collection, if it is present (optional operation). |
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boolean |
removeAll(Collection<?> c)
Removes all of this collection's elements that are also contained in the specified collection (optional operation). |
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boolean |
retainAll(Collection<?> c)
Retains only the elements in this collection that are contained in the specified collection (optional operation). |
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int |
size()
Returns the number of elements in this collection. |
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Object[] |
toArray()
Returns an array containing all of the elements in this collection. |
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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. |
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| Method Detail |
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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 permit 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).
- Returns:
- an Iterator over the elements in this collection
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 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 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().- 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 the specified array is not a supertype of the runtime type of every element in this collectionNullPointerException- if the specified array is null
add
boolean add(E e)
- 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:
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 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 restrictions
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 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 remove operation 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 permit null elements (optional), or 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- collection containing elements to be added to this collection- Returns:
- true if this collection changed as a result of the call
- Throws:
UnsupportedOperationException- 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 restrictions- See Also:
add(Object)
removeAll
boolean removeAll(Collection<?> c)
- 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 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 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- collection containing elements to be retained in this collection- Returns:
- true if this collection changed as a result of the call
- Throws:
UnsupportedOperationException- 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 null- See Also:
remove(Object),contains(Object)
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 the clear operation 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 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.)
- 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.hashCodemethod.
In particular, c1.equals(c2) implies that
c1.hashCode()==c2.hashCode().
- Returns:
- the hash code value for this collection
- See Also:
Object.hashCode(),Object.equals(Object)
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Java™ Platform Standard Ed. 6 |
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