java.util
public 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 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.
Since: 1.2
Version: 1.45, 06/22/03
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. |
| <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.
|
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.
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
Throws: UnsupportedOperationException if the clear method is not supported by this collection.
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).
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
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.)
Parameters: o Object to be compared for equality with this collection.
Returns: true if the specified object is equal to this collection
Returns: the hash code value for this collection
Returns: true if this collection contains no elements
Returns: an Iterator over the elements in this collection
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.
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.
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.
Returns: the number of elements in this collection
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
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.