java.util

Interface Collection<E>

All Superinterfaces:: Iterable<T>

Known Subinterfaces:: Bag, BeanContext, BeanContextServices, BlockingQueue<E>, BoundedCollection, Buffer, HandlerChain, IndexedRecord, List<E>, Queue<E>, Set<E>, SortedBag, SortedSet<E>

Known Implementing Classes:: AbstractBagDecorator, AbstractBufferDecorator, AbstractCollection<E>, AbstractCollectionDecorator, AbstractDualBidiMap.EntrySet, AbstractDualBidiMap.KeySet, AbstractDualBidiMap.Values, AbstractDualBidiMap.View, AbstractHashedMap.EntrySet, AbstractHashedMap.KeySet, AbstractHashedMap.Values, AbstractLinkedList, AbstractLinkedList.LinkedSubList, AbstractList<E>, AbstractListDecorator, AbstractMapBag, AbstractQueue<E>, AbstractSequentialList<E>, AbstractSerializableCollectionDecorator, AbstractSerializableListDecorator, AbstractSerializableSetDecorator, AbstractSet<E>, AbstractSetDecorator, AbstractSortedBagDecorator, AbstractSortedSetDecorator, Anchor, ArrayBlockingQueue<E>, ArrayList<E>, ArrayQueue<T>, ArrayStack, AttributeList, BeanContextServicesSupport, BeanContextSupport, BinaryHeap, BlockingBuffer, BoundedBuffer, BoundedFifoBuffer, BoundedFifoBuffer, BucketingService.CollectedList, ByteCollectionCollection, ByteListList, Chapter, ChapterAutoNumber, CharArraySet, CharCollectionCollection, CharListList, CircularFifoBuffer, CompositeCollection, CompositeSet, ConcurrentLinkedQueue<E>, CopyOnWriteArrayList<E>, CopyOnWriteArraySet<E>, CursorableLinkedList, CursorableLinkedList, DefaultMapBag, DelayQueue<E,extends,Delayed>, DigesterRuleParser.PatternStack, DoubleCollectionCollection, DoubleListList, EnumSet<E,extends,Enum,E>, FastArrayList, FixedSizeList, FloatCollectionCollection, FloatListList, GrowthList, HashBag, HashBag, HashSet<E>, IntCollectionCollection, IntListList, JobStateReasons, LazyDynaList, LazyList, LinkedBlockingQueue<E>, LinkedHashSet<E>, LinkedList<E>, ListItem, ListOrderedSet, LongCollectionCollection, LongListList, ManagedList, ManagedSet, MapBackedSet, NodeCachingLinkedList, Paragraph, PersistentBag, PersistentIdentifierBag, PersistentList, PersistentSet, PersistentSortedSet, Phrase, PredicatedBag, PredicatedBuffer, PredicatedCollection, PredicatedList, PredicatedSet, PredicatedSortedBag, PredicatedSortedSet, PriorityBlockingQueue<E>, PriorityBuffer, PriorityQueue<E>, RoleList, RoleUnresolvedList, Section, SetUniqueList, ShortCollectionCollection, ShortListList, SnmpVarBindList, Stack<E>, SynchronizedBag, SynchronizedBuffer, SynchronizedCollection, SynchronizedList, SynchronizedSet, SynchronizedSortedBag, SynchronizedSortedSet, SynchronousQueue<E>, TransformedBag, TransformedBuffer, TransformedCollection, TransformedList, TransformedSet, TransformedSortedBag, TransformedSortedSet, TreeBag, TreeBag, TreeList, TreeSet<E>, UnboundedFifoBuffer, UnboundedFifoBuffer, UnmodifiableBag, UnmodifiableBoundedCollection, UnmodifiableBuffer, UnmodifiableCollection, UnmodifiableEntrySet, UnmodifiableList, UnmodifiableSet, UnmodifiableSortedBag, UnmodifiableSortedSet, Vector<E>, XfaForm.Stack2, XmpArray

public interface Collection<E>

extends Iterable<T>

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.

This interface is a member of the ../../../guide/collections/index.html"> 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.

Since:: 1.2

See Also:: Set, List, Map, SortedSet, SortedMap, HashSet, TreeSet, ArrayList, LinkedList, Vector, Collections, Arrays, AbstractCollection

Method Summary
	`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.
`boolean`	`add(E o)` Ensures that this collection contains the specified element (optional operation).
`boolean`	`addAll(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`	`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.

Methods inherited from interface java.lang.Iterable<T>
`iterator`

T[] toArray

public  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

public 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.

Usages and Demos :

View More Examples of add(E o)

   1:   {
   2:     Collection c = new LinkedList();
   3: 
   4:         ...
   5:     for ( int i = 0; i < 5; i++ )
   6:       c.add( "" + i );
   7: 
   8:   }
   9: }

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java.util

Interface Collection<E>

Method Summary

Methods inherited from interface java.lang.Iterable<T>

Method Details

T[] toArray

add

addAll

clear

contains

containsAll

equals

hashCode

isEmpty

iterator

remove

removeAll

retainAll

size

toArray