Collectionpublic interface Collection implements Iterable| 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 {@link #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 {@link Object#equals(Object) equals} method. For example,
the specification for the {@link #contains(Object) 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
{@link 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 {@link Object} methods wherever the
implementor deems it appropriate.
This interface is a member of the
Java Collections Framework. |
| Methods Summary |
|---|
| public 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.
| | public boolean | addAll(java.util.Collection 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.)
| | public void | clear()Removes all of the elements from this collection (optional operation).
The collection will be empty after this method returns.
| | public boolean | contains(java.lang.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)).
| | public boolean | containsAll(java.util.Collection c)Returns true if this collection contains all of the elements
in the specified collection.
| | public boolean | equals(java.lang.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.)
| | public 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().
| | public boolean | isEmpty()Returns true if this collection contains no elements.
| | public java.util.Iterator | 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).
| | public boolean | remove(java.lang.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).
| | public boolean | removeAll(java.util.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.
| | public boolean | retainAll(java.util.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.
| | public int | size()Returns the number of elements in this collection. If this collection
contains more than Integer.MAX_VALUE elements, returns
Integer.MAX_VALUE.
| | public java.lang.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.
| | 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 {@link #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().
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