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Interface java.util.Collection
- public interface Collection
The root interface in the collection hierarchy. A Collection
represents a group of Objects, known as its elements. Bags
(unordered collections that may contain duplicate elements) implement this
interface directly, rather than implementing one of its "subinterfaces",
List or Set.
The interfaces that comprise the collection hierarchy are designed to allow
manipulation of collections in an implementation-independent fashion. This
allows for interoperability among unrelated APIs that take collections as
input or return them as output. It reduces the effort in learning new APIs
that would otherwise have their own collection interfaces, reduces the
effort in designing such APIs, and fosters software reuse.
The collection hierarchy consists of four interfaces, the core
collection intefaces. Two of these interfaces, Set and List, are
children of the Collection interface; they add further constraints on the
contracts imposed by the methods in this interface, as well as adding new
methods. The final core collection interface, Map, is not a child of
Collection, as it represents a mapping rather than a true collection.
All of the methods in Collection and the other core interfaces that modify
the collection are labeled optional. Some implementations may not
perform one or more of these operations, throwing a runtime exception,n
UnsupportedOperationException, if they are attempted. Implementations
should specify in their documentation which optional operations they
support. Several terms are introduced to aid in this specification:
- Collections that do not support any modification operations (such
as add, remove, clear) are referred to as unmodifiable.
Collections that are not unmodifiable are referred to modifiable.
- Collections that additionally guarantee that no change in the
Collection will ever be observable via "query" operations (such as
iterator, size, contains) are referred to as immutable.
Collections that are not immutable are referred to as mutable.
- Lists that guarantee that their size will remain constant even though
the elements may change are referred to as fixed-size. Lists that
are not fixed-size are referred to as variable-size.
Some implementations may restrict what elements (or in the case of Maps,
keys and values) may be stored. Possible restrictions include requiring
elements to:
- be of a particular type.
- be comparable to other elements in the collection.
- be non-null.
- obey some arbitrary predicate.
Attempting to add an element that violates an implementation's
restrictions will result in a runtime exception, typically a
ClassCastException, an IllegalArgumentException or a NullPointerException.
Attempting to remove or test for such an element may result in such an
exception, though some "restricted collections" may permit this usage.
All general-purpose Collection implementation classes 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. Similarly, all general-purpose Map implementations
should provide a void (no arguments) constructor and a constructor that
takes a single argument of type Map. There is no way to enforce these
recommendations (as interfaces cannot contain constructors) but all of the
general-purpose Collection and Map implementations in the JDK comply.
Collection implementation classes typically have names of the form
<Implementation><Interface>.
Set implementations in the JDK include HashSet and ArraySet. List
implementations include ArrayList, LinkedList and Vector. Map
implementations include HashMap, TreeMap, ArrayMap, and Hashtable.
(The JDK contains no class that implements Collection directly.)
All of the JDK implementations with "new style"
(<Implementation><Interface>) names
are unsynchronized. The Collections class contains static factories
that may be used to add synchronization to any unsynchronized
collection.
The AbstractCollection, AbstractSet, AbstractList and
AbstractSequentialList classes provide skeletal implementations of the
core collection interfaces, to minimize the effort required to implement
them. The Iterator, ListIterator, Comparable and Comparator interfaces
provide the required "infrastructure."
- Since:
- JDK1.2
- See Also:
- Set, List, Map, ArrayList, LinkedList, Vector, HashSet, ArraySet, HashMap, TreeMap, ArrayMap, Hashtable, Collections, AbstractCollection, AbstractSet, AbstractList, AbstractSequentialList, AbstractMap, Iterator, ListIterator, Comparable, Comparator, UnsupportedOperationException, ConcurrentModificationException
add(Object)
- Ensures that this Collection contains the specified element (optional
operation).
addAll(Collection)
- Adds all of the elements in the specified Collection to this Collection
(optional operation).
clear()
- Removes all of the elements from this Collection (optional operation).
contains(Object)
- Returns true if this Collection contains the specified element.
containsAll(Collection)
- Returns true if this Collection contains all of the elements in the
specified Collection.
equals(Object)
- Compares the specified Object with this Collection for equality.
hashCode()
- Returns the hash code value for this Collection.
isEmpty()
- Returns true if this Collection contains no elements.
iterator()
- Returns an Iterator over the elements in this Collection.
remove(Object)
- Removes a single instance of the specified element from this Collection,
if it is present (optional operation).
removeAll(Collection)
- Removes from this Collection all of its elements that are contained in
the specified Collection (optional operation).
retainAll(Collection)
- Retains only the elements in this Collection that are contained in the
specified Collection (optional operation).
size()
- Returns the number of elements in this Collection.
toArray()
- Returns an array containing all of the elements in this Collection.
size
public abstract int size()
- Returns the number of elements in this Collection.
isEmpty
public abstract boolean isEmpty()
- Returns true if this Collection contains no elements.
contains
public abstract 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.
iterator
public abstract 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).
toArray
public abstract 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 the Collection. (In other words, this method must
allocate a new array even if the 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.
add
public abstract boolean add(Object o)
- Ensures that this Collection contains the specified element (optional
operation). Returns true if the 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 the
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.
- Parameters:
- o - element whose presence in this Collection is to be ensured.
- Returns:
- true if the Collection changed as a result of the call.
- Throws:
UnsupportedOperationException
- add is not supported by this
Collection.
- Throws:
ClassCastException
- class of the specified element
prevents it from being added to this Collection.
- Throws:
IllegalArgumentException
- some aspect of this element prevents
it from being added to this Collection.
remove
public abstract 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 the Collection contains one or more such
elements. Returns true if the Collection contained the specified
element (or equivalently, if the Collection changed as a result of the
call).
- Parameters:
- o - element to be removed from this Collection, if present.
- Returns:
- true if the Collection changed as a result of the call.
- Throws:
UnsupportedOperationException
- remove is not supported
by this Collection.
containsAll
public abstract boolean containsAll(Collection c)
- Returns true if this Collection contains all of the elements in the
specified Collection.
- See Also:
- contains
addAll
public abstract boolean addAll(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 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
- addAll is not supported
by this Collection.
- Throws:
ClassCastException
- class of an element of the specified
Collection prevents it from being added to this Collection.
- Throws:
IllegalArgumentException
- some aspect of an element of
the specified Collection prevents it from being added to
this Collection.
- See Also:
- add
removeAll
public abstract boolean removeAll(Collection c)
- Removes from this Collection all of its elements that are contained in
the specified Collection (optional operation). After this call returns,
this Collection will contains 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
- removeAll is not supported
by this Collection.
- See Also:
- remove, contains
retainAll
public abstract 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
- retainAll is not supported
by this Collection.
- See Also:
- remove, contains
clear
public abstract void clear()
- Removes all of the elements from this Collection (optional operation).
The Collection will be empty after this call returns (unless it throws
an exception).
- Throws:
UnsupportedOperationException
- clear is not supported
by this Collection.
equals
public abstract boolean equals(Object o)
- Compares the specified Object with this Collection for equality.
While Collection adds no stipulations to the general contract for
Object.equals, programmers who implement Collection "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 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."
(Lists and Sets mandate such value comparisons.)
The general contract for Object.equals states that equals
must be reflexive (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
that is neither a List nor a Set must return false when this
Collection is compared to any List or Set.
- Parameters:
- o - Object to be compared for equality with this Collection.
- Returns:
- true if the specified Object is equal to this Collection.
- Overrides:
- equals in class Object
- See Also:
- equals, equals, equals
hashCode
public abstract int hashCode()
- Returns the hash code value for this Collection. While Collection
adds no stipulations to the general contract for Object.hashCode,
programmers should take note that any class that overrides
Object.equals must also override Object.hashCode, in order to satisfy
the general contract for Object.hashCode. In particular,
c1.equals(c2)
implies that
c1.hashCode()==c2.hashCode()
.
- Overrides:
- hashCode in class Object
- See Also:
- hashCode, equals
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