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java.util
Interface Set<E>
-
-
Type Parameters:
-
E - the type of elements maintained by this set
-
All Superinterfaces:
-
Collection
<E>,
Iterable
<E>
-
All Known Subinterfaces:
-
NavigableSet
SortedSet
<E>,
SortedSet
<E>
-
All Known Implementing Classes:
-
AbstractSet
,
ConcurrentSkipListSet
,
CopyOnWriteArraySet
,
EnumSet
,
HashSet
,
JobStateReasons
,
LinkedHashSet
,
TreeSet
-
public interface Set<E>
- extends Collection<E>
A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. As implied by its name, this interface models the mathematical
set
abstraction.
The
Set
interface places additional stipulations, beyond those inherited from the
Collection
interface, on the contracts of all constructors and on the contracts of the
add
,
equals
and
hashCode
methods. Declarations for other inherited methods are also included here for convenience. (The specifications accompanying these declarations have been tailored to the
Set
interface, but they do not contain any additional stipulations.)
The additional stipulation on constructors is, not surprisingly, that all constructors must create a set that contains no duplicate elements (as defined above).
Note: Great care must be exercised if mutable objects are used as set elements. The behavior of a set is not specified if the value of an object is changed in a manner that affects
equals
comparisons while the object is an element in the set. A special case of this prohibition is that it is not permissible for a set to contain itself as an element.
Some set 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 set 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
-
See Also:
-
Collection
,
List
,
SortedSet
,
HashSet
,
TreeSet
,
AbstractSet
,
Collections.singleton(java.lang.Object)
,
Collections.EMPTY_SET
Method Summary
|
boolean |
add
(
E
Adds the specified element to this set if it is not already present (optional operation). |
boolean |
addAll
(
Collection
<? extends
E
> c)
Adds all of the elements in the specified collection to this set if they're not already present (optional operation). |
void |
clear
()
Removes all of the elements from this set (optional operation). |
boolean |
contains
(
Object
o)
Returns
true
if this set contains the specified element. |
boolean |
containsAll
(
Collection
<?> c)
Returns
true
if this set contains all of the elements of the specified collection. |
boolean |
equals
(
Object
o)
Compares the specified object with this set for equality. |
int |
hashCode
()
Returns the hash code value for this set. |
boolean |
isEmpty
()
Returns
true
if this set contains no elements. |
Iterator
<
E
> |
iterator
()
Returns an iterator over the elements in this set. |
boolean |
remove
(
Object
o)
Removes the specified element from this set if it is present (optional operation). |
boolean |
removeAll
(
Collection
<?> c)
Removes from this set all of its elements that are contained in the specified collection (optional operation). |
boolean |
retainAll
(
Collection
<?> c)
Retains only the elements in this set that are contained in the specified collection (optional operation). |
int |
size
()
Returns the number of elements in this set (its cardinality). |
Object
[] |
toArray
()
Returns an array containing all of the elements in this set. |
|
toArray
(T[] a)
Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array. |
size
int size()
-
Returns the number of elements in this set (its cardinality). If this set contains more than
Integer.MAX_VALUE
elements, returns
Integer.MAX_VALUE
.
-
-
Specified by:
-
size
in interface
Collection
<
E
>
-
-
Returns:
-
the number of elements in this set (its
cardinality)
cardinality).
isEmpty
boolean isEmpty()
-
Returns
true
if this set contains no elements.
-
-
Specified by:
-
isEmpty
in interface
Collection
<
E
>
-
-
Returns:
-
true
if this set contains no
elements
elements.
contains
boolean contains(Object o)
-
Returns
true
if this set contains the specified element. More formally, returns
true
if and only if this set contains an element
e
such that
(o==null ? e==null : o.equals(e))
.
(o==null ? e==null : o.equals(e)).
-
-
Specified by:
-
contains
in interface
Collection
<
E
>
-
-
Parameters:
-
o - element whose presence in this set is to be
tested
tested.
-
Returns:
-
true
if this set contains the specified
element
element.
-
Throws:
-
ClassCastException
- if the type of the specified element is incompatible with this set
(optional)
(optional).
-
NullPointerException
- if the specified element is null and this set does not
permit
support
null elements
(optional)
(optional).
iterator
Iterator<E> iterator()
-
Returns an iterator over the elements in this set. The elements are returned in no particular order (unless this set is an instance of some class that provides a guarantee).
-
-
Specified by:
-
iterator
in interface
Collection
<
E
>
-
Specified by:
-
iterator
in interface
Iterable
<
E
>
-
-
Returns:
-
an iterator over the elements in this
set
set.
toArray
Object[] toArray()
-
Returns an array containing all of the elements in this set.
If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.
Obeys the general contract of the
Collection.toArray
method.
The returned array will be "safe" in that no references to it are maintained by this set. (In other words, this method must allocate a new array even if this set 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.
-
-
Specified by:
-
toArray
in interface
Collection
<
E
>
-
-
Returns:
-
an array containing all
of
the elements in this
set
set.
toArray
<T> T[] toArray(T[] a)
-
Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array. If the set 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 set.
If this set fits in the specified array with room to spare (i.e., the array has more elements than this set), the element in the array immediately following the end of the set is set to
Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array. Obeys the general contract of the
null
Collection.toArray(Object[])
. (This is useful in determining the length of this set
only
if the caller knows that this set does not contain any null elements.)
method.
If this set 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 set known to contain only strings. The following code can be used to dump the set 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()
.
-
-
Specified by:
-
toArray
in interface
Collection
<
E
>
-
-
Parameters:
-
a - the array into which the elements of this set 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
the elements
in
of
this
set
set.
-
Throws:
-
ArrayStoreException
-
if
the runtime type of
the specified array
a
is not a supertype of the runtime type of every element in this
set
set.
-
NullPointerException
- if the specified array is
null
.
add
boolean add(E
e) ![](../../diffpics/oold.gif)
o)
-
Adds the specified element to this set if it is not already present (optional operation).
More formally, adds the specified element
More formally, adds the specified element, o, to this set if this set contains no element e such that (o==null ? e==null : o.equals(e)). If this set already contains the specified element, the call leaves this set unchanged and returns
e
to this set if the set contains no element
e2
such that
(e==null ? e2==null : e.equals(e2))
. If this set already contains the element, the call leaves the set unchanged and returns
false
. In combination with the restriction on constructors, this ensures that sets never contain duplicate elements.
The stipulation above does not imply that sets must accept all elements; sets may refuse to add any particular element, including
null
, and
throw
throwing
an exception, as described in the specification for
Collection.add
Collection.add
. Individual set implementations should clearly document any restrictions on the elements that they may contain.
-
-
Specified by:
-
add
in interface
Collection
<
E
>
-
-
Parameters:
-
e
o
- element to be added to this
set
set.
-
Returns:
-
true
if this set did not already contain the specified
element
element.
-
Throws:
-
UnsupportedOperationException
- if the
add
operation
method
is not supported by this
set
set.
-
ClassCastException
- if the class of the specified element prevents it from being added to this
set
set.
-
NullPointerException
- if the specified element is null and this set does not
permit
support
null
elements
elements.
-
IllegalArgumentException
- if some
property
aspect
of the specified element prevents it from being added to this
set
set.
remove
boolean remove(Object o)
-
Removes the specified element from this set if it is present (optional operation). More formally, removes an element
e
such that
(o==null ? e==null : o.equals(e))
,
(o==null ? e==null : o.equals(e)),
if
this
the
set contains such an element. Returns
true
if
this
the
set contained the
specified
element (or equivalently, if
this
the
set changed as a result of the call).
(This
(The
set will not contain the
specified
element once the call returns.)
-
-
Specified by:
-
remove
in interface
Collection
<
E
>
-
-
Parameters:
-
o - object to be removed from this set, if
present
present.
-
Returns:
-
true
if
this
the
set contained the specified
element
element.
-
Throws:
-
ClassCastException
- if the type of the specified element is incompatible with this set
(optional)
(optional).
-
NullPointerException
- if the specified element is null and this set does not
permit
support
null elements
(optional)
(optional).
-
UnsupportedOperationException
- if the
remove
operation
method
is not supported by this
set
set.
containsAll
boolean containsAll(Collection<?> c)
-
Returns
true
if this set contains all of the elements of the specified collection. If the specified collection is also a set, this method returns
true
if it is a
subset
of this set.
-
-
Specified by:
-
containsAll
in interface
Collection
<
E
>
-
-
Parameters:
-
c - collection to be checked for containment in this
set
set.
-
Returns:
-
true
if this set contains all of the elements of the specified
collection
collection.
-
Throws:
-
ClassCastException
- if the types of one or more elements in the specified collection are incompatible with this set
(optional)
(optional).
-
NullPointerException
- if the specified collection contains one or more null elements and this set does not
permit
support
null elements
(optional), or
(optional).
-
NullPointerException
-
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 set if they're not already present (optional operation). If the specified collection is also a set, the
addAll
operation effectively modifies this set so that its value is the
union
of the two sets. The behavior of this operation is
undefined
unspecified
if the specified collection is modified while the operation is in progress.
-
-
Specified by:
-
addAll
in interface
Collection
<
E
>
-
-
Parameters:
-
c - collection
containing
whose
elements
are
to be added to this
set
set.
-
Returns:
-
true
if this set changed as a result of the
call
call.
-
Throws:
-
UnsupportedOperationException
- if the
addAll
operation
method
is not supported by this
set
set.
-
ClassCastException
- if the class of
an
some
element of the specified collection prevents it from being added to this
set
set.
-
NullPointerException
- if the specified collection contains one or more null elements and this set does not
permit
support
null elements, or if the specified collection is
null
.
-
IllegalArgumentException
- if some
property
aspect
of
an
some
element of the specified collection prevents it from being added to this
set
set.
-
See Also:
-
add(Object)
retainAll
boolean retainAll(Collection<?> c)
-
Retains only the elements in this set that are contained in the specified collection (optional operation). In other words, removes from this set all of its elements that are not contained in the specified collection. If the specified collection is also a set, this operation effectively modifies this set so that its value is the
intersection
of the two sets.
-
-
Specified by:
-
retainAll
in interface
Collection
<
E
>
-
-
Parameters:
-
c - collection
containing
that defines which
elements
to be retained in
this set
will retain.
-
Returns:
-
true
if this
set
collection
changed as a result of the
call
call.
-
Throws:
-
UnsupportedOperationException
- if the
retainAll
operation
method
is not supported by this
set
Collection.
-
ClassCastException
- if the
class
types
of
an element of
one or more elements in
this set
is
are
incompatible with the specified collection
(optional)
(optional).
-
NullPointerException
- if this set contains a null element and the specified collection does not
permit
support
null elements
(optional), or
(optional).
-
NullPointerException
-
if the specified collection is
null
.
-
See Also:
-
remove(Object)
removeAll
boolean removeAll(Collection<?> c)
-
Removes from this set all of its elements that are contained in the specified collection (optional operation). If the specified collection is also a set, this operation effectively modifies this set so that its value is the
asymmetric set difference
of the two sets.
-
-
Specified by:
-
removeAll
in interface
Collection
<
E
>
-
-
Parameters:
-
c - collection
containing
that defines which
elements
to
will
be removed from this
set
set.
-
Returns:
-
true
if this set changed as a result of the
call
call.
-
Throws:
-
UnsupportedOperationException
- if the
removeAll
operation
method
is not supported by this
set
Collection.
-
ClassCastException
- if the
class
types
of
an element of
one or more elements in
this set
is
are
incompatible with the specified collection
(optional)
(optional).
-
NullPointerException
- if this set contains a null element and the specified collection does not
permit
support
null elements
(optional), or
(optional).
-
NullPointerException
-
if the specified collection is
null
.
-
See Also:
-
remove(Object)
,
contains(Object)
clear
void clear()
-
Removes all of the elements from this set (optional operation).
The
This
set will be empty after this call
returns.
returns (unless it throws an exception).
-
-
Specified by:
-
clear
in interface
Collection
<
E
>
-
-
Throws:
-
UnsupportedOperationException
- if the
clear
method is not supported by this
set
set.
equals
boolean equals(Object o)
-
Compares the specified object with this set for equality. Returns
true
if the specified object is also a set, the two sets have the same size, and every member of the specified set is contained in this set (or equivalently, every member of this set is contained in the specified set). This definition ensures that the equals method works properly across different implementations of the set interface.
-
-
Specified by:
-
equals
in interface
Collection
<
E
>
-
Overrides:
-
equals
in class
Object
-
-
Parameters:
-
o -
object
Object
to be compared for equality with this
set
set.
-
Returns:
-
true
if the specified
object
Object
is equal to this
set
set.
-
See Also:
-
Object.hashCode()
,
Hashtable
hashCode
int hashCode()
-
Returns the hash code value for this set. The hash code of a set is defined to be the sum of the hash codes of the elements in the set, where the
hash code
hashcode
of a
null
element is defined to be zero.
This ensures that
This ensures that s1.equals(s2) implies that s1.hashCode()==s2.hashCode() for any two sets s1 and s2, as required by the general contract of the
s1.equals(s2)
Object.hashCode
implies that
s1.hashCode()==s2.hashCode()
for any two sets
s1
and
s2
, as required by the general contract of
Object.hashCode()
.
method.
-
-
Specified by:
-
hashCode
in interface
Collection
<
E
>
-
Overrides:
-
hashCode
in class
Object
-
-
Returns:
-
the hash code value for this
set
set.
-
See Also:
-
Object.equals(Object)
Object.hashCode()
,
Object.equals(Object)
,
equals(Object)