Collections
public
class
Collections
extends Object
java.lang.Object | |
↳ | java.util.Collections |
This class consists exclusively of static methods that operate on or return collections. It contains polymorphic algorithms that operate on collections, "wrappers", which return a new collection backed by a specified collection, and a few other odds and ends.
The methods of this class all throw a NullPointerException
if the collections or class objects provided to them are null.
The documentation for the polymorphic algorithms contained in this class
generally includes a brief description of the implementation. Such
descriptions should be regarded as implementation notes, rather than
parts of the specification. Implementors should feel free to
substitute other algorithms, so long as the specification itself is adhered
to. (For example, the algorithm used by sort
does not have to be
a mergesort, but it does have to be stable.)
The "destructive" algorithms contained in this class, that is, the
algorithms that modify the collection on which they operate, are specified
to throw UnsupportedOperationException
if the collection does not
support the appropriate mutation primitive(s), such as the set
method. These algorithms may, but are not required to, throw this
exception if an invocation would have no effect on the collection. For
example, invoking the sort
method on an unmodifiable list that is
already sorted may or may not throw UnsupportedOperationException
.
This class is a member of the Java Collections Framework.
See also:
Summary
Fields | |
---|---|
public
static
final
List<E> |
EMPTY_LIST
The empty list (immutable). |
public
static
final
Map<K, V> |
EMPTY_MAP
The empty map (immutable). |
public
static
final
Set<E> |
EMPTY_SET
The empty set (immutable). |
Public methods | |
---|---|
static
<T>
boolean
|
addAll(Collection<? super T> c, T... elements)
Adds all of the specified elements to the specified collection. |
static
<T>
Queue<T>
|
asLifoQueue(Deque<T> deque)
Returns a view of a |
static
<T>
int
|
binarySearch(List<? extends T> list, T key, Comparator<? super T> c)
Searches the specified list for the specified object using the binary search algorithm. |
static
<T>
int
|
binarySearch(List<? extends Comparable<? super T>> list, T key)
Searches the specified list for the specified object using the binary search algorithm. |
static
<E>
Collection<E>
|
checkedCollection(Collection<E> c, Class<E> type)
Returns a dynamically typesafe view of the specified collection. |
static
<E>
List<E>
|
checkedList(List<E> list, Class<E> type)
Returns a dynamically typesafe view of the specified list. |
static
<K, V>
Map<K, V>
|
checkedMap(Map<K, V> m, Class<K> keyType, Class<V> valueType)
Returns a dynamically typesafe view of the specified map. |
static
<K, V>
NavigableMap<K, V>
|
checkedNavigableMap(NavigableMap<K, V> m, Class<K> keyType, Class<V> valueType)
Returns a dynamically typesafe view of the specified navigable map. |
static
<E>
NavigableSet<E>
|
checkedNavigableSet(NavigableSet<E> s, Class<E> type)
Returns a dynamically typesafe view of the specified navigable set. |
static
<E>
Queue<E>
|
checkedQueue(Queue<E> queue, Class<E> type)
Returns a dynamically typesafe view of the specified queue. |
static
<E>
Set<E>
|
checkedSet(Set<E> s, Class<E> type)
Returns a dynamically typesafe view of the specified set. |
static
<K, V>
SortedMap<K, V>
|
checkedSortedMap(SortedMap<K, V> m, Class<K> keyType, Class<V> valueType)
Returns a dynamically typesafe view of the specified sorted map. |
static
<E>
SortedSet<E>
|
checkedSortedSet(SortedSet<E> s, Class<E> type)
Returns a dynamically typesafe view of the specified sorted set. |
static
<T>
void
|
copy(List<? super T> dest, List<? extends T> src)
Copies all of the elements from one list into another. |
static
boolean
|
disjoint(Collection<?> c1, Collection<?> c2)
Returns |
static
<T>
Enumeration<T>
|
emptyEnumeration()
Returns an enumeration that has no elements. |
static
<T>
Iterator<T>
|
emptyIterator()
Returns an iterator that has no elements. |
static
final
<T>
List<T>
|
emptyList()
Returns an empty list (immutable). |
static
<T>
ListIterator<T>
|
emptyListIterator()
Returns a list iterator that has no elements. |
static
final
<K, V>
Map<K, V>
|
emptyMap()
Returns an empty map (immutable). |
static
final
<K, V>
NavigableMap<K, V>
|
emptyNavigableMap()
Returns an empty navigable map (immutable). |
static
<E>
NavigableSet<E>
|
emptyNavigableSet()
Returns an empty navigable set (immutable). |
static
final
<T>
Set<T>
|
emptySet()
Returns an empty set (immutable). |
static
final
<K, V>
SortedMap<K, V>
|
emptySortedMap()
Returns an empty sorted map (immutable). |
static
<E>
SortedSet<E>
|
emptySortedSet()
Returns an empty sorted set (immutable). |
static
<T>
Enumeration<T>
|
enumeration(Collection<T> c)
Returns an enumeration over the specified collection. |
static
<T>
void
|
fill(List<? super T> list, T obj)
Replaces all of the elements of the specified list with the specified element. |
static
int
|
frequency(Collection<?> c, Object o)
Returns the number of elements in the specified collection equal to the specified object. |
static
int
|
indexOfSubList(List<?> source, List<?> target)
Returns the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. |
static
int
|
lastIndexOfSubList(List<?> source, List<?> target)
Returns the starting position of the last occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. |
static
<T>
ArrayList<T>
|
list(Enumeration<T> e)
Returns an array list containing the elements returned by the specified enumeration in the order they are returned by the enumeration. |
static
<T>
T
|
max(Collection<? extends T> coll, Comparator<? super T> comp)
Returns the maximum element of the given collection, according to the order induced by the specified comparator. |
static
<T extends Comparable<? super T>>
T
|
max(Collection<? extends T> coll)
Returns the maximum element of the given collection, according to the natural ordering of its elements. |
static
<T extends Comparable<? super T>>
T
|
min(Collection<? extends T> coll)
Returns the minimum element of the given collection, according to the natural ordering of its elements. |
static
<T>
T
|
min(Collection<? extends T> coll, Comparator<? super T> comp)
Returns the minimum element of the given collection, according to the order induced by the specified comparator. |
static
<T>
List<T>
|
nCopies(int n, T o)
Returns an immutable list consisting of |
static
<E>
SequencedSet<E>
|
newSequencedSetFromMap(SequencedMap<E, Boolean> map)
Returns a sequenced set backed by the specified map. |
static
<E>
Set<E>
|
newSetFromMap(Map<E, Boolean> map)
Returns a set backed by the specified map. |
static
<T>
boolean
|
replaceAll(List<T> list, T oldVal, T newVal)
Replaces all occurrences of one specified value in a list with another. |
static
void
|
reverse(List<?> list)
Reverses the order of the elements in the specified list. |
static
<T>
Comparator<T>
|
reverseOrder()
Returns a comparator that imposes the reverse of the natural
ordering on a collection of objects that implement the
|
static
<T>
Comparator<T>
|
reverseOrder(Comparator<T> cmp)
Returns a comparator that imposes the reverse ordering of the specified comparator. |
static
void
|
rotate(List<?> list, int distance)
Rotates the elements in the specified list by the specified distance. |
static
void
|
shuffle(List<?> list, Random rnd)
Randomly permute the specified list using the specified source of randomness. |
static
void
|
shuffle(List<?> list)
Randomly permutes the specified list using a default source of randomness. |
static
void
|
shuffle(List<?> list, RandomGenerator rnd)
Randomly permute the specified list using the specified source of randomness. |
static
<T>
Set<T>
|
singleton(T o)
Returns an immutable set containing only the specified object. |
static
<T>
List<T>
|
singletonList(T o)
Returns an immutable list containing only the specified object. |
static
<K, V>
Map<K, V>
|
singletonMap(K key, V value)
Returns an immutable map, mapping only the specified key to the specified value. |
static
<T extends Comparable<? super T>>
void
|
sort(List<T> list)
Sorts the specified list into ascending order, according to the natural ordering of its elements. |
static
<T>
void
|
sort(List<T> list, Comparator<? super T> c)
Sorts the specified list according to the order induced by the specified comparator. |
static
void
|
swap(List<?> list, int i, int j)
Swaps the elements at the specified positions in the specified list. |
static
<T>
Collection<T>
|
synchronizedCollection(Collection<T> c)
Returns a synchronized (thread-safe) collection backed by the specified collection. |
static
<T>
List<T>
|
synchronizedList(List<T> list)
Returns a synchronized (thread-safe) list backed by the specified list. |
static
<K, V>
Map<K, V>
|
synchronizedMap(Map<K, V> m)
Returns a synchronized (thread-safe) map backed by the specified map. |
static
<K, V>
NavigableMap<K, V>
|
synchronizedNavigableMap(NavigableMap<K, V> m)
Returns a synchronized (thread-safe) navigable map backed by the specified navigable map. |
static
<T>
NavigableSet<T>
|
synchronizedNavigableSet(NavigableSet<T> s)
Returns a synchronized (thread-safe) navigable set backed by the specified navigable set. |
static
<T>
Set<T>
|
synchronizedSet(Set<T> s)
Returns a synchronized (thread-safe) set backed by the specified set. |
static
<K, V>
SortedMap<K, V>
|
synchronizedSortedMap(SortedMap<K, V> m)
Returns a synchronized (thread-safe) sorted map backed by the specified sorted map. |
static
<T>
SortedSet<T>
|
synchronizedSortedSet(SortedSet<T> s)
Returns a synchronized (thread-safe) sorted set backed by the specified sorted set. |
static
<T>
Collection<T>
|
unmodifiableCollection(Collection<? extends T> c)
Returns an unmodifiable view of the specified collection. |
static
<T>
List<T>
|
unmodifiableList(List<? extends T> list)
Returns an unmodifiable view of the specified list. |
static
<K, V>
Map<K, V>
|
unmodifiableMap(Map<? extends K, ? extends V> m)
Returns an unmodifiable view of the specified map. |
static
<K, V>
NavigableMap<K, V>
|
unmodifiableNavigableMap(NavigableMap<K, ? extends V> m)
Returns an unmodifiable view of the specified navigable map. |
static
<T>
NavigableSet<T>
|
unmodifiableNavigableSet(NavigableSet<T> s)
Returns an unmodifiable view of the specified navigable set. |
static
<T>
SequencedCollection<T>
|
unmodifiableSequencedCollection(SequencedCollection<? extends T> c)
Returns an unmodifiable view of the
specified |
static
<K, V>
SequencedMap<K, V>
|
unmodifiableSequencedMap(SequencedMap<? extends K, ? extends V> m)
Returns an unmodifiable view of the
specified |
static
<T>
SequencedSet<T>
|
unmodifiableSequencedSet(SequencedSet<? extends T> s)
Returns an unmodifiable view of the
specified |
static
<T>
Set<T>
|
unmodifiableSet(Set<? extends T> s)
Returns an unmodifiable view of the specified set. |
static
<K, V>
SortedMap<K, V>
|
unmodifiableSortedMap(SortedMap<K, ? extends V> m)
Returns an unmodifiable view of the specified sorted map. |
static
<T>
SortedSet<T>
|
unmodifiableSortedSet(SortedSet<T> s)
Returns an unmodifiable view of the specified sorted set. |
Inherited methods | |||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Fields
EMPTY_LIST
public static final List<E> EMPTY_LIST
The empty list (immutable). This list is serializable.
See also:
EMPTY_MAP
public static final Map<K, V> EMPTY_MAP
The empty map (immutable). This map is serializable.
See also:
EMPTY_SET
public static final Set<E> EMPTY_SET
The empty set (immutable). This set is serializable.
See also:
Public methods
addAll
public static boolean addAll (Collection<? super T> c, T... elements)
Adds all of the specified elements to the specified collection.
Elements to be added may be specified individually or as an array.
The behaviour of this convenience method is similar to that of
c.addAll(Collections.unmodifiableList(Arrays.asList(elements)))
.
When elements are specified individually, this method provides a convenient way to add a few elements to an existing collection:
Collections.addAll(flavors, "Peaches 'n Plutonium", "Rocky Racoon");
Parameters | |
---|---|
c |
Collection : the collection into which elements are to be inserted |
elements |
T : the elements to insert into c |
Returns | |
---|---|
boolean |
true if the collection changed as a result of the call |
Throws | |
---|---|
UnsupportedOperationException |
if c does not support
the add operation |
NullPointerException |
if elements contains one or more
null values and c does not permit null elements, or
if c or elements are null |
IllegalArgumentException |
if some property of a value in
elements prevents it from being added to c |
See also:
asLifoQueue
public static Queue<T> asLifoQueue (Deque<T> deque)
Returns a view of a Deque
as a Last-in-first-out (Lifo)
Queue
. Method add
is mapped to push
,
remove
is mapped to pop
and so on. This
view can be useful when you would like to use a method
requiring a Queue
but you need Lifo ordering.
Each method invocation on the queue returned by this method
results in exactly one method invocation on the backing deque, with
one exception. The addAll
method is
implemented as a sequence of addFirst
invocations on the backing deque.
API Note:
- This method provides a view that inverts the sense of certain operations,
but it doesn't reverse the encounter order. To obtain a reverse-ordered
view, use the
Deque.reversed
method.
Parameters | |
---|---|
deque |
Deque : the deque |
Returns | |
---|---|
Queue<T> |
the queue |
binarySearch
public static int binarySearch (List<? extends T> list, T key, Comparator<? super T> c)
Searches the specified list for the specified object using the binary
search algorithm. The list must be sorted into ascending order
according to the specified comparator (as by the
sort(List, Comparator)
method), prior to making this call. If it is
not sorted, the results are undefined. If the list contains multiple
elements equal to the specified object, there is no guarantee which one
will be found.
This method runs in log(n) time for a "random access" list (which
provides near-constant-time positional access). If the specified list
does not implement the RandomAccess
interface and is large,
this method will do an iterator-based binary search that performs
O(n) link traversals and O(log n) element comparisons.
Parameters | |
---|---|
list |
List : the list to be searched. |
key |
T : the key to be searched for. |
c |
Comparator : the comparator by which the list is ordered.
A null value indicates that the elements'
natural ordering should be used. |
Returns | |
---|---|
int |
the index of the search key, if it is contained in the list;
otherwise, (-(insertion point) - 1) . The
insertion point is defined as the point at which the
key would be inserted into the list: the index of the first
element greater than the key, or list.size() if all
elements in the list are less than the specified key. Note
that this guarantees that the return value will be >= 0 if
and only if the key is found. |
Throws | |
---|---|
ClassCastException |
if the list contains elements that are not mutually comparable using the specified comparator, or the search key is not mutually comparable with the elements of the list using this comparator. |
binarySearch
public static int binarySearch (List<? extends Comparable<? super T>> list, T key)
Searches the specified list for the specified object using the binary
search algorithm. The list must be sorted into ascending order
according to the natural ordering of its
elements (as by the sort(java.util.List)
method) prior to making this
call. If it is not sorted, the results are undefined. If the list
contains multiple elements equal to the specified object, there is no
guarantee which one will be found.
This method runs in log(n) time for a "random access" list (which
provides near-constant-time positional access). If the specified list
does not implement the RandomAccess
interface and is large,
this method will do an iterator-based binary search that performs
O(n) link traversals and O(log n) element comparisons.
Parameters | |
---|---|
list |
List : the list to be searched. |
key |
T : the key to be searched for. |
Returns | |
---|---|
int |
the index of the search key, if it is contained in the list;
otherwise, (-(insertion point) - 1) . The
insertion point is defined as the point at which the
key would be inserted into the list: the index of the first
element greater than the key, or list.size() if all
elements in the list are less than the specified key. Note
that this guarantees that the return value will be >= 0 if
and only if the key is found. |
Throws | |
---|---|
ClassCastException |
if the list contains elements that are not mutually comparable (for example, strings and integers), or the search key is not mutually comparable with the elements of the list. |
checkedCollection
public static Collection<E> checkedCollection (Collection<E> c, Class<E> type)
Returns a dynamically typesafe view of the specified collection.
Any attempt to insert an element of the wrong type will result in an
immediate ClassCastException
. Assuming a collection
contains no incorrectly typed elements prior to the time a
dynamically typesafe view is generated, and that all subsequent
access to the collection takes place through the view, it is
guaranteed that the collection cannot contain an incorrectly
typed element.
The generics mechanism in the language provides compile-time (static) type checking, but it is possible to defeat this mechanism with unchecked casts. Usually this is not a problem, as the compiler issues warnings on all such unchecked operations. There are, however, times when static type checking alone is not sufficient. For example, suppose a collection is passed to a third-party library and it is imperative that the library code not corrupt the collection by inserting an element of the wrong type.
Another use of dynamically typesafe views is debugging. Suppose a
program fails with a ClassCastException
, indicating that an
incorrectly typed element was put into a parameterized collection.
Unfortunately, the exception can occur at any time after the erroneous
element is inserted, so it typically provides little or no information
as to the real source of the problem. If the problem is reproducible,
one can quickly determine its source by temporarily modifying the
program to wrap the collection with a dynamically typesafe view.
For example, this declaration:
Collection<String> c = new HashSet<>();
Collection<String> c = Collections.checkedCollection(
new HashSet<>(), String.class);
The returned collection does not pass the hashCode and equals
operations through to the backing collection, but relies on
Object
's equals
and hashCode
methods. This
is necessary to preserve the contracts of these operations in the case
that the backing collection is a set or a list.
The returned collection will be serializable if the specified collection is serializable.
Since null
is considered to be a value of any reference
type, the returned collection permits insertion of null elements
whenever the backing collection does.
Parameters | |
---|---|
c |
Collection : the collection for which a dynamically typesafe view is to be
returned |
type |
Class : the type of element that c is permitted to hold |
Returns | |
---|---|
Collection<E> |
a dynamically typesafe view of the specified collection |
checkedList
public static List<E> checkedList (List<E> list, Class<E> type)
Returns a dynamically typesafe view of the specified list.
Any attempt to insert an element of the wrong type will result in
an immediate ClassCastException
. Assuming a list contains
no incorrectly typed elements prior to the time a dynamically typesafe
view is generated, and that all subsequent access to the list
takes place through the view, it is guaranteed that the
list cannot contain an incorrectly typed element.
A discussion of the use of dynamically typesafe views may be
found in the documentation for the checkedCollection
method.
The returned list will be serializable if the specified list is serializable.
Since null
is considered to be a value of any reference
type, the returned list permits insertion of null elements whenever
the backing list does.
Parameters | |
---|---|
list |
List : the list for which a dynamically typesafe view is to be
returned |
type |
Class : the type of element that list is permitted to hold |
Returns | |
---|---|
List<E> |
a dynamically typesafe view of the specified list |
checkedMap
public static Map<K, V> checkedMap (Map<K, V> m, Class<K> keyType, Class<V> valueType)
Returns a dynamically typesafe view of the specified map.
Any attempt to insert a mapping whose key or value have the wrong
type will result in an immediate ClassCastException
.
Similarly, any attempt to modify the value currently associated with
a key will result in an immediate ClassCastException
,
whether the modification is attempted directly through the map
itself, or through a Map.Entry
instance obtained from the
map's entry set
view.
Assuming a map contains no incorrectly typed keys or values prior to the time a dynamically typesafe view is generated, and that all subsequent access to the map takes place through the view (or one of its collection views), it is guaranteed that the map cannot contain an incorrectly typed key or value.
A discussion of the use of dynamically typesafe views may be
found in the documentation for the checkedCollection
method.
The returned map will be serializable if the specified map is serializable.
Since null
is considered to be a value of any reference
type, the returned map permits insertion of null keys or values
whenever the backing map does.
Parameters | |
---|---|
m |
Map : the map for which a dynamically typesafe view is to be
returned |
keyType |
Class : the type of key that m is permitted to hold |
valueType |
Class : the type of value that m is permitted to hold |
Returns | |
---|---|
Map<K, V> |
a dynamically typesafe view of the specified map |
checkedNavigableMap
public static NavigableMap<K, V> checkedNavigableMap (NavigableMap<K, V> m, Class<K> keyType, Class<V> valueType)
Returns a dynamically typesafe view of the specified navigable map.
Any attempt to insert a mapping whose key or value have the wrong
type will result in an immediate ClassCastException
.
Similarly, any attempt to modify the value currently associated with
a key will result in an immediate ClassCastException
,
whether the modification is attempted directly through the map
itself, or through a Map.Entry
instance obtained from the
map's entry set
view.
Assuming a map contains no incorrectly typed keys or values prior to the time a dynamically typesafe view is generated, and that all subsequent access to the map takes place through the view (or one of its collection views), it is guaranteed that the map cannot contain an incorrectly typed key or value.
A discussion of the use of dynamically typesafe views may be
found in the documentation for the checkedCollection
method.
The returned map will be serializable if the specified map is serializable.
Since null
is considered to be a value of any reference
type, the returned map permits insertion of null keys or values
whenever the backing map does.
Parameters | |
---|---|
m |
NavigableMap : the map for which a dynamically typesafe view is to be
returned |
keyType |
Class : the type of key that m is permitted to hold |
valueType |
Class : the type of value that m is permitted to hold |
Returns | |
---|---|
NavigableMap<K, V> |
a dynamically typesafe view of the specified map |
checkedNavigableSet
public static NavigableSet<E> checkedNavigableSet (NavigableSet<E> s, Class<E> type)
Returns a dynamically typesafe view of the specified navigable set.
Any attempt to insert an element of the wrong type will result in an
immediate ClassCastException
. Assuming a navigable set
contains no incorrectly typed elements prior to the time a
dynamically typesafe view is generated, and that all subsequent
access to the navigable set takes place through the view, it is
guaranteed that the navigable set cannot contain an incorrectly
typed element.
A discussion of the use of dynamically typesafe views may be
found in the documentation for the checkedCollection
method.
The returned navigable set will be serializable if the specified navigable set is serializable.
Since null
is considered to be a value of any reference
type, the returned navigable set permits insertion of null elements
whenever the backing sorted set does.
Parameters | |
---|---|
s |
NavigableSet : the navigable set for which a dynamically typesafe view is to be
returned |
type |
Class : the type of element that s is permitted to hold |
Returns | |
---|---|
NavigableSet<E> |
a dynamically typesafe view of the specified navigable set |
checkedQueue
public static Queue<E> checkedQueue (Queue<E> queue, Class<E> type)
Returns a dynamically typesafe view of the specified queue.
Any attempt to insert an element of the wrong type will result in
an immediate ClassCastException
. Assuming a queue contains
no incorrectly typed elements prior to the time a dynamically typesafe
view is generated, and that all subsequent access to the queue
takes place through the view, it is guaranteed that the
queue cannot contain an incorrectly typed element.
A discussion of the use of dynamically typesafe views may be
found in the documentation for the checkedCollection
method.
The returned queue will be serializable if the specified queue is serializable.
Since null
is considered to be a value of any reference
type, the returned queue permits insertion of null
elements
whenever the backing queue does.
Parameters | |
---|---|
queue |
Queue : the queue for which a dynamically typesafe view is to be
returned |
type |
Class : the type of element that queue is permitted to hold |
Returns | |
---|---|
Queue<E> |
a dynamically typesafe view of the specified queue |
checkedSet
public static Set<E> checkedSet (Set<E> s, Class<E> type)
Returns a dynamically typesafe view of the specified set.
Any attempt to insert an element of the wrong type will result in
an immediate ClassCastException
. Assuming a set contains
no incorrectly typed elements prior to the time a dynamically typesafe
view is generated, and that all subsequent access to the set
takes place through the view, it is guaranteed that the
set cannot contain an incorrectly typed element.
A discussion of the use of dynamically typesafe views may be
found in the documentation for the checkedCollection
method.
The returned set will be serializable if the specified set is serializable.
Since null
is considered to be a value of any reference
type, the returned set permits insertion of null elements whenever
the backing set does.
Parameters | |
---|---|
s |
Set : the set for which a dynamically typesafe view is to be
returned |
type |
Class : the type of element that s is permitted to hold |
Returns | |
---|---|
Set<E> |
a dynamically typesafe view of the specified set |
checkedSortedMap
public static SortedMap<K, V> checkedSortedMap (SortedMap<K, V> m, Class<K> keyType, Class<V> valueType)
Returns a dynamically typesafe view of the specified sorted map.
Any attempt to insert a mapping whose key or value have the wrong
type will result in an immediate ClassCastException
.
Similarly, any attempt to modify the value currently associated with
a key will result in an immediate ClassCastException
,
whether the modification is attempted directly through the map
itself, or through a Map.Entry
instance obtained from the
map's entry set
view.
Assuming a map contains no incorrectly typed keys or values prior to the time a dynamically typesafe view is generated, and that all subsequent access to the map takes place through the view (or one of its collection views), it is guaranteed that the map cannot contain an incorrectly typed key or value.
A discussion of the use of dynamically typesafe views may be
found in the documentation for the checkedCollection
method.
The returned map will be serializable if the specified map is serializable.
Since null
is considered to be a value of any reference
type, the returned map permits insertion of null keys or values
whenever the backing map does.
Parameters | |
---|---|
m |
SortedMap : the map for which a dynamically typesafe view is to be
returned |
keyType |
Class : the type of key that m is permitted to hold |
valueType |
Class : the type of value that m is permitted to hold |
Returns | |
---|---|
SortedMap<K, V> |
a dynamically typesafe view of the specified map |
checkedSortedSet
public static SortedSet<E> checkedSortedSet (SortedSet<E> s, Class<E> type)
Returns a dynamically typesafe view of the specified sorted set.
Any attempt to insert an element of the wrong type will result in an
immediate ClassCastException
. Assuming a sorted set
contains no incorrectly typed elements prior to the time a
dynamically typesafe view is generated, and that all subsequent
access to the sorted set takes place through the view, it is
guaranteed that the sorted set cannot contain an incorrectly
typed element.
A discussion of the use of dynamically typesafe views may be
found in the documentation for the checkedCollection
method.
The returned sorted set will be serializable if the specified sorted set is serializable.
Since null
is considered to be a value of any reference
type, the returned sorted set permits insertion of null elements
whenever the backing sorted set does.
Parameters | |
---|---|
s |
SortedSet : the sorted set for which a dynamically typesafe view is to be
returned |
type |
Class : the type of element that s is permitted to hold |
Returns | |
---|---|
SortedSet<E> |
a dynamically typesafe view of the specified sorted set |
copy
public static void copy (List<? super T> dest, List<? extends T> src)
Copies all of the elements from one list into another. After the operation, the index of each copied element in the destination list will be identical to its index in the source list. The destination list's size must be greater than or equal to the source list's size. If it is greater, the remaining elements in the destination list are unaffected.
This method runs in linear time.
Parameters | |
---|---|
dest |
List : The destination list. |
src |
List : The source list. |
Throws | |
---|---|
IndexOutOfBoundsException |
if the destination list is too small to contain the entire source List. |
UnsupportedOperationException |
if the destination list's
list-iterator does not support the set operation. |
disjoint
public static boolean disjoint (Collection<?> c1, Collection<?> c2)
Returns true
if the two specified collections have no
elements in common.
Care must be exercised if this method is used on collections that
do not comply with the general contract for Collection
.
Implementations may elect to iterate over either collection and test
for containment in the other collection (or to perform any equivalent
computation). If either collection uses a nonstandard equality test
(as does a SortedSet
whose ordering is not compatible with
equals, or the key set of an IdentityHashMap
), both
collections must use the same nonstandard equality test, or the
result of this method is undefined.
Care must also be exercised when using collections that have restrictions on the elements that they may contain. Collection implementations are allowed to throw exceptions for any operation involving elements they deem ineligible. For absolute safety the specified collections should contain only elements which are eligible elements for both collections.
Note that it is permissible to pass the same collection in both
parameters, in which case the method will return true
if and
only if the collection is empty.
Parameters | |
---|---|
c1 |
Collection : a collection |
c2 |
Collection : a collection |
Returns | |
---|---|
boolean |
true if the two specified collections have no
elements in common. |
Throws | |
---|---|
NullPointerException |
if either collection is null . |
NullPointerException |
if one collection contains a null
element and null is not an eligible element for the other collection.
(optional) |
ClassCastException |
if one collection contains an element that is of a type which is ineligible for the other collection. (optional) |
emptyEnumeration
public static Enumeration<T> emptyEnumeration ()
Returns an enumeration that has no elements. More precisely,
hasMoreElements
always returnsfalse
.-
nextElement
always throwsNoSuchElementException
.
Implementations of this method are permitted, but not required, to return the same object from multiple invocations.
Returns | |
---|---|
Enumeration<T> |
an empty enumeration |
emptyIterator
public static Iterator<T> emptyIterator ()
Returns an iterator that has no elements. More precisely,
hasNext
always returnsfalse
.next
always throwsNoSuchElementException
.remove
always throwsIllegalStateException
.
Implementations of this method are permitted, but not required, to return the same object from multiple invocations.
Returns | |
---|---|
Iterator<T> |
an empty iterator |
emptyList
public static final List<T> emptyList ()
Returns an empty list (immutable). This list is serializable.
This example illustrates the type-safe way to obtain an empty list:
List<String> s = Collections.emptyList();
Implementation Note:
- Implementations of this method need not create a separate
List
object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.)
Returns | |
---|---|
List<T> |
an empty immutable list |
See also:
emptyListIterator
public static ListIterator<T> emptyListIterator ()
Returns a list iterator that has no elements. More precisely,
hasNext
andhasPrevious
always returnfalse
.next
andprevious
always throwNoSuchElementException
.remove
andset
always throwIllegalStateException
.add
always throwsUnsupportedOperationException
.nextIndex
always returns0
.previousIndex
always returns-1
.
Implementations of this method are permitted, but not required, to return the same object from multiple invocations.
Returns | |
---|---|
ListIterator<T> |
an empty list iterator |
emptyMap
public static final Map<K, V> emptyMap ()
Returns an empty map (immutable). This map is serializable.
This example illustrates the type-safe way to obtain an empty map:
Map<String, Date> s = Collections.emptyMap();
Implementation Note:
- Implementations of this method need not create a separate
Map
object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.)
Returns | |
---|---|
Map<K, V> |
an empty map |
See also:
emptyNavigableMap
public static final NavigableMap<K, V> emptyNavigableMap ()
Returns an empty navigable map (immutable). This map is serializable.
This example illustrates the type-safe way to obtain an empty map:
NavigableMap<String, Date> s = Collections.emptyNavigableMap();
Implementation Note:
- Implementations of this method need not create a separate
NavigableMap
object for each call.
Returns | |
---|---|
NavigableMap<K, V> |
an empty navigable map |
emptyNavigableSet
public static NavigableSet<E> emptyNavigableSet ()
Returns an empty navigable set (immutable). This set is serializable.
This example illustrates the type-safe way to obtain an empty navigable set:
NavigableSet<String> s = Collections.emptyNavigableSet();
Implementation Note:
- Implementations of this method need not
create a separate
NavigableSet
object for each call.
Returns | |
---|---|
NavigableSet<E> |
the empty navigable set |
emptySet
public static final Set<T> emptySet ()
Returns an empty set (immutable). This set is serializable. Unlike the like-named field, this method is parameterized.
This example illustrates the type-safe way to obtain an empty set:
Set<String> s = Collections.emptySet();
Implementation Note:
- Implementations of this method need not create a separate
Set
object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.)
Returns | |
---|---|
Set<T> |
the empty set |
See also:
emptySortedMap
public static final SortedMap<K, V> emptySortedMap ()
Returns an empty sorted map (immutable). This map is serializable.
This example illustrates the type-safe way to obtain an empty map:
SortedMap<String, Date> s = Collections.emptySortedMap();
Implementation Note:
- Implementations of this method need not create a separate
SortedMap
object for each call.
Returns | |
---|---|
SortedMap<K, V> |
an empty sorted map |
emptySortedSet
public static SortedSet<E> emptySortedSet ()
Returns an empty sorted set (immutable). This set is serializable.
This example illustrates the type-safe way to obtain an empty sorted set:
SortedSet<String> s = Collections.emptySortedSet();
Implementation Note:
- Implementations of this method need not create a separate
SortedSet
object for each call.
Returns | |
---|---|
SortedSet<E> |
the empty sorted set |
enumeration
public static Enumeration<T> enumeration (Collection<T> c)
Returns an enumeration over the specified collection. This provides interoperability with legacy APIs that require an enumeration as input.
The iterator returned from a call to Enumeration#asIterator()
does not support removal of elements from the specified collection. This
is necessary to avoid unintentionally increasing the capabilities of the
returned enumeration.
Parameters | |
---|---|
c |
Collection : the collection for which an enumeration is to be returned. |
Returns | |
---|---|
Enumeration<T> |
an enumeration over the specified collection. |
See also:
fill
public static void fill (List<? super T> list, T obj)
Replaces all of the elements of the specified list with the specified element.
This method runs in linear time.
Parameters | |
---|---|
list |
List : the list to be filled with the specified element. |
obj |
T : The element with which to fill the specified list. |
Throws | |
---|---|
UnsupportedOperationException |
if the specified list or its
list-iterator does not support the set operation. |
frequency
public static int frequency (Collection<?> c, Object o)
Returns the number of elements in the specified collection equal to the
specified object. More formally, returns the number of elements
e
in the collection such that
Objects.equals(o, e)
.
Parameters | |
---|---|
c |
Collection : the collection in which to determine the frequency
of o |
o |
Object : the object whose frequency is to be determined |
Returns | |
---|---|
int |
the number of elements in c equal to o |
Throws | |
---|---|
NullPointerException |
if c is null |
indexOfSubList
public static int indexOfSubList (List<?> source, List<?> target)
Returns the starting position of the first occurrence of the specified
target list within the specified source list, or -1 if there is no
such occurrence. More formally, returns the lowest index i
such that source.subList(i, i+target.size()).equals(target)
,
or -1 if there is no such index. (Returns -1 if
target.size() > source.size()
)
This implementation uses the "brute force" technique of scanning over the source list, looking for a match with the target at each location in turn.
Parameters | |
---|---|
source |
List : the list in which to search for the first occurrence
of target . |
target |
List : the list to search for as a subList of source . |
Returns | |
---|---|
int |
the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. |
lastIndexOfSubList
public static int lastIndexOfSubList (List<?> source, List<?> target)
Returns the starting position of the last occurrence of the specified
target list within the specified source list, or -1 if there is no such
occurrence. More formally, returns the highest index i
such that source.subList(i, i+target.size()).equals(target)
,
or -1 if there is no such index. (Returns -1 if
target.size() > source.size()
)
This implementation uses the "brute force" technique of iterating over the source list, looking for a match with the target at each location in turn.
Parameters | |
---|---|
source |
List : the list in which to search for the last occurrence
of target . |
target |
List : the list to search for as a subList of source . |
Returns | |
---|---|
int |
the starting position of the last occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. |
list
public static ArrayList<T> list (Enumeration<T> e)
Returns an array list containing the elements returned by the specified enumeration in the order they are returned by the enumeration. This method provides interoperability between legacy APIs that return enumerations and new APIs that require collections.
Parameters | |
---|---|
e |
Enumeration : enumeration providing elements for the returned
array list |
Returns | |
---|---|
ArrayList<T> |
an array list containing the elements returned by the specified enumeration. |
See also:
max
public static T max (Collection<? extends T> coll, Comparator<? super T> comp)
Returns the maximum element of the given collection, according to the
order induced by the specified comparator. All elements in the
collection must be mutually comparable by the specified
comparator (that is, comp.compare(e1, e2)
must not throw a
ClassCastException
for any elements e1
and
e2
in the collection).
This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
Parameters | |
---|---|
coll |
Collection : the collection whose maximum element is to be determined. |
comp |
Comparator : the comparator with which to determine the maximum element.
A null value indicates that the elements' natural
ordering should be used. |
Returns | |
---|---|
T |
the maximum element of the given collection, according to the specified comparator. |
Throws | |
---|---|
ClassCastException |
if the collection contains elements that are not mutually comparable using the specified comparator. |
NoSuchElementException |
if the collection is empty. |
See also:
max
public static T max (Collection<? extends T> coll)
Returns the maximum element of the given collection, according to the
natural ordering of its elements. All elements in the
collection must implement the Comparable
interface.
Furthermore, all elements in the collection must be mutually
comparable (that is, e1.compareTo(e2)
must not throw a
ClassCastException
for any elements e1
and
e2
in the collection).
This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
Parameters | |
---|---|
coll |
Collection : the collection whose maximum element is to be determined. |
Returns | |
---|---|
T |
the maximum element of the given collection, according to the natural ordering of its elements. |
Throws | |
---|---|
ClassCastException |
if the collection contains elements that are not mutually comparable (for example, strings and integers). |
NoSuchElementException |
if the collection is empty. |
See also:
min
public static T min (Collection<? extends T> coll)
Returns the minimum element of the given collection, according to the
natural ordering of its elements. All elements in the
collection must implement the Comparable
interface.
Furthermore, all elements in the collection must be mutually
comparable (that is, e1.compareTo(e2)
must not throw a
ClassCastException
for any elements e1
and
e2
in the collection).
This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
Parameters | |
---|---|
coll |
Collection : the collection whose minimum element is to be determined. |
Returns | |
---|---|
T |
the minimum element of the given collection, according to the natural ordering of its elements. |
Throws | |
---|---|
ClassCastException |
if the collection contains elements that are not mutually comparable (for example, strings and integers). |
NoSuchElementException |
if the collection is empty. |
See also:
min
public static T min (Collection<? extends T> coll, Comparator<? super T> comp)
Returns the minimum element of the given collection, according to the
order induced by the specified comparator. All elements in the
collection must be mutually comparable by the specified
comparator (that is, comp.compare(e1, e2)
must not throw a
ClassCastException
for any elements e1
and
e2
in the collection).
This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
Parameters | |
---|---|
coll |
Collection : the collection whose minimum element is to be determined. |
comp |
Comparator : the comparator with which to determine the minimum element.
A null value indicates that the elements' natural
ordering should be used. |
Returns | |
---|---|
T |
the minimum element of the given collection, according to the specified comparator. |
Throws | |
---|---|
ClassCastException |
if the collection contains elements that are not mutually comparable using the specified comparator. |
NoSuchElementException |
if the collection is empty. |
See also:
nCopies
public static List<T> nCopies (int n, T o)
Returns an immutable list consisting of n
copies of the
specified object. The newly allocated data object is tiny (it contains
a single reference to the data object). This method is useful in
combination with the List.addAll
method to grow lists.
The returned list is serializable.
Parameters | |
---|---|
n |
int : the number of elements in the returned list. |
o |
T : the element to appear repeatedly in the returned list. |
Returns | |
---|---|
List<T> |
an immutable list consisting of n copies of the
specified object. |
Throws | |
---|---|
IllegalArgumentException |
if n < 0 |
newSequencedSetFromMap
public static SequencedSet<E> newSequencedSetFromMap (SequencedMap<E, Boolean> map)
Returns a sequenced set backed by the specified map. The resulting set displays
the same ordering, concurrency, and performance characteristics as the
backing map. In essence, this factory method provides a SequencedSet
implementation corresponding to any SequencedMap
implementation.
Each method invocation on the set returned by this method results in
exactly one method invocation on the backing map or its keySet
view, with one exception. The addAll
method is implemented
as a sequence of put
invocations on the backing map.
The specified map must be empty at the time this method is invoked, and should not be accessed directly after this method returns. These conditions are ensured if the map is created empty, passed directly to this method, and no reference to the map is retained.
API Note:
- The following example code creates a
SequencedSet
from aLinkedHashMap
. This differs from aLinkedHashSet
in that the map'sremoveEldestEntry
is overridden to provide an eviction policy, which is not possible with aLinkedHashSet
.: SequencedSet<String> set = Collections.newSequencedSetFromMap( new LinkedHashMap<String, Boolean>() { protected boolean removeEldestEntry(Map.Entry<String, Boolean> e) { return this.size() > 5; } });
Parameters | |
---|---|
map |
SequencedMap : the backing map |
Returns | |
---|---|
SequencedSet<E> |
the set backed by the map |
Throws | |
---|---|
IllegalArgumentException |
if map is not empty |
newSetFromMap
public static Set<E> newSetFromMap (Map<E, Boolean> map)
Returns a set backed by the specified map. The resulting set displays
the same ordering, concurrency, and performance characteristics as the
backing map. In essence, this factory method provides a Set
implementation corresponding to any Map
implementation. There
is no need to use this method on a Map
implementation that
already has a corresponding Set
implementation (such as HashMap
or TreeMap
).
Each method invocation on the set returned by this method results in
exactly one method invocation on the backing map or its keySet
view, with one exception. The addAll
method is implemented
as a sequence of put
invocations on the backing map.
The specified map must be empty at the time this method is invoked, and should not be accessed directly after this method returns. These conditions are ensured if the map is created empty, passed directly to this method, and no reference to the map is retained, as illustrated in the following code fragment:
Set<Object> weakHashSet = Collections.newSetFromMap( new WeakHashMap<Object, Boolean>());
Parameters | |
---|---|
map |
Map : the backing map |
Returns | |
---|---|
Set<E> |
the set backed by the map |
Throws | |
---|---|
IllegalArgumentException |
if map is not empty |
replaceAll
public static boolean replaceAll (List<T> list, T oldVal, T newVal)
Replaces all occurrences of one specified value in a list with another.
More formally, replaces with newVal
each element e
in list
such that
(oldVal==null ? e==null : oldVal.equals(e))
.
(This method has no effect on the size of the list.)
Parameters | |
---|---|
list |
List : the list in which replacement is to occur. |
oldVal |
T : the old value to be replaced. |
newVal |
T : the new value with which oldVal is to be
replaced. |
Returns | |
---|---|
boolean |
true if list contained one or more elements
e such that
(oldVal==null ? e==null : oldVal.equals(e)) . |
Throws | |
---|---|
UnsupportedOperationException |
if the specified list or
its list-iterator does not support the set operation. |
reverse
public static void reverse (List<?> list)
Reverses the order of the elements in the specified list.
This method runs in linear time.
API Note:
- This method mutates the specified list in-place. To obtain a
reverse-ordered view of a list without mutating it, use the
List.reversed
method.
Parameters | |
---|---|
list |
List : the list whose elements are to be reversed. |
Throws | |
---|---|
UnsupportedOperationException |
if the specified list or
its list-iterator does not support the set operation. |
See also:
reverseOrder
public static Comparator<T> reverseOrder ()
Returns a comparator that imposes the reverse of the natural
ordering on a collection of objects that implement the
Comparable
interface. (The natural ordering is the ordering
imposed by the objects' own compareTo
method.) This enables a
simple idiom for sorting (or maintaining) collections (or arrays) of
objects that implement the Comparable
interface in
reverse-natural-order. For example, suppose a
is an array of
strings. Then:
Arrays.sort(a, Collections.reverseOrder());
The returned comparator is serializable.
API Note:
- This method returns a
Comparator
that is suitable for sorting elements in reverse order. To obtain a reverse-ordered view of a sequenced collection, use theSequencedCollection.reversed
method. Or, to obtain a reverse-ordered view of a sequenced map, use theSequencedMap.reversed
method.
Returns | |
---|---|
Comparator<T> |
A comparator that imposes the reverse of the natural
ordering on a collection of objects that implement
the Comparable interface. |
See also:
reverseOrder
public static Comparator<T> reverseOrder (Comparator<T> cmp)
Returns a comparator that imposes the reverse ordering of the specified
comparator. If the specified comparator is null
, this method is
equivalent to reverseOrder()
(in other words, it returns a
comparator that imposes the reverse of the natural ordering on
a collection of objects that implement the Comparable interface).
The returned comparator is serializable (assuming the specified
comparator is also serializable or null
).
API Note:
- This method returns a
Comparator
that is suitable for sorting elements in reverse order. To obtain a reverse-ordered view of a sequenced collection, use theSequencedCollection.reversed
method. Or, to obtain a reverse-ordered view of a sequenced map, use theSequencedMap.reversed
method.
Parameters | |
---|---|
cmp |
Comparator : a comparator who's ordering is to be reversed by the returned
comparator or null |
Returns | |
---|---|
Comparator<T> |
A comparator that imposes the reverse ordering of the specified comparator. |
rotate
public static void rotate (List<?> list, int distance)
Rotates the elements in the specified list by the specified distance.
After calling this method, the element at index i
will be
the element previously at index (i - distance)
mod
list.size()
, for all values of i
between 0
and list.size()-1
, inclusive. (This method has no effect on
the size of the list.)
For example, suppose list
comprises[t, a, n, k, s]
.
After invoking Collections.rotate(list, 1)
(or
Collections.rotate(list, -4)
), list
will comprise
[s, t, a, n, k]
.
Note that this method can usefully be applied to sublists to
move one or more elements within a list while preserving the
order of the remaining elements. For example, the following idiom
moves the element at index j
forward to position
k
(which must be greater than or equal to j
):
Collections.rotate(list.subList(j, k+1), -1);
list
comprises
[a, b, c, d, e]
. To move the element at index 1
(b
) forward two positions, perform the following invocation:
Collections.rotate(l.subList(1, 4), -1);
[a, c, d, b, e]
.
To move more than one element forward, increase the absolute value of the rotation distance. To move elements backward, use a positive shift distance.
If the specified list is small or implements the RandomAccess
interface, this implementation exchanges the first
element into the location it should go, and then repeatedly exchanges
the displaced element into the location it should go until a displaced
element is swapped into the first element. If necessary, the process
is repeated on the second and successive elements, until the rotation
is complete. If the specified list is large and doesn't implement the
RandomAccess
interface, this implementation breaks the
list into two sublist views around index -distance mod size
.
Then the reverse(java.util.List)
method is invoked on each sublist view,
and finally it is invoked on the entire list. For a more complete
description of both algorithms, see Section 2.3 of Jon Bentley's
Programming Pearls (Addison-Wesley, 1986).
Parameters | |
---|---|
list |
List : the list to be rotated. |
distance |
int : the distance to rotate the list. There are no
constraints on this value; it may be zero, negative, or
greater than list.size() . |
Throws | |
---|---|
UnsupportedOperationException |
if the specified list or
its list-iterator does not support the set operation. |
shuffle
public static void shuffle (List<?> list, Random rnd)
Randomly permute the specified list using the specified source of randomness.
This method is equivalent to shuffle(java.util.List, java.util.random.RandomGenerator)
and exists for backward compatibility. The shuffle(java.util.List, java.util.random.RandomGenerator)
method is preferred, as it is not limited to random generators
that extend the Random
class.
Parameters | |
---|---|
list |
List : the list to be shuffled. |
rnd |
Random : the source of randomness to use to shuffle the list. |
Throws | |
---|---|
UnsupportedOperationException |
if the specified list or its
list-iterator does not support the set operation. |
shuffle
public static void shuffle (List<?> list)
Randomly permutes the specified list using a default source of randomness. All permutations occur with approximately equal likelihood.
The hedge "approximately" is used in the foregoing description because default source of randomness is only approximately an unbiased source of independently chosen bits. If it were a perfect source of randomly chosen bits, then the algorithm would choose permutations with perfect uniformity.
This implementation traverses the list backwards, from the last element up to the second, repeatedly swapping a randomly selected element into the "current position". Elements are randomly selected from the portion of the list that runs from the first element to the current position, inclusive.
Implementation Requirements:
- This method runs in linear time. If the specified list does
not implement the
RandomAccess
interface and is large, this implementation dumps the specified list into an array before shuffling it, and dumps the shuffled array back into the list. This avoids the quadratic behavior that would result from shuffling a "sequential access" list in place.
Parameters | |
---|---|
list |
List : the list to be shuffled. |
Throws | |
---|---|
UnsupportedOperationException |
if the specified list or
its list-iterator does not support the set operation. |
shuffle
public static void shuffle (List<?> list, RandomGenerator rnd)
Randomly permute the specified list using the specified source of randomness. All permutations occur with equal likelihood assuming that the source of randomness is fair.
This implementation traverses the list backwards, from the last element up to the second, repeatedly swapping a randomly selected element into the "current position". Elements are randomly selected from the portion of the list that runs from the first element to the current position, inclusive.
Implementation Requirements:
- This method runs in linear time. If the specified list does
not implement the
RandomAccess
interface and is large, this implementation dumps the specified list into an array before shuffling it, and dumps the shuffled array back into the list. This avoids the quadratic behavior that would result from shuffling a "sequential access" list in place.
Parameters | |
---|---|
list |
List : the list to be shuffled. |
rnd |
RandomGenerator : the source of randomness to use to shuffle the list. |
Throws | |
---|---|
UnsupportedOperationException |
if the specified list or its
list-iterator does not support the set operation. |
singleton
public static Set<T> singleton (T o)
Returns an immutable set containing only the specified object. The returned set is serializable.
Parameters | |
---|---|
o |
T : the sole object to be stored in the returned set. |
Returns | |
---|---|
Set<T> |
an immutable set containing only the specified object. |
singletonList
public static List<T> singletonList (T o)
Returns an immutable list containing only the specified object. The returned list is serializable.
Parameters | |
---|---|
o |
T : the sole object to be stored in the returned list. |
Returns | |
---|---|
List<T> |
an immutable list containing only the specified object. |
singletonMap
public static Map<K, V> singletonMap (K key, V value)
Returns an immutable map, mapping only the specified key to the specified value. The returned map is serializable.
Parameters | |
---|---|
key |
K : the sole key to be stored in the returned map. |
value |
V : the value to which the returned map maps key . |
Returns | |
---|---|
Map<K, V> |
an immutable map containing only the specified key-value mapping. |
sort
public static void sort (List<T> list)
Sorts the specified list into ascending order, according to the
natural ordering of its elements.
All elements in the list must implement the Comparable
interface. Furthermore, all elements in the list must be
mutually comparable (that is, e1.compareTo(e2)
must not throw a ClassCastException
for any elements
e1
and e2
in the list).
This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort.
The specified list must be modifiable, but need not be resizable.
Implementation Note:
- This implementation defers to the
List#sort(Comparator)
method using the specified list and anull
comparator. Do not call this method fromList.sort()
since that can lead to infinite recursion. Apps targeting APIs<= 25
observe backwards compatibility behavior where this method was implemented on top ofList#toArray()
,ListIterator#next()
andListIterator#set(Object)
.
Parameters | |
---|---|
list |
List : the list to be sorted. |
Throws | |
---|---|
ClassCastException |
if the list contains elements that are not mutually comparable (for example, strings and integers). |
UnsupportedOperationException |
if the specified list's
list-iterator does not support the set operation. |
IllegalArgumentException |
(optional) if the implementation
detects that the natural ordering of the list elements is
found to violate the Comparable contract |
See also:
sort
public static void sort (List<T> list, Comparator<? super T> c)
Sorts the specified list according to the order induced by the
specified comparator. All elements in the list must be mutually
comparable using the specified comparator (that is,
c.compare(e1, e2)
must not throw a ClassCastException
for any elements e1
and e2
in the list).
This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort.
The specified list must be modifiable, but need not be resizable.
Implementation Note:
- This implementation defers to the
List#sort(Comparator)
method using the specified list and comparator. Do not call this method fromList.sort()
since that can lead to infinite recursion. Apps targeting APIs<= 25
observe backwards compatibility behavior where this method was implemented on top ofList#toArray()
,ListIterator#next()
andListIterator#set(Object)
.
Parameters | |
---|---|
list |
List : the list to be sorted. |
c |
Comparator : the comparator to determine the order of the list. A
null value indicates that the elements' natural
ordering should be used. |
Throws | |
---|---|
ClassCastException |
if the list contains elements that are not mutually comparable using the specified comparator. |
UnsupportedOperationException |
if the specified list's
list-iterator does not support the set operation. |
IllegalArgumentException |
(optional) if the comparator is
found to violate the Comparator contract |
See also:
swap
public static void swap (List<?> list, int i, int j)
Swaps the elements at the specified positions in the specified list. (If the specified positions are equal, invoking this method leaves the list unchanged.)
Parameters | |
---|---|
list |
List : The list in which to swap elements. |
i |
int : the index of one element to be swapped. |
j |
int : the index of the other element to be swapped. |
Throws | |
---|---|
IndexOutOfBoundsException |
if either i or j
is out of range (i < 0 || i >= list.size()
|| j < 0 || j >= list.size()). |
synchronizedCollection
public static Collection<T> synchronizedCollection (Collection<T> c)
Returns a synchronized (thread-safe) collection backed by the specified collection. In order to guarantee serial access, it is critical that all access to the backing collection is accomplished through the returned collection.
It is imperative that the user manually synchronize on the returned
collection when traversing it via Iterator
, Spliterator
or Stream
:
Collection c = Collections.synchronizedCollection(myCollection); ... synchronized (c) { Iterator i = c.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }
The returned collection does not pass the hashCode
and equals
operations through to the backing collection, but
relies on Object
's equals and hashCode methods. This is
necessary to preserve the contracts of these operations in the case
that the backing collection is a set or a list.
The returned collection will be serializable if the specified collection is serializable.
Parameters | |
---|---|
c |
Collection : the collection to be "wrapped" in a synchronized collection. |
Returns | |
---|---|
Collection<T> |
a synchronized view of the specified collection. |
synchronizedList
public static List<T> synchronizedList (List<T> list)
Returns a synchronized (thread-safe) list backed by the specified list. In order to guarantee serial access, it is critical that all access to the backing list is accomplished through the returned list.
It is imperative that the user manually synchronize on the returned
list when traversing it via Iterator
, Spliterator
or Stream
:
List list = Collections.synchronizedList(new ArrayList()); ... synchronized (list) { Iterator i = list.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }
The returned list will be serializable if the specified list is serializable.
Parameters | |
---|---|
list |
List : the list to be "wrapped" in a synchronized list. |
Returns | |
---|---|
List<T> |
a synchronized view of the specified list. |
synchronizedMap
public static Map<K, V> synchronizedMap (Map<K, V> m)
Returns a synchronized (thread-safe) map backed by the specified map. In order to guarantee serial access, it is critical that all access to the backing map is accomplished through the returned map.
It is imperative that the user manually synchronize on the returned
map when traversing any of its collection views via Iterator
,
Spliterator
or Stream
:
Map m = Collections.synchronizedMap(new HashMap()); ... Set s = m.keySet(); // Needn't be in synchronized block ... synchronized (m) { // Synchronizing on m, not s! Iterator i = s.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }
The returned map will be serializable if the specified map is serializable.
Parameters | |
---|---|
m |
Map : the map to be "wrapped" in a synchronized map. |
Returns | |
---|---|
Map<K, V> |
a synchronized view of the specified map. |
synchronizedNavigableMap
public static NavigableMap<K, V> synchronizedNavigableMap (NavigableMap<K, V> m)
Returns a synchronized (thread-safe) navigable map backed by the specified navigable map. In order to guarantee serial access, it is critical that all access to the backing navigable map is accomplished through the returned navigable map (or its views).
It is imperative that the user manually synchronize on the returned
navigable map when traversing any of its collection views, or the
collections views of any of its subMap
, headMap
or
tailMap
views, via Iterator
, Spliterator
or
Stream
:
NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap()); ... Set s = m.keySet(); // Needn't be in synchronized block ... synchronized (m) { // Synchronizing on m, not s! Iterator i = s.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }
NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap()); NavigableMap m2 = m.subMap(foo, true, bar, false); ... Set s2 = m2.keySet(); // Needn't be in synchronized block ... synchronized (m) { // Synchronizing on m, not m2 or s2! Iterator i = s2.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }
The returned navigable map will be serializable if the specified navigable map is serializable.
Parameters | |
---|---|
m |
NavigableMap : the navigable map to be "wrapped" in a synchronized navigable
map |
Returns | |
---|---|
NavigableMap<K, V> |
a synchronized view of the specified navigable map. |
synchronizedNavigableSet
public static NavigableSet<T> synchronizedNavigableSet (NavigableSet<T> s)
Returns a synchronized (thread-safe) navigable set backed by the specified navigable set. In order to guarantee serial access, it is critical that all access to the backing navigable set is accomplished through the returned navigable set (or its views).
It is imperative that the user manually synchronize on the returned
navigable set when traversing it, or any of its subSet
,
headSet
, or tailSet
views, via Iterator
,
Spliterator
or Stream
:
NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet()); ... synchronized (s) { Iterator i = s.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }
NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet()); NavigableSet s2 = s.headSet(foo, true); ... synchronized (s) { // Note: s, not s2!!! Iterator i = s2.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }
The returned navigable set will be serializable if the specified navigable set is serializable.
Parameters | |
---|---|
s |
NavigableSet : the navigable set to be "wrapped" in a synchronized navigable
set |
Returns | |
---|---|
NavigableSet<T> |
a synchronized view of the specified navigable set |
synchronizedSet
public static Set<T> synchronizedSet (Set<T> s)
Returns a synchronized (thread-safe) set backed by the specified set. In order to guarantee serial access, it is critical that all access to the backing set is accomplished through the returned set.
It is imperative that the user manually synchronize on the returned
collection when traversing it via Iterator
, Spliterator
or Stream
:
Set s = Collections.synchronizedSet(new HashSet()); ... synchronized (s) { Iterator i = s.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }
The returned set will be serializable if the specified set is serializable.
Parameters | |
---|---|
s |
Set : the set to be "wrapped" in a synchronized set. |
Returns | |
---|---|
Set<T> |
a synchronized view of the specified set. |
synchronizedSortedMap
public static SortedMap<K, V> synchronizedSortedMap (SortedMap<K, V> m)
Returns a synchronized (thread-safe) sorted map backed by the specified sorted map. In order to guarantee serial access, it is critical that all access to the backing sorted map is accomplished through the returned sorted map (or its views).
It is imperative that the user manually synchronize on the returned
sorted map when traversing any of its collection views, or the
collections views of any of its subMap
, headMap
or
tailMap
views, via Iterator
, Spliterator
or
Stream
:
SortedMap m = Collections.synchronizedSortedMap(new TreeMap()); ... Set s = m.keySet(); // Needn't be in synchronized block ... synchronized (m) { // Synchronizing on m, not s! Iterator i = s.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }
SortedMap m = Collections.synchronizedSortedMap(new TreeMap()); SortedMap m2 = m.subMap(foo, bar); ... Set s2 = m2.keySet(); // Needn't be in synchronized block ... synchronized (m) { // Synchronizing on m, not m2 or s2! Iterator i = s2.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }
The returned sorted map will be serializable if the specified sorted map is serializable.
Parameters | |
---|---|
m |
SortedMap : the sorted map to be "wrapped" in a synchronized sorted map. |
Returns | |
---|---|
SortedMap<K, V> |
a synchronized view of the specified sorted map. |
synchronizedSortedSet
public static SortedSet<T> synchronizedSortedSet (SortedSet<T> s)
Returns a synchronized (thread-safe) sorted set backed by the specified sorted set. In order to guarantee serial access, it is critical that all access to the backing sorted set is accomplished through the returned sorted set (or its views).
It is imperative that the user manually synchronize on the returned
sorted set when traversing it or any of its subSet
,
headSet
, or tailSet
views via Iterator
,
Spliterator
or Stream
:
SortedSet s = Collections.synchronizedSortedSet(new TreeSet()); ... synchronized (s) { Iterator i = s.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }
SortedSet s = Collections.synchronizedSortedSet(new TreeSet()); SortedSet s2 = s.headSet(foo); ... synchronized (s) { // Note: s, not s2!!! Iterator i = s2.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }
The returned sorted set will be serializable if the specified sorted set is serializable.
Parameters | |
---|---|
s |
SortedSet : the sorted set to be "wrapped" in a synchronized sorted set. |
Returns | |
---|---|
SortedSet<T> |
a synchronized view of the specified sorted set. |
unmodifiableCollection
public static Collection<T> unmodifiableCollection (Collection<? extends T> c)
Returns an unmodifiable view of the
specified collection. Query operations on the returned collection "read through"
to the specified collection, and attempts to modify the returned
collection, whether direct or via its iterator, result in an
UnsupportedOperationException
.
The returned collection does not pass the hashCode and equals
operations through to the backing collection, but relies on
Object
's equals
and hashCode
methods. This
is necessary to preserve the contracts of these operations in the case
that the backing collection is a set or a list.
The returned collection will be serializable if the specified collection is serializable.
Implementation Note:
- This method may return its argument if the argument is already unmodifiable.
Parameters | |
---|---|
c |
Collection : the collection for which an unmodifiable view is to be
returned. |
Returns | |
---|---|
Collection<T> |
an unmodifiable view of the specified collection. |
unmodifiableList
public static List<T> unmodifiableList (List<? extends T> list)
Returns an unmodifiable view of the
specified list. Query operations on the returned list "read through" to the
specified list, and attempts to modify the returned list, whether
direct or via its iterator, result in an
UnsupportedOperationException
.
The returned list will be serializable if the specified list
is serializable. Similarly, the returned list will implement
RandomAccess
if the specified list does.
Implementation Note:
- This method may return its argument if the argument is already unmodifiable.
Parameters | |
---|---|
list |
List : the list for which an unmodifiable view is to be returned. |
Returns | |
---|---|
List<T> |
an unmodifiable view of the specified list. |
unmodifiableMap
public static Map<K, V> unmodifiableMap (Map<? extends K, ? extends V> m)
Returns an unmodifiable view of the
specified map. Query operations on the returned map "read through"
to the specified map, and attempts to modify the returned
map, whether direct or via its collection views, result in an
UnsupportedOperationException
.
The returned map will be serializable if the specified map is serializable.
Implementation Note:
- This method may return its argument if the argument is already unmodifiable.
Parameters | |
---|---|
m |
Map : the map for which an unmodifiable view is to be returned. |
Returns | |
---|---|
Map<K, V> |
an unmodifiable view of the specified map. |
unmodifiableNavigableMap
public static NavigableMap<K, V> unmodifiableNavigableMap (NavigableMap<K, ? extends V> m)
Returns an unmodifiable view of the
specified navigable map. Query operations on the returned navigable map "read
through" to the specified navigable map. Attempts to modify the returned
navigable map, whether direct, via its collection views, or via its
subMap
, headMap
, or tailMap
views, result in
an UnsupportedOperationException
.
The returned navigable map will be serializable if the specified navigable map is serializable.
Implementation Note:
- This method may return its argument if the argument is already unmodifiable.
Parameters | |
---|---|
m |
NavigableMap : the navigable map for which an unmodifiable view is to be
returned |
Returns | |
---|---|
NavigableMap<K, V> |
an unmodifiable view of the specified navigable map |
unmodifiableNavigableSet
public static NavigableSet<T> unmodifiableNavigableSet (NavigableSet<T> s)
Returns an unmodifiable view of the
specified navigable set. Query operations on the returned navigable set "read
through" to the specified navigable set. Attempts to modify the returned
navigable set, whether direct, via its iterator, or via its
subSet
, headSet
, or tailSet
views, result in
an UnsupportedOperationException
.
The returned navigable set will be serializable if the specified navigable set is serializable.
Implementation Note:
- This method may return its argument if the argument is already unmodifiable.
Parameters | |
---|---|
s |
NavigableSet : the navigable set for which an unmodifiable view is to be
returned |
Returns | |
---|---|
NavigableSet<T> |
an unmodifiable view of the specified navigable set |
unmodifiableSequencedCollection
public static SequencedCollection<T> unmodifiableSequencedCollection (SequencedCollection<? extends T> c)
Returns an unmodifiable view of the
specified SequencedCollection
. Query operations on the returned collection
"read through" to the specified collection, and attempts to modify the returned
collection, whether direct or via its iterator, result in an
UnsupportedOperationException
.
The returned collection does not pass the hashCode
and
equals
operations through to the backing collection, but relies on
Object
's equals
and hashCode
methods. This
is necessary to preserve the contracts of these operations in the case
that the backing collection is a set or a list.
The returned collection will be serializable if the specified collection is serializable.
Implementation Note:
- This method may return its argument if the argument is already unmodifiable.
Parameters | |
---|---|
c |
SequencedCollection : the collection for which an unmodifiable view is to be
returned. |
Returns | |
---|---|
SequencedCollection<T> |
an unmodifiable view of the specified collection. |
unmodifiableSequencedMap
public static SequencedMap<K, V> unmodifiableSequencedMap (SequencedMap<? extends K, ? extends V> m)
Returns an unmodifiable view of the
specified SequencedMap
. Query operations on the returned map
"read through" to the specified map, and attempts to modify the returned
map, whether direct or via its collection views, result in an
UnsupportedOperationException
.
The returned map will be serializable if the specified map is serializable.
Implementation Note:
- This method may return its argument if the argument is already unmodifiable.
Parameters | |
---|---|
m |
SequencedMap : the map for which an unmodifiable view is to be returned. |
Returns | |
---|---|
SequencedMap<K, V> |
an unmodifiable view of the specified map. |
unmodifiableSequencedSet
public static SequencedSet<T> unmodifiableSequencedSet (SequencedSet<? extends T> s)
Returns an unmodifiable view of the
specified SequencedSet
. Query operations on the returned set
"read through" to the specified set, and attempts to modify the returned
set, whether direct or via its iterator, result in an
UnsupportedOperationException
.
The returned set will be serializable if the specified set is serializable.
Implementation Note:
- This method may return its argument if the argument is already unmodifiable.
Parameters | |
---|---|
s |
SequencedSet : the set for which an unmodifiable view is to be returned. |
Returns | |
---|---|
SequencedSet<T> |
an unmodifiable view of the specified sequenced set. |
unmodifiableSet
public static Set<T> unmodifiableSet (Set<? extends T> s)
Returns an unmodifiable view of the
specified set. Query operations on the returned set "read through" to the specified
set, and attempts to modify the returned set, whether direct or via its
iterator, result in an UnsupportedOperationException
.
The returned set will be serializable if the specified set is serializable.
Implementation Note:
- This method may return its argument if the argument is already unmodifiable.
Parameters | |
---|---|
s |
Set : the set for which an unmodifiable view is to be returned. |
Returns | |
---|---|
Set<T> |
an unmodifiable view of the specified set. |
unmodifiableSortedMap
public static SortedMap<K, V> unmodifiableSortedMap (SortedMap<K, ? extends V> m)
Returns an unmodifiable view of the
specified sorted map. Query operations on the returned sorted map "read through"
to the specified sorted map. Attempts to modify the returned
sorted map, whether direct, via its collection views, or via its
subMap
, headMap
, or tailMap
views, result in
an UnsupportedOperationException
.
The returned sorted map will be serializable if the specified sorted map is serializable.
Implementation Note:
- This method may return its argument if the argument is already unmodifiable.
Parameters | |
---|---|
m |
SortedMap : the sorted map for which an unmodifiable view is to be
returned. |
Returns | |
---|---|
SortedMap<K, V> |
an unmodifiable view of the specified sorted map. |
unmodifiableSortedSet
public static SortedSet<T> unmodifiableSortedSet (SortedSet<T> s)
Returns an unmodifiable view of the
specified sorted set. Query operations on the returned sorted set "read
through" to the specified sorted set. Attempts to modify the returned
sorted set, whether direct, via its iterator, or via its
subSet
, headSet
, or tailSet
views, result in
an UnsupportedOperationException
.
The returned sorted set will be serializable if the specified sorted set is serializable.
Implementation Note:
- This method may return its argument if the argument is already unmodifiable.
Parameters | |
---|---|
s |
SortedSet : the sorted set for which an unmodifiable view is to be
returned. |
Returns | |
---|---|
SortedSet<T> |
an unmodifiable view of the specified sorted set. |