public class Collections extends Object
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.
Collection
,
Set
,
List
,
Map
Modifier and Type | Field and Description |
---|---|
static List |
EMPTY_LIST
The empty list (immutable).
|
static Map |
EMPTY_MAP
The empty map (immutable).
|
static Set |
EMPTY_SET
The empty set (immutable).
|
Modifier and Type | Method and Description |
---|---|
static <T> boolean |
addAll(Collection
Adds all of the specified elements to the specified collection.
|
static <T> Queue |
asLifoQueue(Deque
|
static <T> int |
binarySearch(List
Searches the specified list for the specified object using the binary search algorithm.
|
static <T> int |
binarySearch(List
Searches the specified list for the specified object using the binary search algorithm.
|
static <E> Collection |
checkedCollection(Collection
Returns a dynamically typesafe view of the specified collection.
|
static <E> List |
checkedList(List
Returns a dynamically typesafe view of the specified list.
|
static <K |
checkedMap(Map
Returns a dynamically typesafe view of the specified map.
|
static <K |
checkedNavigableMap(NavigableMap
Returns a dynamically typesafe view of the specified navigable map.
|
static <E> NavigableSet |
checkedNavigableSet(NavigableSet
Returns a dynamically typesafe view of the specified navigable set.
|
static <E> Queue |
checkedQueue(Queue
Returns a dynamically typesafe view of the specified queue.
|
static <E> Set |
checkedSet(Set
Returns a dynamically typesafe view of the specified set.
|
static <K |
checkedSortedMap(SortedMap
Returns a dynamically typesafe view of the specified sorted map.
|
static <E> SortedSet |
checkedSortedSet(SortedSet
Returns a dynamically typesafe view of the specified sorted set.
|
static <T> void |
copy(List
Copies all of the elements from one list into another.
|
static boolean |
disjoint(Collection
Returns
true if the two specified collections have no elements in common.
|
static <T> Enumeration |
emptyEnumeration()
Returns an enumeration that has no elements.
|
static <T> Iterator |
emptyIterator()
Returns an iterator that has no elements.
|
static <T> List |
emptyList()
Returns an empty list (immutable).
|
static <T> ListIterator |
emptyListIterator()
Returns a list iterator that has no elements.
|
static <K |
emptyMap()
Returns an empty map (immutable).
|
static <K |
emptyNavigableMap()
Returns an empty navigable map (immutable).
|
static <E> NavigableSet |
emptyNavigableSet()
Returns an empty navigable set (immutable).
|
static <T> Set |
emptySet()
Returns an empty set (immutable).
|
static <K |
emptySortedMap()
Returns an empty sorted map (immutable).
|
static <E> SortedSet |
emptySortedSet()
Returns an empty sorted set (immutable).
|
static <T> Enumeration |
enumeration(Collection
Returns an enumeration over the specified collection.
|
static <T> void |
fill(List
Replaces all of the elements of the specified list with the specified element.
|
static int |
frequency(Collection
Returns the number of elements in the specified collection equal to the specified object.
|
static int |
indexOfSubList(List
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
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 |
list(Enumeration
Returns an array list containing the elements returned by the specified enumeration in the order they are returned by the enumeration.
|
static <T extends Object |
max(Collection
Returns the maximum element of the given collection, according to the
natural ordering of its elements.
|
static <T> T |
max(Collection
Returns the maximum element of the given collection, according to the order induced by the specified comparator.
|
static <T extends Object |
min(Collection
Returns the minimum element of the given collection, according to the
natural ordering of its elements.
|
static <T> T |
min(Collection
Returns the minimum element of the given collection, according to the order induced by the specified comparator.
|
static <T> List |
nCopies(int n, T o)
Returns an immutable list consisting of
n copies of the specified object.
|
static <E> Set |
newSetFromMap(Map
Returns a set backed by the specified map.
|
static <T> boolean |
replaceAll(List
Replaces all occurrences of one specified value in a list with another.
|
static void |
reverse(List
Reverses the order of the elements in the specified list.
|
static <T> Comparator |
reverseOrder()
Returns a comparator that imposes the reverse of the
natural ordering on a collection of objects that implement the
Comparable interface.
|
static <T> Comparator |
reverseOrder(Comparator
Returns a comparator that imposes the reverse ordering of the specified comparator.
|
static void |
rotate(List
Rotates the elements in the specified list by the specified distance.
|
static void |
shuffle(List
Randomly permutes the specified list using a default source of randomness.
|
static void |
shuffle(List
Randomly permute the specified list using the specified source of randomness.
|
static <T> Set |
singleton(T o)
Returns an immutable set containing only the specified object.
|
static <T> List |
singletonList(T o)
Returns an immutable list containing only the specified object.
|
static <K |
singletonMap(K key, V value)
Returns an immutable map, mapping only the specified key to the specified value.
|
static <T extends Comparable |
sort(List
Sorts the specified list into ascending order, according to the
natural ordering of its elements.
|
static <T> void |
sort(List
Sorts the specified list according to the order induced by the specified comparator.
|
static void |
swap(List
Swaps the elements at the specified positions in the specified list.
|
static <T> Collection |
synchronizedCollection(Collection
Returns a synchronized (thread-safe) collection backed by the specified collection.
|
static <T> List |
synchronizedList(List
Returns a synchronized (thread-safe) list backed by the specified list.
|
static <K |
synchronizedMap(Map
Returns a synchronized (thread-safe) map backed by the specified map.
|
static <K |
synchronizedNavigableMap(NavigableMap
Returns a synchronized (thread-safe) navigable map backed by the specified navigable map.
|
static <T> NavigableSet |
synchronizedNavigableSet(NavigableSet
Returns a synchronized (thread-safe) navigable set backed by the specified navigable set.
|
static <T> Set |
synchronizedSet(Set
Returns a synchronized (thread-safe) set backed by the specified set.
|
static <K |
synchronizedSortedMap(SortedMap
Returns a synchronized (thread-safe) sorted map backed by the specified sorted map.
|
static <T> SortedSet |
synchronizedSortedSet(SortedSet
Returns a synchronized (thread-safe) sorted set backed by the specified sorted set.
|
static <T> Collection |
unmodifiableCollection(Collection
Returns an unmodifiable view of the specified collection.
|
static <T> List |
unmodifiableList(List
Returns an unmodifiable view of the specified list.
|
static <K |
unmodifiableMap(Map
Returns an unmodifiable view of the specified map.
|
static <K |
unmodifiableNavigableMap(NavigableMap
Returns an unmodifiable view of the specified navigable map.
|
static <T> NavigableSet |
unmodifiableNavigableSet(NavigableSet
Returns an unmodifiable view of the specified navigable set.
|
static <T> Set |
unmodifiableSet(Set
Returns an unmodifiable view of the specified set.
|
static <K |
unmodifiableSortedMap(SortedMap
Returns an unmodifiable view of the specified sorted map.
|
static <T> SortedSet |
unmodifiableSortedSet(SortedSet
Returns an unmodifiable view of the specified sorted set.
|
public static final SetEMPTY_SET
emptySet()
public static final ListEMPTY_LIST
emptyList()
public static final MapEMPTY_MAP
emptyMap()
public static <T extends Comparable<? super T>> void sort(List <T> list)
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 is a stable, adaptive, iterative mergesort that requires far fewer than n lg(n) comparisons when the input array is partially sorted, while offering the performance of a traditional mergesort when the input array is randomly ordered. If the input array is nearly sorted, the implementation requires approximately n comparisons. Temporary storage requirements vary from a small constant for nearly sorted input arrays to n/2 object references for randomly ordered input arrays.
The implementation takes equal advantage of ascending and descending order in its input array, and can take advantage of ascending and descending order in different parts of the same input array. It is well-suited to merging two or more sorted arrays: simply concatenate the arrays and sort the resulting array.
The implementation was adapted from Tim Peters's list sort for Python ( TimSort). It uses techniques from Peter McIlroy's "Optimistic Sorting and Information Theoretic Complexity", in Proceedings of the Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474, January 1993.
This implementation dumps the specified list into an array, sorts the array, and iterates over the list resetting each element from the corresponding position in the array. This avoids the n2 log(n) performance that would result from attempting to sort a linked list in place.
T
- the class of the objects in the list
list
- the list to be sorted.
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
public static <T> void sort(List<T> list, Comparator <? super T> c)
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 is a stable, adaptive, iterative mergesort that requires far fewer than n lg(n) comparisons when the input array is partially sorted, while offering the performance of a traditional mergesort when the input array is randomly ordered. If the input array is nearly sorted, the implementation requires approximately n comparisons. Temporary storage requirements vary from a small constant for nearly sorted input arrays to n/2 object references for randomly ordered input arrays.
The implementation takes equal advantage of ascending and descending order in its input array, and can take advantage of ascending and descending order in different parts of the same input array. It is well-suited to merging two or more sorted arrays: simply concatenate the arrays and sort the resulting array.
The implementation was adapted from Tim Peters's list sort for Python ( TimSort). It uses techniques from Peter McIlroy's "Optimistic Sorting and Information Theoretic Complexity", in Proceedings of the Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474, January 1993.
This implementation dumps the specified list into an array, sorts the array, and iterates over the list resetting each element from the corresponding position in the array. This avoids the n2 log(n) performance that would result from attempting to sort a linked list in place.
T
- the class of the objects in the list
list
- the list to be sorted.
c
- the comparator to determine the order of the list. A
null
value indicates that the elements'
natural ordering should be used.
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
public static <T> int binarySearch(List<? extends Comparable <? super T>> list, T key)
sort(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.
T
- the class of the objects in the list
list
- the list to be searched.
key
- the key to be searched for.
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.
public static <T> int binarySearch(List<? extends T> list, T key, Comparator <? super T> c)
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.
T
- the class of the objects in the list
list
- the list to be searched.
key
- the key to be searched for.
c
- the comparator by which the list is ordered. A
null value indicates that the elements'
natural ordering should be used.
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.
public static void reverse(List<?> list)
This method runs in linear time.
list
- the list whose elements are to be reversed.
UnsupportedOperationException
- if the specified list or its list-iterator does not support the
set operation.
public static void shuffle(List<?> list)
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.
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.
list
- the list to be shuffled.
UnsupportedOperationException
- if the specified list or its list-iterator does not support the
set operation.
public static void shuffle(List<?> list, Random rnd)
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.
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.
list
- the list to be shuffled.
rnd
- the source of randomness to use to shuffle the list.
UnsupportedOperationException
- if the specified list or its list-iterator does not support the
set operation.
public static void swap(List<?> list, int i, int j)
list
- The list in which to swap elements.
i
- the index of one element to be swapped.
j
- the index of the other element to be swapped.
IndexOutOfBoundsException
- if either
i or
j is out of range (i < 0 || i >= list.size() || j < 0 || j >= list.size()).
public static <T> void fill(List<? super T> list, T obj)
This method runs in linear time.
T
- the class of the objects in the list
list
- the list to be filled with the specified element.
obj
- The element with which to fill the specified list.
UnsupportedOperationException
- if the specified list or its list-iterator does not support the
set operation.
public static <T> void copy(List<? super T> dest, List <? extends T> src)
This method runs in linear time.
T
- the class of the objects in the lists
dest
- The destination list.
src
- The source list.
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.
public static <T extends Object& Comparable <? super T>> T min(Collection <? extends T> coll)
This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
T
- the class of the objects in the collection
coll
- the collection whose minimum element is to be determined.
ClassCastException
- if the collection contains elements that are not
mutually comparable (for example, strings and integers).
NoSuchElementException
- if the collection is empty.
Comparable
public static <T> T min(Collection<? extends T> coll, Comparator <? super T> comp)
This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
T
- the class of the objects in the collection
coll
- the collection whose minimum element is to be determined.
comp
- the comparator with which to determine the minimum element. A
null value indicates that the elements'
natural ordering should be used.
ClassCastException
- if the collection contains elements that are not
mutually comparable using the specified comparator.
NoSuchElementException
- if the collection is empty.
Comparable
public static <T extends Object& Comparable <? super T>> T max(Collection <? extends T> coll)
This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
T
- the class of the objects in the collection
coll
- the collection whose maximum element is to be determined.
ClassCastException
- if the collection contains elements that are not
mutually comparable (for example, strings and integers).
NoSuchElementException
- if the collection is empty.
Comparable
public static <T> T max(Collection<? extends T> coll, Comparator <? super T> comp)
This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
T
- the class of the objects in the collection
coll
- the collection whose maximum element is to be determined.
comp
- the comparator with which to determine the maximum element. A
null value indicates that the elements'
natural ordering should be used.
ClassCastException
- if the collection contains elements that are not
mutually comparable using the specified comparator.
NoSuchElementException
- if the collection is empty.
Comparable
public static void rotate(List<?> list, int distance)
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);To make this concrete, suppose 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);The resulting list is [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(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).
list
- the list to be rotated.
distance
- the distance to rotate the list. There are no constraints on this value; it may be zero, negative, or greater than
list.size().
UnsupportedOperationException
- if the specified list or its list-iterator does not support the
set operation.
public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal)
T
- the class of the objects in the list
list
- the list in which replacement is to occur.
oldVal
- the old value to be replaced.
newVal
- the new value with which
oldVal is to be replaced.
UnsupportedOperationException
- if the specified list or its list-iterator does not support the
set operation.
public static int indexOfSubList(List<?> source, List <?> target)
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.
source
- the list in which to search for the first occurrence of
target.
target
- the list to search for as a subList of
source.
public static int lastIndexOfSubList(List<?> source, List <?> target)
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.
source
- the list in which to search for the last occurrence of
target.
target
- the list to search for as a subList of
source.
public static <T> Collection<T> unmodifiableCollection(Collection <? extends T> c)
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.
T
- the class of the objects in the collection
c
- the collection for which an unmodifiable view is to be returned.
public static <T> Set<T> unmodifiableSet(Set <? extends T> s)
The returned set will be serializable if the specified set is serializable.
T
- the class of the objects in the set
s
- the set for which an unmodifiable view is to be returned.
public static <T> SortedSet<T> unmodifiableSortedSet(SortedSet <T> s)
The returned sorted set will be serializable if the specified sorted set is serializable.
T
- the class of the objects in the set
s
- the sorted set for which an unmodifiable view is to be returned.
public static <T> NavigableSet<T> unmodifiableNavigableSet(NavigableSet <T> s)
subSet
,
headSet
, or
tailSet
views, result in an
UnsupportedOperationException
.
The returned navigable set will be serializable if the specified navigable set is serializable.
T
- the class of the objects in the set
s
- the navigable set for which an unmodifiable view is to be returned
public static <T> List<T> unmodifiableList(List <? extends T> list)
The returned list will be serializable if the specified list is serializable. Similarly, the returned list will implement RandomAccess
if the specified list does.
T
- the class of the objects in the list
list
- the list for which an unmodifiable view is to be returned.
public static <K,V> Map <K ,V> unmodifiableMap(Map <? extends K ,? extends V> m)
The returned map will be serializable if the specified map is serializable.
K
- the class of the map keys
V
- the class of the map values
m
- the map for which an unmodifiable view is to be returned.
public static <K,V> SortedMap <K ,V> unmodifiableSortedMap(SortedMap <K ,? extends V> m)
The returned sorted map will be serializable if the specified sorted map is serializable.
K
- the class of the map keys
V
- the class of the map values
m
- the sorted map for which an unmodifiable view is to be returned.
public static <K,V> NavigableMap <K ,V> unmodifiableNavigableMap(NavigableMap <K ,? extends V> m)
subMap
,
headMap
, or
tailMap
views, result in an
UnsupportedOperationException
.
The returned navigable map will be serializable if the specified navigable map is serializable.
K
- the class of the map keys
V
- the class of the map values
m
- the navigable map for which an unmodifiable view is to be returned
public static <T> Collection<T> synchronizedCollection(Collection <T> c)
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()); }Failure to follow this advice may result in non-deterministic behavior.
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.
T
- the class of the objects in the collection
c
- the collection to be "wrapped" in a synchronized collection.
public static <T> Set<T> synchronizedSet(Set <T> s)
It is imperative that the user manually synchronize on the returned set when iterating over it:
Set s = Collections.synchronizedSet(new HashSet()); ... synchronized (s) { Iterator i = s.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }Failure to follow this advice may result in non-deterministic behavior.
The returned set will be serializable if the specified set is serializable.
T
- the class of the objects in the set
s
- the set to be "wrapped" in a synchronized set.
public static <T> SortedSet<T> synchronizedSortedSet(SortedSet <T> s)
It is imperative that the user manually synchronize on the returned sorted set when iterating over it or any of its subSet, headSet, or tailSet views.
SortedSet s = Collections.synchronizedSortedSet(new TreeSet()); ... synchronized (s) { Iterator i = s.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }or:
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()); }Failure to follow this advice may result in non-deterministic behavior.
The returned sorted set will be serializable if the specified sorted set is serializable.
T
- the class of the objects in the set
s
- the sorted set to be "wrapped" in a synchronized sorted set.
public static <T> NavigableSet<T> synchronizedNavigableSet(NavigableSet <T> s)
It is imperative that the user manually synchronize on the returned navigable set when iterating over it or any of its subSet
, headSet
, or tailSet
views.
NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet()); ... synchronized (s) { Iterator i = s.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }or:
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()); }Failure to follow this advice may result in non-deterministic behavior.
The returned navigable set will be serializable if the specified navigable set is serializable.
T
- the class of the objects in the set
s
- the navigable set to be "wrapped" in a synchronized navigable set
public static <T> List<T> synchronizedList(List <T> list)
It is imperative that the user manually synchronize on the returned list when iterating over it:
List list = Collections.synchronizedList(new ArrayList()); ... synchronized (list) { Iterator i = list.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }Failure to follow this advice may result in non-deterministic behavior.
The returned list will be serializable if the specified list is serializable.
T
- the class of the objects in the list
list
- the list to be "wrapped" in a synchronized list.
public static <K,V> Map <K ,V> synchronizedMap(Map <K ,V> m)
It is imperative that the user manually synchronize on the returned map when iterating over any of its collection views:
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()); }Failure to follow this advice may result in non-deterministic behavior.
The returned map will be serializable if the specified map is serializable.
K
- the class of the map keys
V
- the class of the map values
m
- the map to be "wrapped" in a synchronized map.
public static <K,V> SortedMap <K ,V> synchronizedSortedMap(SortedMap <K ,V> m)
It is imperative that the user manually synchronize on the returned sorted map when iterating over any of its collection views, or the collections views of any of its subMap, headMap or tailMap views.
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()); }or:
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 = s.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }Failure to follow this advice may result in non-deterministic behavior.
The returned sorted map will be serializable if the specified sorted map is serializable.
K
- the class of the map keys
V
- the class of the map values
m
- the sorted map to be "wrapped" in a synchronized sorted map.
public static <K,V> NavigableMap <K ,V> synchronizedNavigableMap(NavigableMap <K ,V> m)
It is imperative that the user manually synchronize on the returned navigable map when iterating over any of its collection views, or the collections views of any of its subMap
, headMap
or tailMap
views.
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()); }or:
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 = s.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }Failure to follow this advice may result in non-deterministic behavior.
The returned navigable map will be serializable if the specified navigable map is serializable.
K
- the class of the map keys
V
- the class of the map values
m
- the navigable map to be "wrapped" in a synchronized navigable map
public static <E> Collection<E> checkedCollection(Collection <E> c, Class <E> type)
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<>();
may be replaced temporarily by this one:
Collection<String> c = Collections.checkedCollection( new HashSet<>(), String.class);
Running the program again will cause it to fail at the point where an incorrectly typed element is inserted into the collection, clearly identifying the source of the problem. Once the problem is fixed, the modified declaration may be reverted back to the original.
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.
E
- the class of the objects in the collection
c
- the collection for which a dynamically typesafe view is to be returned
type
- the type of element that
c
is permitted to hold
public static <E> Queue<E> checkedQueue(Queue <E> queue, Class <E> type)
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.
E
- the class of the objects in the queue
queue
- the queue for which a dynamically typesafe view is to be returned
type
- the type of element that
queue
is permitted to hold
public static <E> Set<E> checkedSet(Set <E> s, Class <E> type)
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.
E
- the class of the objects in the set
s
- the set for which a dynamically typesafe view is to be returned
type
- the type of element that
s
is permitted to hold
public static <E> SortedSet<E> checkedSortedSet(SortedSet <E> s, Class <E> type)
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.
E
- the class of the objects in the set
s
- the sorted set for which a dynamically typesafe view is to be returned
type
- the type of element that
s
is permitted to hold
public static <E> NavigableSet<E> checkedNavigableSet(NavigableSet <E> s, Class <E> type)
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.
E
- the class of the objects in the set
s
- the navigable set for which a dynamically typesafe view is to be returned
type
- the type of element that
s
is permitted to hold
public static <E> List<E> checkedList(List <E> list, Class <E> type)
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.
E
- the class of the objects in the list
list
- the list for which a dynamically typesafe view is to be returned
type
- the type of element that
list
is permitted to hold
public static <K,V> Map <K ,V> checkedMap(Map <K ,V> m, Class <K> keyType, Class <V> valueType)
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.
K
- the class of the map keys
V
- the class of the map values
m
- the map for which a dynamically typesafe view is to be returned
keyType
- the type of key that
m
is permitted to hold
valueType
- the type of value that
m
is permitted to hold
public static <K,V> SortedMap <K ,V> checkedSortedMap(SortedMap <K ,V> m, Class <K> keyType, Class <V> valueType)
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.
K
- the class of the map keys
V
- the class of the map values
m
- the map for which a dynamically typesafe view is to be returned
keyType
- the type of key that
m
is permitted to hold
valueType
- the type of value that
m
is permitted to hold
public static <K,V> NavigableMap <K ,V> checkedNavigableMap(NavigableMap <K ,V> m, Class <K> keyType, Class <V> valueType)
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.
K
- type of map keys
V
- type of map values
m
- the map for which a dynamically typesafe view is to be returned
keyType
- the type of key that
m
is permitted to hold
valueType
- the type of value that
m
is permitted to hold
public static <T> Iterator<T> emptyIterator()
hasNext
always returns false
.next
always throws NoSuchElementException
.remove
always throws IllegalStateException
.Implementations of this method are permitted, but not required, to return the same object from multiple invocations.
T
- type of elements, if there were any, in the iterator
public static <T> ListIterator<T> emptyListIterator()
hasNext
and hasPrevious
always return false
.next
and previous
always throw NoSuchElementException
.remove
and set
always throw IllegalStateException
.add
always throws UnsupportedOperationException
.nextIndex
always returns 0
.previousIndex
always returns -1
.Implementations of this method are permitted, but not required, to return the same object from multiple invocations.
T
- type of elements, if there were any, in the iterator
public static <T> Enumeration<T> emptyEnumeration()
hasMoreElements
always returns false
.nextElement
always throws NoSuchElementException
.Implementations of this method are permitted, but not required, to return the same object from multiple invocations.
T
- the class of the objects in the enumeration
public static final <T> Set<T> emptySet()
This example illustrates the type-safe way to obtain an empty set:
Set<String> s = Collections.emptySet();
T
- the class of the objects in the set
EMPTY_SET
public static <E> SortedSet<E> emptySortedSet()
This example illustrates the type-safe way to obtain an empty sorted set:
SortedSet<String> s = Collections.emptySortedSet();
E
- type of elements, if there were any, in the set
public static <E> NavigableSet<E> emptyNavigableSet()
This example illustrates the type-safe way to obtain an empty navigable set:
NavigableSet<String> s = Collections.emptyNavigableSet();
E
- type of elements, if there were any, in the set
public static final <T> List<T> emptyList()
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.)
T
- type of elements, if there were any, in the list
EMPTY_LIST
public static final <K,V> Map <K ,V> emptyMap()
This example illustrates the type-safe way to obtain an empty map:
Map<String, Date> s = Collections.emptyMap();
K
- the class of the map keys
V
- the class of the map values
EMPTY_MAP
public static final <K,V> SortedMap <K ,V> emptySortedMap()
This example illustrates the type-safe way to obtain an empty map:
SortedMap<String, Date> s = Collections.emptySortedMap();
K
- the class of the map keys
V
- the class of the map values
public static final <K,V> NavigableMap <K ,V> emptyNavigableMap()
This example illustrates the type-safe way to obtain an empty map:
NavigableMap<String, Date> s = Collections.emptyNavigableMap();
K
- the class of the map keys
V
- the class of the map values
public static <T> Set<T> singleton(T o)
T
- the class of the objects in the set
o
- the sole object to be stored in the returned set.
public static <T> List<T> singletonList(T o)
T
- the class of the objects in the list
o
- the sole object to be stored in the returned list.
public static <K,V> Map <K ,V> singletonMap(K key, V value)
K
- the class of the map keys
V
- the class of the map values
key
- the sole key to be stored in the returned map.
value
- the value to which the returned map maps
key.
public static <T> List<T> nCopies(int n, T o)
T
- the class of the object to copy and of the objects in the returned list.
n
- the number of elements in the returned list.
o
- the element to appear repeatedly in the returned list.
IllegalArgumentException
- if
n < 0
List.addAll(Collection)
,
List.addAll(int, Collection)
public static <T> Comparator<T> reverseOrder()
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());sorts the array in reverse-lexicographic (alphabetical) order.
The returned comparator is serializable.
T
- the class of the objects compared by the comparator
Comparable
public static <T> Comparator<T> reverseOrder(Comparator <T> cmp)
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
).
T
- the class of the objects compared by the comparator
cmp
- a comparator who's ordering is to be reversed by the returned comparator or
null
public static <T> Enumeration<T> enumeration(Collection <T> c)
T
- the class of the objects in the collection
c
- the collection for which an enumeration is to be returned.
Enumeration
public static <T> ArrayList<T> list(Enumeration <T> e)
T
- the class of the objects returned by the enumeration
e
- enumeration providing elements for the returned array list
Enumeration
,
ArrayList
public static int frequency(Collection<?> c, Object o)
c
- the collection in which to determine the frequency of
o
o
- the object whose frequency is to be determined
c
equal to
o
NullPointerException
- if
c is null
public static boolean disjoint(Collection<?> c1, Collection <?> c2)
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.
c1
- a collection
c2
- a collection
true
if the two specified collections have no elements in common.
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)
@SafeVarargs public static <T> boolean addAll(Collection<? super T> c, T... 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");
T
- the class of the elements to add and of the collection
c
- the collection into which
elements are to be inserted
elements
- the elements to insert into
c
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
Collection.addAll(Collection)
public static <E> Set<E> newSetFromMap(Map <E ,Boolean > map)
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>());
E
- the class of the map keys and of the objects in the returned set
map
- the backing map
IllegalArgumentException
- if
map is not empty
public static <T> Queue<T> asLifoQueue(Deque <T> deque)
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.
T
- the class of the objects in the deque
deque
- the deque