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.
Modifier and Type | Field | Description |
---|---|---|
static final List |
EMPTY_LIST |
The empty list (immutable). |
static final Map |
EMPTY_MAP |
The empty map (immutable). |
static final Set |
EMPTY_SET |
The empty set (immutable). |
Modifier and Type | Method | Description |
---|---|---|
static <T> boolean |
addAll |
Adds all of the specified elements to the specified collection. |
static <T> Queue |
asLifoQueue |
|
static <T> int |
binarySearch |
Searches the specified list for the specified object using the binary search algorithm. |
static <T> int |
binarySearch |
Searches the specified list for the specified object using the binary search algorithm. |
static <E> Collection |
checkedCollection |
Returns a dynamically typesafe view of the specified collection. |
static <E> List |
checkedList |
Returns a dynamically typesafe view of the specified list. |
static <K, |
checkedMap |
Returns a dynamically typesafe view of the specified map. |
static <K, |
checkedNavigableMap |
Returns a dynamically typesafe view of the specified navigable map. |
static <E> NavigableSet |
checkedNavigableSet |
Returns a dynamically typesafe view of the specified navigable set. |
static <E> Queue |
checkedQueue |
Returns a dynamically typesafe view of the specified queue. |
static <E> Set |
checkedSet |
Returns a dynamically typesafe view of the specified set. |
static <K, |
checkedSortedMap |
Returns a dynamically typesafe view of the specified sorted map. |
static <E> SortedSet |
checkedSortedSet |
Returns a dynamically typesafe view of the specified sorted set. |
static <T> void |
copy |
Copies all of the elements from one list into another. |
static boolean |
disjoint |
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 final <T> List |
emptyList() |
Returns an empty list (immutable). |
static <T> ListIterator |
emptyListIterator() |
Returns a list iterator that has no elements. |
static final <K, |
emptyMap() |
Returns an empty map (immutable). |
static final <K, |
emptyNavigableMap() |
Returns an empty navigable map (immutable). |
static <E> NavigableSet |
emptyNavigableSet() |
Returns an empty navigable set (immutable). |
static final <T> Set |
emptySet() |
Returns an empty set (immutable). |
static final <K, |
emptySortedMap() |
Returns an empty sorted map (immutable). |
static <E> SortedSet |
emptySortedSet() |
Returns an empty sorted set (immutable). |
static <T> Enumeration |
enumeration |
Returns an enumeration over the specified collection. |
static <T> void |
fill |
Replaces all of the elements of the specified list with the specified element. |
static int |
frequency |
Returns the number of elements in the specified collection equal to the specified object. |
static int |
indexOfSubList |
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 |
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 |
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 & Comparable<? super T>> |
max |
Returns the maximum element of the given collection, according to the natural ordering of its elements. |
static <T> T |
max |
Returns the maximum element of the given collection, according to the order induced by the specified comparator. |
static <T extends Object & Comparable<? super T>> |
min |
Returns the minimum element of the given collection, according to the natural ordering of its elements. |
static <T> T |
min |
Returns the minimum element of the given collection, according to the order induced by the specified comparator. |
static <T> List |
nCopies |
Returns an immutable list consisting of n copies of the specified object. |
static <E> SequencedSet |
newSequencedSetFromMap |
Returns a sequenced set backed by the specified map. |
static <E> Set |
newSetFromMap |
Returns a set backed by the specified map. |
static <T> boolean |
replaceAll |
Replaces all occurrences of one specified value in a list with another. |
static void |
reverse |
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 |
Returns a comparator that imposes the reverse ordering of the specified comparator. |
static void |
rotate |
Rotates the elements in the specified list by the specified distance. |
static void |
shuffle |
Randomly permutes the specified list using a default source of randomness. |
static void |
shuffle |
Randomly permute the specified list using the specified source of randomness. |
static void |
shuffle |
Randomly permute the specified list using the specified source of randomness. |
static <T> Set |
singleton |
Returns an immutable set containing only the specified object. |
static <T> List |
singletonList |
Returns an immutable list containing only the specified object. |
static <K, |
singletonMap |
Returns an immutable map, mapping only the specified key to the specified value. |
static <T extends Comparable<? super T>> |
sort |
Sorts the specified list into ascending order, according to the natural ordering of its elements. |
static <T> void |
sort |
Sorts the specified list according to the order induced by the specified comparator. |
static void |
swap |
Swaps the elements at the specified positions in the specified list. |
static <T> Collection |
synchronizedCollection |
Returns a synchronized (thread-safe) collection backed by the specified collection. |
static <T> List |
synchronizedList |
Returns a synchronized (thread-safe) list backed by the specified list. |
static <K, |
synchronizedMap |
Returns a synchronized (thread-safe) map backed by the specified map. |
static <K, |
synchronizedNavigableMap |
Returns a synchronized (thread-safe) navigable map backed by the specified navigable map. |
static <T> NavigableSet |
synchronizedNavigableSet |
Returns a synchronized (thread-safe) navigable set backed by the specified navigable set. |
static <T> Set |
synchronizedSet |
Returns a synchronized (thread-safe) set backed by the specified set. |
static <K, |
synchronizedSortedMap |
Returns a synchronized (thread-safe) sorted map backed by the specified sorted map. |
static <T> SortedSet |
synchronizedSortedSet |
Returns a synchronized (thread-safe) sorted set backed by the specified sorted set. |
static <T> Collection |
unmodifiableCollection |
Returns an unmodifiable view of the specified collection. |
static <T> List |
unmodifiableList |
Returns an unmodifiable view of the specified list. |
static <K, |
unmodifiableMap |
Returns an unmodifiable view of the specified map. |
static <K, |
unmodifiableNavigableMap |
Returns an unmodifiable view of the specified navigable map. |
static <T> NavigableSet |
unmodifiableNavigableSet |
Returns an unmodifiable view of the specified navigable set. |
static <T> SequencedCollection |
unmodifiableSequencedCollection |
Returns an unmodifiable view of the specified SequencedCollection . |
static <K, |
unmodifiableSequencedMap |
Returns an unmodifiable view of the specified SequencedMap . |
static <T> SequencedSet |
unmodifiableSequencedSet |
Returns an unmodifiable view of the specified SequencedSet . |
static <T> Set |
unmodifiableSet |
Returns an unmodifiable view of the specified set. |
static <K, |
unmodifiableSortedMap |
Returns an unmodifiable view of the specified sorted map. |
static <T> SortedSet |
unmodifiableSortedSet |
Returns an unmodifiable view of the specified sorted set. |
public static final Set EMPTY_SET
public static final List EMPTY_LIST
public static final Map EMPTY_MAP
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.
List.sort(Comparator)
method using the specified list and a null
comparator.T
- the class of the objects in the listlist
- 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
contractpublic 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.
List.sort(Comparator)
method using the specified list and comparator.T
- the class of the objects in the listlist
- 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
contractpublic 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 listlist
- the list to be searched.key
- the key to be searched for.(-(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.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 listlist
- 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.(-(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.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.reversed
method.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.
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 method is equivalent to shuffle(List, RandomGenerator)
and exists for backward compatibility. The shuffle(List, RandomGenerator)
method is preferred, as it is not limited to random generators that extend the Random
class.
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 shuffle(List<?> list, RandomGenerator 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.
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 listlist
- 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 listsdest
- 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)
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.
T
- the class of the objects in the collectioncoll
- 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.public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp)
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.
T
- the class of the objects in the collectioncoll
- 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.public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll)
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.
T
- the class of the objects in the collectioncoll
- 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.public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp)
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.
T
- the class of the objects in the collectioncoll
- 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.public static void rotate(List<?> list, int distance)
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);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)
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.)T
- the class of the objects in the listlist
- 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.true
if list
contained one or more elements e
such that (oldVal==null ? e==null : oldVal.equals(e))
.UnsupportedOperationException
- if the specified list or its list-iterator does not support the set
operation.public static int indexOfSubList(List<?> source, List<?> target)
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.
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)
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.
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)
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.
T
- the class of the objects in the collectionc
- the collection for which an unmodifiable view is to be returned.public static <T> SequencedCollection<T> unmodifiableSequencedCollection(SequencedCollection<? extends T> c)
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.
T
- the class of the objects in the collectionc
- the collection for which an unmodifiable view is to be returned.public static <T> Set<T> unmodifiableSet(Set<? extends T> s)
UnsupportedOperationException
.The returned set will be serializable if the specified set is serializable.
T
- the class of the objects in the sets
- the set for which an unmodifiable view is to be returned.public static <T> SequencedSet<T> unmodifiableSequencedSet(SequencedSet<? extends T> s)
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.
T
- the class of the objects in the sets
- the set for which an unmodifiable view is to be returned.public static <T> SortedSet<T> unmodifiableSortedSet(SortedSet<T> s)
subSet
, headSet
, or tailSet
views, result in an UnsupportedOperationException
.The returned sorted set will be serializable if the specified sorted set is serializable.
T
- the class of the objects in the sets
- the sorted set for which an unmodifiable view is to be returned.public static <T> List<T> unmodifiableList(List<? extends T> list)
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.
T
- the class of the objects in the listlist
- 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)
UnsupportedOperationException
.The returned map will be serializable if the specified map is serializable.
K
- the class of the map keysV
- the class of the map valuesm
- the map for which an unmodifiable view is to be returned.public static <K, V> SequencedMap<K,V> unmodifiableSequencedMap(SequencedMap<? extends K,? extends V> m)
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.
K
- the class of the map keysV
- the class of the map valuesm
- the map for which an unmodifiable view is to be returned.public static <K, V> SortedMap<K,V> unmodifiableSortedMap(SortedMap<K,? extends V> m)
subMap
, headMap
, or tailMap
views, result in an UnsupportedOperationException
.The returned sorted map will be serializable if the specified sorted map is serializable.
K
- the class of the map keysV
- the class of the map valuesm
- the sorted 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 collectionc
- 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 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()); }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 sets
- 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 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()); }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 sets
- the sorted set to be "wrapped" in a synchronized sorted set.public static <T> List<T> synchronizedList(List<T> 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()); }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 listlist
- 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 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()); }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 keysV
- the class of the map valuesm
- 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 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()); }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 = s2.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 keysV
- the class of the map valuesm
- the sorted map to be "wrapped" in a synchronized sorted 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 collectionc
- the collection for which a dynamically typesafe view is to be returnedtype
- the type of element that c
is permitted to holdpublic 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 queuequeue
- the queue for which a dynamically typesafe view is to be returnedtype
- the type of element that queue
is permitted to holdpublic 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 sets
- the set for which a dynamically typesafe view is to be returnedtype
- the type of element that s
is permitted to holdpublic 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 sets
- the sorted set for which a dynamically typesafe view is to be returnedtype
- the type of element that s
is permitted to holdpublic 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 listlist
- the list for which a dynamically typesafe view is to be returnedtype
- the type of element that list
is permitted to holdpublic 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 keysV
- the class of the map valuesm
- the map for which a dynamically typesafe view is to be returnedkeyType
- the type of key that m
is permitted to holdvalueType
- the type of value that m
is permitted to holdpublic 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 keysV
- the class of the map valuesm
- the map for which a dynamically typesafe view is to be returnedkeyType
- the type of key that m
is permitted to holdvalueType
- the type of value that m
is permitted to holdpublic 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 iteratorpublic 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 iteratorpublic 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 enumerationpublic static final <T> Set<T> emptySet()
This example illustrates the type-safe way to obtain an empty set:
Set<String> s = Collections.emptySet();
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.)T
- the class of the objects in the setpublic static <E> SortedSet<E> emptySortedSet()
This example illustrates the type-safe way to obtain an empty sorted set:
SortedSet<String> s = Collections.emptySortedSet();
SortedSet
object for each call.E
- type of elements, if there were any, in the setpublic static final <T> List<T> emptyList()
This example illustrates the type-safe way to obtain an empty list:
List<String> s = Collections.emptyList();
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 listpublic 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();
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.)K
- the class of the map keysV
- the class of the map valuespublic 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();
SortedMap
object for each call.K
- the class of the map keysV
- the class of the map valuespublic static <T> Set<T> singleton(T o)
T
- the class of the objects in the seto
- 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 listo
- 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 keysV
- the class of the map valueskey
- 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)
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.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.n
copies of the specified object.IllegalArgumentException
- if n < 0
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.
Comparator
that is suitable for sorting elements in reverse order. To obtain a reverse-ordered view of a sequenced collection, use the SequencedCollection.reversed
method. Or, to obtain a reverse-ordered view of a sequenced map, use the SequencedMap.reversed
method.T
- the class of the objects compared by the comparatorComparable
interface.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
).
Comparator
that is suitable for sorting elements in reverse order. To obtain a reverse-ordered view of a sequenced collection, use the SequencedCollection.reversed
method. Or, to obtain a reverse-ordered view of a sequenced map, use the SequencedMap.reversed
method.T
- the class of the objects compared by the comparatorcmp
- a comparator who's ordering is to be reversed by the returned comparator or null
public static <T> Enumeration<T> enumeration(Collection<T> c)
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.
T
- the class of the objects in the collectionc
- the collection for which an enumeration is to be returned.public static <T> ArrayList<T> list(Enumeration<T> e)
T
- the class of the objects returned by the enumeratione
- enumeration providing elements for the returned array listpublic static int frequency(Collection<?> c, Object o)
e
in the collection such that Objects.equals(o, e)
.c
- the collection in which to determine the frequency of o
o
- the object whose frequency is to be determinedc
equal to o
NullPointerException
- if c
is nullpublic 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 collectionc2
- a collectiontrue
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)
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");
T
- the class of the elements to add and of the collectionc
- the collection into which elements
are to be insertedelements
- the elements to insert into c
true
if the collection changed as a result of the callUnsupportedOperationException
- if c
does not support the add
operationNullPointerException
- 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
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 setmap
- the backing mapIllegalArgumentException
- if map
is not emptypublic static <E> SequencedSet<E> newSequencedSetFromMap(SequencedMap<E,Boolean> map)
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.
SequencedSet
from a LinkedHashMap
. This differs from a LinkedHashSet
in that the map's removeEldestEntry
is overridden to provide an eviction policy, which is not possible with a LinkedHashSet
. SequencedSet<String> set = Collections.newSequencedSetFromMap(
new LinkedHashMap<String, Boolean>() {
protected boolean removeEldestEntry(Map.Entry<String, Boolean> e) {
return this.size() > 5;
}
});
E
- the class of the map keys and of the objects in the returned setmap
- the backing mapIllegalArgumentException
- if map
is not emptypublic 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.
Deque.reversed
method.T
- the class of the objects in the dequedeque
- the deque
© 1993, 2023, Oracle and/or its affiliates. All rights reserved.
Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Collections.html