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.
- Since:
- 1.2
- See Also:
Collection
,Set
,List
,Map
-
Field Summary
-
Method Summary
Modifier and Type Method Description static <T> boolean
addAll(Collection<? super T> c, T... elements)
Adds all of the specified elements to the specified collection.static <T> Queue<T>
asLifoQueue(Deque<T> deque)
static <T> int
binarySearch(List<? extends Comparable<? super T>> list, T key)
Searches the specified list for the specified object using the binary search algorithm.static <T> int
binarySearch(List<? extends T> list, T key, Comparator<? super T> c)
Searches the specified list for the specified object using the binary search algorithm.static <E> Collection<E>
checkedCollection(Collection<E> c, Class<E> type)
Returns a dynamically typesafe view of the specified collection.static <E> List<E>
checkedList(List<E> list, Class<E> type)
Returns a dynamically typesafe view of the specified list.static <K, V> Map<K,V>
checkedMap(Map<K,V> m, Class<K> keyType, Class<V> valueType)
Returns a dynamically typesafe view of the specified map.static <K, V> NavigableMap<K,V>
checkedNavigableMap(NavigableMap<K,V> m, Class<K> keyType, Class<V> valueType)
Returns a dynamically typesafe view of the specified navigable map.static <E> NavigableSet<E>
checkedNavigableSet(NavigableSet<E> s, Class<E> type)
Returns a dynamically typesafe view of the specified navigable set.static <E> Queue<E>
checkedQueue(Queue<E> queue, Class<E> type)
Returns a dynamically typesafe view of the specified queue.static <E> Set<E>
checkedSet(Set<E> s, Class<E> type)
Returns a dynamically typesafe view of the specified set.static <K, V> SortedMap<K,V>
checkedSortedMap(SortedMap<K,V> m, Class<K> keyType, Class<V> valueType)
Returns a dynamically typesafe view of the specified sorted map.static <E> SortedSet<E>
checkedSortedSet(SortedSet<E> s, Class<E> type)
Returns a dynamically typesafe view of the specified sorted set.static <T> void
copy(List<? super T> dest, List<? extends T> src)
Copies all of the elements from one list into another.static boolean
disjoint(Collection<?> c1, Collection<?> c2)
Returnstrue
if the two specified collections have no elements in common.static <T> Enumeration<T>
emptyEnumeration()
Returns an enumeration that has no elements.static <T> Iterator<T>
emptyIterator()
Returns an iterator that has no elements.static <T> List<T>
emptyList()
Returns an empty list (immutable).static <T> ListIterator<T>
emptyListIterator()
Returns a list iterator that has no elements.static <K, V> Map<K,V>
emptyMap()
Returns an empty map (immutable).static <K, V> NavigableMap<K,V>
emptyNavigableMap()
Returns an empty navigable map (immutable).static <E> NavigableSet<E>
emptyNavigableSet()
Returns an empty navigable set (immutable).static <T> Set<T>
emptySet()
Returns an empty set (immutable).static <K, V> SortedMap<K,V>
emptySortedMap()
Returns an empty sorted map (immutable).static <E> SortedSet<E>
emptySortedSet()
Returns an empty sorted set (immutable).static <T> Enumeration<T>
enumeration(Collection<T> c)
Returns an enumeration over the specified collection.static <T> void
fill(List<? super T> list, T obj)
Replaces all of the elements of the specified list with the specified element.static int
frequency(Collection<?> c, Object o)
Returns the number of elements in the specified collection equal to the specified object.static int
indexOfSubList(List<?> source, List<?> target)
Returns the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.static int
lastIndexOfSubList(List<?> source, List<?> target)
Returns the starting position of the last occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.static <T> ArrayList<T>
list(Enumeration<T> e)
Returns an array list containing the elements returned by the specified enumeration in the order they are returned by the enumeration.static <T extends Object & Comparable<? super T>>
Tmax(Collection<? extends T> coll)
Returns the maximum element of the given collection, according to the natural ordering of its elements.static <T> T
max(Collection<? extends T> coll, Comparator<? super T> comp)
Returns the maximum element of the given collection, according to the order induced by the specified comparator.static <T extends Object & Comparable<? super T>>
Tmin(Collection<? extends T> coll)
Returns the minimum element of the given collection, according to the natural ordering of its elements.static <T> T
min(Collection<? extends T> coll, Comparator<? super T> comp)
Returns the minimum element of the given collection, according to the order induced by the specified comparator.static <T> List<T>
nCopies(int n, T o)
Returns an immutable list consisting ofn
copies of the specified object.static <E> Set<E>
newSetFromMap(Map<E,Boolean> map)
Returns a set backed by the specified map.static <T> boolean
replaceAll(List<T> list, T oldVal, T newVal)
Replaces all occurrences of one specified value in a list with another.static void
reverse(List<?> list)
Reverses the order of the elements in the specified list.static <T> Comparator<T>
reverseOrder()
Returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement theComparable
interface.static <T> Comparator<T>
reverseOrder(Comparator<T> cmp)
Returns a comparator that imposes the reverse ordering of the specified comparator.static void
rotate(List<?> list, int distance)
Rotates the elements in the specified list by the specified distance.static void
shuffle(List<?> list)
Randomly permutes the specified list using a default source of randomness.static void
shuffle(List<?> list, Random rnd)
Randomly permute the specified list using the specified source of randomness.static <T> Set<T>
singleton(T o)
Returns an immutable set containing only the specified object.static <T> List<T>
singletonList(T o)
Returns an immutable list containing only the specified object.static <K, V> Map<K,V>
singletonMap(K key, V value)
Returns an immutable map, mapping only the specified key to the specified value.static <T extends Comparable<? super T>>
voidsort(List<T> list)
Sorts the specified list into ascending order, according to the natural ordering of its elements.static <T> void
sort(List<T> list, Comparator<? super T> c)
Sorts the specified list according to the order induced by the specified comparator.static void
swap(List<?> list, int i, int j)
Swaps the elements at the specified positions in the specified list.static <T> Collection<T>
synchronizedCollection(Collection<T> c)
Returns a synchronized (thread-safe) collection backed by the specified collection.static <T> List<T>
synchronizedList(List<T> list)
Returns a synchronized (thread-safe) list backed by the specified list.static <K, V> Map<K,V>
synchronizedMap(Map<K,V> m)
Returns a synchronized (thread-safe) map backed by the specified map.static <K, V> NavigableMap<K,V>
synchronizedNavigableMap(NavigableMap<K,V> m)
Returns a synchronized (thread-safe) navigable map backed by the specified navigable map.static <T> NavigableSet<T>
synchronizedNavigableSet(NavigableSet<T> s)
Returns a synchronized (thread-safe) navigable set backed by the specified navigable set.static <T> Set<T>
synchronizedSet(Set<T> s)
Returns a synchronized (thread-safe) set backed by the specified set.static <K, V> SortedMap<K,V>
synchronizedSortedMap(SortedMap<K,V> m)
Returns a synchronized (thread-safe) sorted map backed by the specified sorted map.static <T> SortedSet<T>
synchronizedSortedSet(SortedSet<T> s)
Returns a synchronized (thread-safe) sorted set backed by the specified sorted set.static <T> Collection<T>
unmodifiableCollection(Collection<? extends T> c)
Returns an unmodifiable view of the specified collection.static <T> List<T>
unmodifiableList(List<? extends T> list)
Returns an unmodifiable view of the specified list.static <K, V> Map<K,V>
unmodifiableMap(Map<? extends K,? extends V> m)
Returns an unmodifiable view of the specified map.static <K, V> NavigableMap<K,V>
unmodifiableNavigableMap(NavigableMap<K,? extends V> m)
Returns an unmodifiable view of the specified navigable map.static <T> NavigableSet<T>
unmodifiableNavigableSet(NavigableSet<T> s)
Returns an unmodifiable view of the specified navigable set.static <T> Set<T>
unmodifiableSet(Set<? extends T> s)
Returns an unmodifiable view of the specified set.static <K, V> SortedMap<K,V>
unmodifiableSortedMap(SortedMap<K,? extends V> m)
Returns an unmodifiable view of the specified sorted map.static <T> SortedSet<T>
unmodifiableSortedSet(SortedSet<T> s)
Returns an unmodifiable view of the specified sorted set.
-
Field Details
-
EMPTY_SET
The empty set (immutable). This set is serializable.- See Also:
emptySet()
-
EMPTY_LIST
The empty list (immutable). This list is serializable.- See Also:
emptyList()
-
EMPTY_MAP
The empty map (immutable). This map is serializable.- Since:
- 1.3
- See Also:
emptyMap()
-
-
Method Details
-
sort
Sorts the specified list into ascending order, according to the natural ordering of its elements. All elements in the list must implement theComparable
interface. Furthermore, all elements in the list must be mutually comparable (that is,e1.compareTo(e2)
must not throw aClassCastException
for any elementse1
ande2
in the list).This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort.
The specified list must be modifiable, but need not be resizable.
- Implementation Note:
- This implementation defers to the
List.sort(Comparator)
method using the specified list and anull
comparator. - Type Parameters:
T
- the class of the objects in the list- Parameters:
list
- the list to be sorted.- Throws:
ClassCastException
- if the list contains elements that are not mutually comparable (for example, strings and integers).UnsupportedOperationException
- if the specified list's list-iterator does not support theset
operation.IllegalArgumentException
- (optional) if the implementation detects that the natural ordering of the list elements is found to violate theComparable
contract- See Also:
List.sort(Comparator)
-
sort
Sorts the specified list according to the order induced by the specified comparator. All elements in the list must be mutually comparable using the specified comparator (that is,c.compare(e1, e2)
must not throw aClassCastException
for any elementse1
ande2
in the list).This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort.
The specified list must be modifiable, but need not be resizable.
- Implementation Note:
- This implementation defers to the
List.sort(Comparator)
method using the specified list and comparator. - Type Parameters:
T
- the class of the objects in the list- Parameters:
list
- the list to be sorted.c
- the comparator to determine the order of the list. Anull
value indicates that the elements' natural ordering should be used.- Throws:
ClassCastException
- if the list contains elements that are not mutually comparable using the specified comparator.UnsupportedOperationException
- if the specified list's list-iterator does not support theset
operation.IllegalArgumentException
- (optional) if the comparator is found to violate theComparator
contract- See Also:
List.sort(Comparator)
-
binarySearch
Searches the specified list for the specified object using the binary search algorithm. The list must be sorted into ascending order according to the natural ordering of its elements (as by thesort(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.- Type Parameters:
T
- the class of the objects in the list- Parameters:
list
- the list to be searched.key
- the key to be searched for.- Returns:
- the index of the search key, if it is contained in the list;
otherwise,
(-(insertion point) - 1)
. The insertion point is defined as the point at which the key would be inserted into the list: the index of the first element greater than the key, orlist.size()
if all elements in the list are less than the specified key. Note that this guarantees that the return value will be >= 0 if and only if the key is found. - Throws:
ClassCastException
- if the list contains elements that are not mutually comparable (for example, strings and integers), or the search key is not mutually comparable with the elements of the list.
-
binarySearch
Searches the specified list for the specified object using the binary search algorithm. The list must be sorted into ascending order according to the specified comparator (as by thesort(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.- Type Parameters:
T
- the class of the objects in the list- Parameters:
list
- the list to be searched.key
- the key to be searched for.c
- the comparator by which the list is ordered. Anull
value indicates that the elements' natural ordering should be used.- Returns:
- the index of the search key, if it is contained in the list;
otherwise,
(-(insertion point) - 1)
. The insertion point is defined as the point at which the key would be inserted into the list: the index of the first element greater than the key, orlist.size()
if all elements in the list are less than the specified key. Note that this guarantees that the return value will be >= 0 if and only if the key is found. - Throws:
ClassCastException
- if the list contains elements that are not mutually comparable using the specified comparator, or the search key is not mutually comparable with the elements of the list using this comparator.
-
reverse
Reverses the order of the elements in the specified list.This method runs in linear time.
- Parameters:
list
- the list whose elements are to be reversed.- Throws:
UnsupportedOperationException
- if the specified list or its list-iterator does not support theset
operation.
-
shuffle
Randomly permutes the specified list using a default source of randomness. All permutations occur with approximately equal likelihood.The hedge "approximately" is used in the foregoing description because default source of randomness is only approximately an unbiased source of independently chosen bits. If it were a perfect source of randomly chosen bits, then the algorithm would choose permutations with perfect uniformity.
This implementation traverses the list backwards, from the last element up to the second, repeatedly swapping a randomly selected element into the "current position". Elements are randomly selected from the portion of the list that runs from the first element to the current position, inclusive.
This method runs in linear time. If the specified list does not implement the
RandomAccess
interface and is large, this implementation dumps the specified list into an array before shuffling it, and dumps the shuffled array back into the list. This avoids the quadratic behavior that would result from shuffling a "sequential access" list in place.- Parameters:
list
- the list to be shuffled.- Throws:
UnsupportedOperationException
- if the specified list or its list-iterator does not support theset
operation.
-
shuffle
Randomly permute the specified list using the specified source of randomness. All permutations occur with equal likelihood assuming that the source of randomness is fair.This implementation traverses the list backwards, from the last element up to the second, repeatedly swapping a randomly selected element into the "current position". Elements are randomly selected from the portion of the list that runs from the first element to the current position, inclusive.
This method runs in linear time. If the specified list does not implement the
RandomAccess
interface and is large, this implementation dumps the specified list into an array before shuffling it, and dumps the shuffled array back into the list. This avoids the quadratic behavior that would result from shuffling a "sequential access" list in place.- Parameters:
list
- the list to be shuffled.rnd
- the source of randomness to use to shuffle the list.- Throws:
UnsupportedOperationException
- if the specified list or its list-iterator does not support theset
operation.
-
swap
Swaps the elements at the specified positions in the specified list. (If the specified positions are equal, invoking this method leaves the list unchanged.)- Parameters:
list
- 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.- Throws:
IndexOutOfBoundsException
- if eitheri
orj
is out of range (i < 0 || i >= list.size() || j < 0 || j >= list.size()).- Since:
- 1.4
-
fill
Replaces all of the elements of the specified list with the specified element.This method runs in linear time.
- Type Parameters:
T
- the class of the objects in the list- Parameters:
list
- the list to be filled with the specified element.obj
- The element with which to fill the specified list.- Throws:
UnsupportedOperationException
- if the specified list or its list-iterator does not support theset
operation.
-
copy
Copies all of the elements from one list into another. After the operation, the index of each copied element in the destination list will be identical to its index in the source list. The destination list's size must be greater than or equal to the source list's size. If it is greater, the remaining elements in the destination list are unaffected.This method runs in linear time.
- Type Parameters:
T
- the class of the objects in the lists- Parameters:
dest
- The destination list.src
- The source list.- Throws:
IndexOutOfBoundsException
- if the destination list is too small to contain the entire source List.UnsupportedOperationException
- if the destination list's list-iterator does not support theset
operation.
-
min
Returns the minimum element of the given collection, according to the natural ordering of its elements. All elements in the collection must implement theComparable
interface. Furthermore, all elements in the collection must be mutually comparable (that is,e1.compareTo(e2)
must not throw aClassCastException
for any elementse1
ande2
in the collection).This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
- Type Parameters:
T
- the class of the objects in the collection- Parameters:
coll
- the collection whose minimum element is to be determined.- Returns:
- the minimum element of the given collection, according to the natural ordering of its elements.
- Throws:
ClassCastException
- if the collection contains elements that are not mutually comparable (for example, strings and integers).NoSuchElementException
- if the collection is empty.- See Also:
Comparable
-
min
Returns the minimum element of the given collection, according to the order induced by the specified comparator. All elements in the collection must be mutually comparable by the specified comparator (that is,comp.compare(e1, e2)
must not throw aClassCastException
for any elementse1
ande2
in the collection).This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
- Type Parameters:
T
- the class of the objects in the collection- Parameters:
coll
- the collection whose minimum element is to be determined.comp
- the comparator with which to determine the minimum element. Anull
value indicates that the elements' natural ordering should be used.- Returns:
- the minimum element of the given collection, according to the specified comparator.
- Throws:
ClassCastException
- if the collection contains elements that are not mutually comparable using the specified comparator.NoSuchElementException
- if the collection is empty.- See Also:
Comparable
-
max
Returns the maximum element of the given collection, according to the natural ordering of its elements. All elements in the collection must implement theComparable
interface. Furthermore, all elements in the collection must be mutually comparable (that is,e1.compareTo(e2)
must not throw aClassCastException
for any elementse1
ande2
in the collection).This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
- Type Parameters:
T
- the class of the objects in the collection- Parameters:
coll
- the collection whose maximum element is to be determined.- Returns:
- the maximum element of the given collection, according to the natural ordering of its elements.
- Throws:
ClassCastException
- if the collection contains elements that are not mutually comparable (for example, strings and integers).NoSuchElementException
- if the collection is empty.- See Also:
Comparable
-
max
Returns the maximum element of the given collection, according to the order induced by the specified comparator. All elements in the collection must be mutually comparable by the specified comparator (that is,comp.compare(e1, e2)
must not throw aClassCastException
for any elementse1
ande2
in the collection).This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
- Type Parameters:
T
- the class of the objects in the collection- Parameters:
coll
- the collection whose maximum element is to be determined.comp
- the comparator with which to determine the maximum element. Anull
value indicates that the elements' natural ordering should be used.- Returns:
- the maximum element of the given collection, according to the specified comparator.
- Throws:
ClassCastException
- if the collection contains elements that are not mutually comparable using the specified comparator.NoSuchElementException
- if the collection is empty.- See Also:
Comparable
-
rotate
Rotates the elements in the specified list by the specified distance. After calling this method, the element at indexi
will be the element previously at index(i - distance)
modlist.size()
, for all values ofi
between0
andlist.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 invokingCollections.rotate(list, 1)
(orCollections.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 positionk
(which must be greater than or equal toj
):Collections.rotate(list.subList(j, k+1), -1);
To make this concrete, supposelist
comprises[a, b, c, d, e]
. To move the element at index1
(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 theRandomAccess
interface, this implementation breaks the list into two sublist views around index-distance mod size
. Then thereverse(List)
method is invoked on each sublist view, and finally it is invoked on the entire list. For a more complete description of both algorithms, see Section 2.3 of Jon Bentley's Programming Pearls (Addison-Wesley, 1986).- Parameters:
list
- 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 thanlist.size()
.- Throws:
UnsupportedOperationException
- if the specified list or its list-iterator does not support theset
operation.- Since:
- 1.4
-
replaceAll
Replaces all occurrences of one specified value in a list with another. More formally, replaces withnewVal
each elemente
inlist
such that(oldVal==null ? e==null : oldVal.equals(e))
. (This method has no effect on the size of the list.)- Type Parameters:
T
- the class of the objects in the list- Parameters:
list
- the list in which replacement is to occur.oldVal
- the old value to be replaced.newVal
- the new value with whicholdVal
is to be replaced.- Returns:
true
iflist
contained one or more elementse
such that(oldVal==null ? e==null : oldVal.equals(e))
.- Throws:
UnsupportedOperationException
- if the specified list or its list-iterator does not support theset
operation.- Since:
- 1.4
-
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. More formally, returns the lowest indexi
such thatsource.subList(i, i+target.size()).equals(target)
, or -1 if there is no such index. (Returns -1 iftarget.size() > source.size()
)This implementation uses the "brute force" technique of scanning over the source list, looking for a match with the target at each location in turn.
- Parameters:
source
- the list in which to search for the first occurrence oftarget
.target
- the list to search for as a subList ofsource
.- 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.
- Since:
- 1.4
-
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. More formally, returns the highest indexi
such thatsource.subList(i, i+target.size()).equals(target)
, or -1 if there is no such index. (Returns -1 iftarget.size() > source.size()
)This implementation uses the "brute force" technique of iterating over the source list, looking for a match with the target at each location in turn.
- Parameters:
source
- the list in which to search for the last occurrence oftarget
.target
- the list to search for as a subList ofsource
.- 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.
- Since:
- 1.4
-
unmodifiableCollection
Returns an unmodifiable view of the specified collection. Query operations on the returned collection "read through" to the specified collection, and attempts to modify the returned collection, whether direct or via its iterator, result in anUnsupportedOperationException
.The returned collection does not pass the hashCode and equals operations through to the backing collection, but relies on
Object
'sequals
andhashCode
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.
- Type Parameters:
T
- the class of the objects in the collection- Parameters:
c
- the collection for which an unmodifiable view is to be returned.- Returns:
- an unmodifiable view of the specified collection.
-
unmodifiableSet
Returns an unmodifiable view of the specified set. Query operations on the returned set "read through" to the specified set, and attempts to modify the returned set, whether direct or via its iterator, result in anUnsupportedOperationException
.The returned set will be serializable if the specified set is serializable.
- Type Parameters:
T
- the class of the objects in the set- Parameters:
s
- the set for which an unmodifiable view is to be returned.- Returns:
- an unmodifiable view of the specified set.
-
unmodifiableSortedSet
Returns an unmodifiable view of the specified sorted set. Query operations on the returned sorted set "read through" to the specified sorted set. Attempts to modify the returned sorted set, whether direct, via its iterator, or via itssubSet
,headSet
, ortailSet
views, result in anUnsupportedOperationException
.The returned sorted set will be serializable if the specified sorted set is serializable.
- Type Parameters:
T
- the class of the objects in the set- Parameters:
s
- the sorted set for which an unmodifiable view is to be returned.- Returns:
- an unmodifiable view of the specified sorted set.
-
unmodifiableList
Returns an unmodifiable view of the specified list. Query operations on the returned list "read through" to the specified list, and attempts to modify the returned list, whether direct or via its iterator, result in anUnsupportedOperationException
.The returned list will be serializable if the specified list is serializable. Similarly, the returned list will implement
RandomAccess
if the specified list does.- Type Parameters:
T
- the class of the objects in the list- Parameters:
list
- the list for which an unmodifiable view is to be returned.- Returns:
- an unmodifiable view of the specified list.
-
unmodifiableMap
Returns an unmodifiable view of the specified map. Query operations on the returned map "read through" to the specified map, and attempts to modify the returned map, whether direct or via its collection views, result in anUnsupportedOperationException
.The returned map will be serializable if the specified map is serializable.
- Type Parameters:
K
- the class of the map keysV
- the class of the map values- Parameters:
m
- the map for which an unmodifiable view is to be returned.- Returns:
- an unmodifiable view of the specified map.
-
unmodifiableSortedMap
Returns an unmodifiable view of the specified sorted map. Query operations on the returned sorted map "read through" to the specified sorted map. Attempts to modify the returned sorted map, whether direct, via its collection views, or via itssubMap
,headMap
, ortailMap
views, result in anUnsupportedOperationException
.The returned sorted map will be serializable if the specified sorted map is serializable.
- Type Parameters:
K
- the class of the map keysV
- the class of the map values- Parameters:
m
- the sorted map for which an unmodifiable view is to be returned.- Returns:
- an unmodifiable view of the specified sorted map.
-
synchronizedCollection
Returns a synchronized (thread-safe) collection backed by the specified collection. In order to guarantee serial access, it is critical that all access to the backing collection is accomplished through the returned collection.It is imperative that the user manually synchronize on the returned collection when traversing it via
Iterator
,Spliterator
orStream
: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
andequals
operations through to the backing collection, but relies onObject
'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.
- Type Parameters:
T
- the class of the objects in the collection- Parameters:
c
- the collection to be "wrapped" in a synchronized collection.- Returns:
- a synchronized view of the specified collection.
-
synchronizedSet
Returns a synchronized (thread-safe) set backed by the specified set. In order to guarantee serial access, it is critical that all access to the backing set is accomplished through the returned set.It is imperative that the user manually synchronize on the returned collection when traversing it via
Iterator
,Spliterator
orStream
: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.
- Type Parameters:
T
- the class of the objects in the set- Parameters:
s
- the set to be "wrapped" in a synchronized set.- Returns:
- a synchronized view of the specified set.
-
synchronizedSortedSet
Returns a synchronized (thread-safe) sorted set backed by the specified sorted set. In order to guarantee serial access, it is critical that all access to the backing sorted set is accomplished through the returned sorted set (or its views).It is imperative that the user manually synchronize on the returned sorted set when traversing it or any of its
subSet
,headSet
, ortailSet
views viaIterator
,Spliterator
orStream
: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.
- Type Parameters:
T
- the class of the objects in the set- Parameters:
s
- the sorted set to be "wrapped" in a synchronized sorted set.- Returns:
- a synchronized view of the specified sorted set.
-
synchronizedList
Returns a synchronized (thread-safe) list backed by the specified list. In order to guarantee serial access, it is critical that all access to the backing list is accomplished through the returned list.It is imperative that the user manually synchronize on the returned list when traversing it via
Iterator
,Spliterator
orStream
: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.
- Type Parameters:
T
- the class of the objects in the list- Parameters:
list
- the list to be "wrapped" in a synchronized list.- Returns:
- a synchronized view of the specified list.
-
synchronizedMap
Returns a synchronized (thread-safe) map backed by the specified map. In order to guarantee serial access, it is critical that all access to the backing map is accomplished through the returned map.It is imperative that the user manually synchronize on the returned map when traversing any of its collection views via
Iterator
,Spliterator
orStream
: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.
- Type Parameters:
K
- the class of the map keysV
- the class of the map values- Parameters:
m
- the map to be "wrapped" in a synchronized map.- Returns:
- a synchronized view of the specified map.
-
synchronizedSortedMap
Returns a synchronized (thread-safe) sorted map backed by the specified sorted map. In order to guarantee serial access, it is critical that all access to the backing sorted map is accomplished through the returned sorted map (or its views).It is imperative that the user manually synchronize on the returned sorted map when traversing any of its collection views, or the collections views of any of its
subMap
,headMap
ortailMap
views, viaIterator
,Spliterator
orStream
: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.
- Type Parameters:
K
- the class of the map keysV
- the class of the map values- Parameters:
m
- the sorted map to be "wrapped" in a synchronized sorted map.- Returns:
- a synchronized view of the specified sorted map.
-
checkedCollection
Returns a dynamically typesafe view of the specified collection. Any attempt to insert an element of the wrong type will result in an immediateClassCastException
. Assuming a collection contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the collection takes place through the view, it is guaranteed that the collection cannot contain an incorrectly typed element.The generics mechanism in the language provides compile-time (static) type checking, but it is possible to defeat this mechanism with unchecked casts. Usually this is not a problem, as the compiler issues warnings on all such unchecked operations. There are, however, times when static type checking alone is not sufficient. For example, suppose a collection is passed to a third-party library and it is imperative that the library code not corrupt the collection by inserting an element of the wrong type.
Another use of dynamically typesafe views is debugging. Suppose a program fails with a
ClassCastException
, indicating that an incorrectly typed element was put into a parameterized collection. Unfortunately, the exception can occur at any time after the erroneous element is inserted, so it typically provides little or no information as to the real source of the problem. If the problem is reproducible, one can quickly determine its source by temporarily modifying the program to wrap the collection with a dynamically typesafe view. For example, this declaration:Collection<String> c = new HashSet<>();
Collection<String> c = Collections.checkedCollection( new HashSet<>(), String.class);
The returned collection does not pass the hashCode and equals operations through to the backing collection, but relies on
Object
'sequals
andhashCode
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.- Type Parameters:
E
- the class of the objects in the collection- Parameters:
c
- the collection for which a dynamically typesafe view is to be returnedtype
- the type of element thatc
is permitted to hold- Returns:
- a dynamically typesafe view of the specified collection
- Since:
- 1.5
-
checkedQueue
Returns a dynamically typesafe view of the specified queue. Any attempt to insert an element of the wrong type will result in an immediateClassCastException
. 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 ofnull
elements whenever the backing queue does.- Type Parameters:
E
- the class of the objects in the queue- Parameters:
queue
- the queue for which a dynamically typesafe view is to be returnedtype
- the type of element thatqueue
is permitted to hold- Returns:
- a dynamically typesafe view of the specified queue
- Since:
- 1.8
-
checkedSet
Returns a dynamically typesafe view of the specified set. Any attempt to insert an element of the wrong type will result in an immediateClassCastException
. 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.- Type Parameters:
E
- the class of the objects in the set- Parameters:
s
- the set for which a dynamically typesafe view is to be returnedtype
- the type of element thats
is permitted to hold- Returns:
- a dynamically typesafe view of the specified set
- Since:
- 1.5
-
checkedSortedSet
Returns a dynamically typesafe view of the specified sorted set. Any attempt to insert an element of the wrong type will result in an immediateClassCastException
. 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.- Type Parameters:
E
- the class of the objects in the set- Parameters:
s
- the sorted set for which a dynamically typesafe view is to be returnedtype
- the type of element thats
is permitted to hold- Returns:
- a dynamically typesafe view of the specified sorted set
- Since:
- 1.5
-
checkedList
Returns a dynamically typesafe view of the specified list. Any attempt to insert an element of the wrong type will result in an immediateClassCastException
. 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.- Type Parameters:
E
- the class of the objects in the list- Parameters:
list
- the list for which a dynamically typesafe view is to be returnedtype
- the type of element thatlist
is permitted to hold- Returns:
- a dynamically typesafe view of the specified list
- Since:
- 1.5
-
checkedMap
Returns a dynamically typesafe view of the specified map. Any attempt to insert a mapping whose key or value have the wrong type will result in an immediateClassCastException
. Similarly, any attempt to modify the value currently associated with a key will result in an immediateClassCastException
, whether the modification is attempted directly through the map itself, or through aMap.Entry
instance obtained from the map'sentry 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.- Type Parameters:
K
- the class of the map keysV
- the class of the map values- Parameters:
m
- the map for which a dynamically typesafe view is to be returnedkeyType
- the type of key thatm
is permitted to holdvalueType
- the type of value thatm
is permitted to hold- Returns:
- a dynamically typesafe view of the specified map
- Since:
- 1.5
-
checkedSortedMap
public static <K, V> SortedMap<K,V> checkedSortedMap(SortedMap<K,V> m, Class<K> keyType, Class<V> valueType)Returns a dynamically typesafe view of the specified sorted map. Any attempt to insert a mapping whose key or value have the wrong type will result in an immediateClassCastException
. Similarly, any attempt to modify the value currently associated with a key will result in an immediateClassCastException
, whether the modification is attempted directly through the map itself, or through aMap.Entry
instance obtained from the map'sentry 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.- Type Parameters:
K
- the class of the map keysV
- the class of the map values- Parameters:
m
- the map for which a dynamically typesafe view is to be returnedkeyType
- the type of key thatm
is permitted to holdvalueType
- the type of value thatm
is permitted to hold- Returns:
- a dynamically typesafe view of the specified map
- Since:
- 1.5
-
emptyIterator
Returns an iterator that has no elements. More precisely,hasNext
always returnsfalse
.next
always throwsNoSuchElementException
.remove
always throwsIllegalStateException
.
Implementations of this method are permitted, but not required, to return the same object from multiple invocations.
- Type Parameters:
T
- type of elements, if there were any, in the iterator- Returns:
- an empty iterator
- Since:
- 1.7
-
emptyListIterator
Returns a list iterator that has no elements. More precisely,hasNext
andhasPrevious
always returnfalse
.next
andprevious
always throwNoSuchElementException
.remove
andset
always throwIllegalStateException
.add
always throwsUnsupportedOperationException
.nextIndex
always returns0
.previousIndex
always returns-1
.
Implementations of this method are permitted, but not required, to return the same object from multiple invocations.
- Type Parameters:
T
- type of elements, if there were any, in the iterator- Returns:
- an empty list iterator
- Since:
- 1.7
-
emptyEnumeration
Returns an enumeration that has no elements. More precisely,hasMoreElements
always returnsfalse
.-
nextElement
always throwsNoSuchElementException
.
Implementations of this method are permitted, but not required, to return the same object from multiple invocations.
- Type Parameters:
T
- the class of the objects in the enumeration- Returns:
- an empty enumeration
- Since:
- 1.7
-
emptySet
Returns an empty set (immutable). This set is serializable. Unlike the like-named field, this method is parameterized.This example illustrates the type-safe way to obtain an empty set:
Set<String> s = Collections.emptySet();
- Implementation Note:
- Implementations of this method need not create a separate
Set
object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.) - Type Parameters:
T
- the class of the objects in the set- Returns:
- the empty set
- Since:
- 1.5
- See Also:
EMPTY_SET
-
emptySortedSet
Returns an empty sorted set (immutable). This set is serializable.This example illustrates the type-safe way to obtain an empty sorted set:
SortedSet<String> s = Collections.emptySortedSet();
- Implementation Note:
- Implementations of this method need not create a separate
SortedSet
object for each call. - Type Parameters:
E
- type of elements, if there were any, in the set- Returns:
- the empty sorted set
- Since:
- 1.8
-
emptyList
Returns an empty list (immutable). This list is serializable.This example illustrates the type-safe way to obtain an empty list:
List<String> s = Collections.emptyList();
- Implementation Note:
- Implementations of this method need not create a separate
List
object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.) - Type Parameters:
T
- type of elements, if there were any, in the list- Returns:
- an empty immutable list
- Since:
- 1.5
- See Also:
EMPTY_LIST
-
emptyMap
Returns an empty map (immutable). This map is serializable.This example illustrates the type-safe way to obtain an empty map:
Map<String, Date> s = Collections.emptyMap();
- Implementation Note:
- Implementations of this method need not create a separate
Map
object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.) - Type Parameters:
K
- the class of the map keysV
- the class of the map values- Returns:
- an empty map
- Since:
- 1.5
- See Also:
EMPTY_MAP
-
emptySortedMap
Returns an empty sorted map (immutable). This map is serializable.This example illustrates the type-safe way to obtain an empty map:
SortedMap<String, Date> s = Collections.emptySortedMap();
- Implementation Note:
- Implementations of this method need not create a separate
SortedMap
object for each call. - Type Parameters:
K
- the class of the map keysV
- the class of the map values- Returns:
- an empty sorted map
- Since:
- 1.8
-
singleton
Returns an immutable set containing only the specified object. The returned set is serializable.- Type Parameters:
T
- the class of the objects in the set- Parameters:
o
- the sole object to be stored in the returned set.- Returns:
- an immutable set containing only the specified object.
-
singletonList
Returns an immutable list containing only the specified object. The returned list is serializable.- Type Parameters:
T
- the class of the objects in the list- Parameters:
o
- the sole object to be stored in the returned list.- Returns:
- an immutable list containing only the specified object.
- Since:
- 1.3
-
singletonMap
Returns an immutable map, mapping only the specified key to the specified value. The returned map is serializable.- Type Parameters:
K
- the class of the map keysV
- the class of the map values- Parameters:
key
- the sole key to be stored in the returned map.value
- the value to which the returned map mapskey
.- Returns:
- an immutable map containing only the specified key-value mapping.
- Since:
- 1.3
-
nCopies
Returns an immutable list consisting ofn
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 theList.addAll
method to grow lists. The returned list is serializable.- Type Parameters:
T
- the class of the object to copy and of the objects in the returned list.- Parameters:
n
- the number of elements in the returned list.o
- the element to appear repeatedly in the returned list.- Returns:
- an immutable list consisting of
n
copies of the specified object. - Throws:
IllegalArgumentException
- ifn < 0
- See Also:
List.addAll(Collection)
,List.addAll(int, Collection)
-
reverseOrder
Returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement theComparable
interface. (The natural ordering is the ordering imposed by the objects' owncompareTo
method.) This enables a simple idiom for sorting (or maintaining) collections (or arrays) of objects that implement theComparable
interface in reverse-natural-order. For example, supposea
is an array of strings. Then:Arrays.sort(a, Collections.reverseOrder());
sorts the array in reverse-lexicographic (alphabetical) order.The returned comparator is serializable.
- Type Parameters:
T
- the class of the objects compared by the comparator- Returns:
- A comparator that imposes the reverse of the natural
ordering on a collection of objects that implement
the
Comparable
interface. - See Also:
Comparable
-
reverseOrder
Returns a comparator that imposes the reverse ordering of the specified comparator. If the specified comparator isnull
, this method is equivalent toreverseOrder()
(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
).- Type Parameters:
T
- the class of the objects compared by the comparator- Parameters:
cmp
- a comparator who's ordering is to be reversed by the returned comparator ornull
- Returns:
- A comparator that imposes the reverse ordering of the specified comparator.
- Since:
- 1.5
-
enumeration
Returns an enumeration over the specified collection. This provides interoperability with legacy APIs that require an enumeration as input.The iterator returned from a call to
Enumeration.asIterator()
does not support removal of elements from the specified collection. This is necessary to avoid unintentionally increasing the capabilities of the returned enumeration.- Type Parameters:
T
- the class of the objects in the collection- Parameters:
c
- the collection for which an enumeration is to be returned.- Returns:
- an enumeration over the specified collection.
- See Also:
Enumeration
-
list
Returns an array list containing the elements returned by the specified enumeration in the order they are returned by the enumeration. This method provides interoperability between legacy APIs that return enumerations and new APIs that require collections.- Type Parameters:
T
- the class of the objects returned by the enumeration- Parameters:
e
- enumeration providing elements for the returned array list- Returns:
- an array list containing the elements returned by the specified enumeration.
- Since:
- 1.4
- See Also:
Enumeration
,ArrayList
-
frequency
Returns the number of elements in the specified collection equal to the specified object. More formally, returns the number of elementse
in the collection such thatObjects.equals(o, e)
.- Parameters:
c
- the collection in which to determine the frequency ofo
o
- the object whose frequency is to be determined- Returns:
- the number of elements in
c
equal too
- Throws:
NullPointerException
- ifc
is null- Since:
- 1.5
-
disjoint
Returnstrue
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 aSortedSet
whose ordering is not compatible with equals, or the key set of anIdentityHashMap
), both collections must use the same nonstandard equality test, or the result of this method is undefined.Care must also be exercised when using collections that have restrictions on the elements that they may contain. Collection implementations are allowed to throw exceptions for any operation involving elements they deem ineligible. For absolute safety the specified collections should contain only elements which are eligible elements for both collections.
Note that it is permissible to pass the same collection in both parameters, in which case the method will return
true
if and only if the collection is empty.- Parameters:
c1
- a collectionc2
- a collection- Returns:
true
if the two specified collections have no elements in common.- Throws:
NullPointerException
- if either collection isnull
.NullPointerException
- if one collection contains anull
element andnull
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)- Since:
- 1.5
-
addAll
Adds all of the specified elements to the specified collection. Elements to be added may be specified individually or as an array. The behavior of this convenience method is identical to that ofc.addAll(Arrays.asList(elements))
, but this method is likely to run significantly faster under most implementations.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");
- Type Parameters:
T
- the class of the elements to add and of the collection- Parameters:
c
- the collection into whichelements
are to be insertedelements
- the elements to insert intoc
- Returns:
true
if the collection changed as a result of the call- Throws:
UnsupportedOperationException
- ifc
does not support theadd
operationNullPointerException
- ifelements
contains one or more null values andc
does not permit null elements, or ifc
orelements
arenull
IllegalArgumentException
- if some property of a value inelements
prevents it from being added toc
- Since:
- 1.5
- See Also:
Collection.addAll(Collection)
-
newSetFromMap
Returns a set backed by the specified map. The resulting set displays the same ordering, concurrency, and performance characteristics as the backing map. In essence, this factory method provides aSet
implementation corresponding to anyMap
implementation. There is no need to use this method on aMap
implementation that already has a correspondingSet
implementation (such asHashMap
orTreeMap
).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. TheaddAll
method is implemented as a sequence ofput
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>());
- Type Parameters:
E
- the class of the map keys and of the objects in the returned set- Parameters:
map
- the backing map- Returns:
- the set backed by the map
- Throws:
IllegalArgumentException
- ifmap
is not empty- Since:
- 1.6
-
asLifoQueue
Returns a view of aDeque
as a Last-in-first-out (Lifo)Queue
. Methodadd
is mapped topush
,remove
is mapped topop
and so on. This view can be useful when you would like to use a method requiring aQueue
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 ofaddFirst
invocations on the backing deque.- Type Parameters:
T
- the class of the objects in the deque- Parameters:
deque
- the deque- Returns:
- the queue
- Since:
- 1.6
-