Added in API level 1

Summary: Fields | Methods | Inherited Methods

Collections

public class Collections
extends Object

java.lang.Object
↳	java.util.Collections

---

This class consists exclusively of static methods that operate on or return collections. It contains polymorphic algorithms that operate on collections, "wrappers", which return a new collection backed by a specified collection, and a few other odds and ends.

The methods of this class all throw a NullPointerException if the collections or class objects provided to them are null.

The documentation for the polymorphic algorithms contained in this class generally includes a brief description of the implementation. Such descriptions should be regarded as implementation notes, rather than parts of the specification. Implementors should feel free to substitute other algorithms, so long as the specification itself is adhered to. (For example, the algorithm used by sort does not have to be a mergesort, but it does have to be stable.)

The "destructive" algorithms contained in this class, that is, the algorithms that modify the collection on which they operate, are specified to throw UnsupportedOperationException if the collection does not support the appropriate mutation primitive(s), such as the set method. These algorithms may, but are not required to, throw this exception if an invocation would have no effect on the collection. For example, invoking the sort method on an unmodifiable list that is already sorted may or may not throw UnsupportedOperationException.

This class is a member of the Java Collections Framework.

See also:

• Collection
• Set
• List
• Map

Summary

Fields
public static final List<E>	EMPTY_LIST The empty list (immutable).
public static final Map<K, V>	EMPTY_MAP The empty map (immutable).
public static final Set<E>	EMPTY_SET The empty set (immutable).

Public methods
static <T> boolean	addAll(Collection<? super T> c, T... elements) Adds all of the specified elements to the specified collection.
static <T> Queue<T>	asLifoQueue(Deque<T> deque) Returns a view of a Deque as a Last-in-first-out (Lifo) Queue.
static <T> int	binarySearch(List<? extends T> list, T key, Comparator<? super T> c) Searches the specified list for the specified object using the binary search algorithm.
static <T> int	binarySearch(List<? extends Comparable<? super T>> list, T key) Searches the specified list for the specified object using the binary search algorithm.
static <E> Collection<E>	checkedCollection(Collection<E> c, Class<E> type) Returns a dynamically typesafe view of the specified collection.
static <E> List<E>	checkedList(List<E> list, Class<E> type) Returns a dynamically typesafe view of the specified list.
static <K, V> Map<K, V>	checkedMap(Map<K, V> m, Class<K> keyType, Class<V> valueType) Returns a dynamically typesafe view of the specified map.
static <K, V> NavigableMap<K, V>	checkedNavigableMap(NavigableMap<K, V> m, Class<K> keyType, Class<V> valueType) Returns a dynamically typesafe view of the specified navigable map.
static <E> NavigableSet<E>	checkedNavigableSet(NavigableSet<E> s, Class<E> type) Returns a dynamically typesafe view of the specified navigable set.
static <E> Queue<E>	checkedQueue(Queue<E> queue, Class<E> type) Returns a dynamically typesafe view of the specified queue.
static <E> Set<E>	checkedSet(Set<E> s, Class<E> type) Returns a dynamically typesafe view of the specified set.
static <K, V> SortedMap<K, V>	checkedSortedMap(SortedMap<K, V> m, Class<K> keyType, Class<V> valueType) Returns a dynamically typesafe view of the specified sorted map.
static <E> SortedSet<E>	checkedSortedSet(SortedSet<E> s, Class<E> type) Returns a dynamically typesafe view of the specified sorted set.
static <T> void	copy(List<? super T> dest, List<? extends T> src) Copies all of the elements from one list into another.
static boolean	disjoint(Collection<?> c1, Collection<?> c2) Returns true 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 final <T> List<T>	emptyList() Returns an empty list (immutable).
static <T> ListIterator<T>	emptyListIterator() Returns a list iterator that has no elements.
static final <K, V> Map<K, V>	emptyMap() Returns an empty map (immutable).
static final <K, V> NavigableMap<K, V>	emptyNavigableMap() Returns an empty navigable map (immutable).
static <E> NavigableSet<E>	emptyNavigableSet() Returns an empty navigable set (immutable).
static final <T> Set<T>	emptySet() Returns an empty set (immutable).
static final <K, V> SortedMap<K, V>	emptySortedMap() Returns an empty sorted map (immutable).
static <E> SortedSet<E>	emptySortedSet() Returns an empty sorted set (immutable).
static <T> Enumeration<T>	enumeration(Collection<T> c) Returns an enumeration over the specified collection.
static <T> void	fill(List<? super T> list, T obj) Replaces all of the elements of the specified list with the specified element.
static int	frequency(Collection<?> c, Object o) Returns the number of elements in the specified collection equal to the specified object.
static int	indexOfSubList(List<?> source, List<?> target) Returns the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.
static int	lastIndexOfSubList(List<?> source, List<?> target) Returns the starting position of the last occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.
static <T> ArrayList<T>	list(Enumeration<T> e) Returns an array list containing the elements returned by the specified enumeration in the order they are returned by the enumeration.
static <T> T	max(Collection<? extends T> coll, Comparator<? super T> comp) Returns the maximum element of the given collection, according to the order induced by the specified comparator.
static <T extends Comparable<? super T>> T	max(Collection<? extends T> coll) Returns the maximum element of the given collection, according to the natural ordering of its elements.
static <T extends Comparable<? super T>> T	min(Collection<? extends T> coll) Returns the minimum element of the given collection, according to the natural ordering of its elements.
static <T> T	min(Collection<? extends T> coll, Comparator<? super T> comp) Returns the minimum element of the given collection, according to the order induced by the specified comparator.
static <T> List<T>	nCopies(int n, T o) Returns an immutable list consisting of n copies of the specified object.
static <E> SequencedSet<E>	newSequencedSetFromMap(SequencedMap<E, Boolean> map) Returns a sequenced set backed by the specified map.
static <E> Set<E>	newSetFromMap(Map<E, Boolean> map) Returns a set backed by the specified map.
static <T> boolean	replaceAll(List<T> list, T oldVal, T newVal) Replaces all occurrences of one specified value in a list with another.
static void	reverse(List<?> list) Reverses the order of the elements in the specified list.
static <T> Comparator<T>	reverseOrder() Returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable 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, Random rnd) Randomly permute the specified list using the specified source of randomness.
static void	shuffle(List<?> list) Randomly permutes the specified list using a default source of randomness.
static void	shuffle(List<?> list, RandomGenerator rnd) Randomly permute the specified list using the specified source of randomness.
static <T> Set<T>	singleton(T o) Returns an immutable set containing only the specified object.
static <T> List<T>	singletonList(T o) Returns an immutable list containing only the specified object.
static <K, V> Map<K, V>	singletonMap(K key, V value) Returns an immutable map, mapping only the specified key to the specified value.
static <T extends Comparable<? super T>> void	sort(List<T> list) Sorts the specified list into ascending order, according to the natural ordering of its elements.
static <T> void	sort(List<T> list, Comparator<? super T> c) Sorts the specified list according to the order induced by the specified comparator.
static void	swap(List<?> list, int i, int j) Swaps the elements at the specified positions in the specified list.
static <T> Collection<T>	synchronizedCollection(Collection<T> c) Returns a synchronized (thread-safe) collection backed by the specified collection.
static <T> List<T>	synchronizedList(List<T> list) Returns a synchronized (thread-safe) list backed by the specified list.
static <K, V> Map<K, V>	synchronizedMap(Map<K, V> m) Returns a synchronized (thread-safe) map backed by the specified map.
static <K, V> NavigableMap<K, V>	synchronizedNavigableMap(NavigableMap<K, V> m) Returns a synchronized (thread-safe) navigable map backed by the specified navigable map.
static <T> NavigableSet<T>	synchronizedNavigableSet(NavigableSet<T> s) Returns a synchronized (thread-safe) navigable set backed by the specified navigable set.
static <T> Set<T>	synchronizedSet(Set<T> s) Returns a synchronized (thread-safe) set backed by the specified set.
static <K, V> SortedMap<K, V>	synchronizedSortedMap(SortedMap<K, V> m) Returns a synchronized (thread-safe) sorted map backed by the specified sorted map.
static <T> SortedSet<T>	synchronizedSortedSet(SortedSet<T> s) Returns a synchronized (thread-safe) sorted set backed by the specified sorted set.
static <T> Collection<T>	unmodifiableCollection(Collection<? extends T> c) Returns an unmodifiable view of the specified collection.
static <T> List<T>	unmodifiableList(List<? extends T> list) Returns an unmodifiable view of the specified list.
static <K, V> Map<K, V>	unmodifiableMap(Map<? extends K, ? extends V> m) Returns an unmodifiable view of the specified map.
static <K, V> NavigableMap<K, V>	unmodifiableNavigableMap(NavigableMap<K, ? extends V> m) Returns an unmodifiable view of the specified navigable map.
static <T> NavigableSet<T>	unmodifiableNavigableSet(NavigableSet<T> s) Returns an unmodifiable view of the specified navigable set.
static <T> SequencedCollection<T>	unmodifiableSequencedCollection(SequencedCollection<? extends T> c) Returns an unmodifiable view of the specified SequencedCollection.
static <K, V> SequencedMap<K, V>	unmodifiableSequencedMap(SequencedMap<? extends K, ? extends V> m) Returns an unmodifiable view of the specified SequencedMap.
static <T> SequencedSet<T>	unmodifiableSequencedSet(SequencedSet<? extends T> s) Returns an unmodifiable view of the specified SequencedSet.
static <T> Set<T>	unmodifiableSet(Set<? extends T> s) Returns an unmodifiable view of the specified set.
static <K, V> SortedMap<K, V>	unmodifiableSortedMap(SortedMap<K, ? extends V> m) Returns an unmodifiable view of the specified sorted map.
static <T> SortedSet<T>	unmodifiableSortedSet(SortedSet<T> s) Returns an unmodifiable view of the specified sorted set.

Inherited methods

From class java.lang.Object Object clone() Creates and returns a copy of this object. boolean equals(Object obj) Indicates whether some other object is "equal to" this one. void finalize() Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. final Class<?> getClass() Returns the runtime class of this Object. int hashCode() Returns a hash code value for the object. final void notify() Wakes up a single thread that is waiting on this object's monitor. final void notifyAll() Wakes up all threads that are waiting on this object's monitor. String toString() Returns a string representation of the object. final void wait(long timeoutMillis, int nanos) Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed. final void wait(long timeoutMillis) Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed. final void wait() Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

Fields

EMPTY_LIST

Added in API level 1

public static final List<E> EMPTY_LIST

The empty list (immutable). This list is serializable.

See also:

• emptyList()

EMPTY_MAP

Added in API level 1

public static final Map<K, V> EMPTY_MAP

The empty map (immutable). This map is serializable.

See also:

• emptyMap()

EMPTY_SET

Added in API level 1

public static final Set<E> EMPTY_SET

The empty set (immutable). This set is serializable.

See also:

• emptySet()

Public methods

addAll

Added in API level 1

public static boolean addAll (Collection<? super T> c, 
                T... elements)

Adds all of the specified elements to the specified collection. Elements to be added may be specified individually or as an array. The behaviour of this convenience method is similar to that of c.addAll(Collections.unmodifiableList(Arrays.asList(elements))).

When elements are specified individually, this method provides a convenient way to add a few elements to an existing collection:

     Collections.addAll(flavors, "Peaches 'n Plutonium", "Rocky Racoon");
 

Parameters
c	Collection: the collection into which elements are to be inserted
elements	T: the elements to insert into c

Returns
boolean	true if the collection changed as a result of the call

Throws
UnsupportedOperationException	if c does not support the add operation
NullPointerException	if elements contains one or more null values and c does not permit null elements, or if c or elements are null
IllegalArgumentException	if some property of a value in elements prevents it from being added to c

See also:

• Collection.addAll(Collection)

asLifoQueue

Added in API level 9

public static Queue<T> asLifoQueue (Deque<T> deque)

Returns a view of a Deque as a Last-in-first-out (Lifo) Queue. Method add is mapped to push, remove is mapped to pop and so on. This view can be useful when you would like to use a method requiring a Queue but you need Lifo ordering.

Each method invocation on the queue returned by this method results in exactly one method invocation on the backing deque, with one exception. The addAll method is implemented as a sequence of addFirst invocations on the backing deque.

API Note:

• This method provides a view that inverts the sense of certain operations, but it doesn't reverse the encounter order. To obtain a reverse-ordered view, use the Deque.reversed method.

Parameters
deque	Deque: the deque

Returns
Queue<T>	the queue

binarySearch

Added in API level 1

public static int binarySearch (List<? extends T> list, 
                T key, 
                Comparator<? super T> c)

Searches the specified list for the specified object using the binary search algorithm. The list must be sorted into ascending order according to the specified comparator (as by the sort(List, Comparator) method), prior to making this call. If it is not sorted, the results are undefined. If the list contains multiple elements equal to the specified object, there is no guarantee which one will be found.

This method runs in log(n) time for a "random access" list (which provides near-constant-time positional access). If the specified list does not implement the RandomAccess interface and is large, this method will do an iterator-based binary search that performs O(n) link traversals and O(log n) element comparisons.

Parameters
list	List: the list to be searched.
key	T: the key to be searched for.
c	Comparator: the comparator by which the list is ordered. A null value indicates that the elements' natural ordering should be used.

Returns

int

the index of the search key, if it is contained in the list; otherwise, (-(insertion point) - 1). The insertion point is defined as the point at which the key would be inserted into the list: the index of the first element greater than the key, or list.size() if all elements in the list are less than the specified key. Note that this guarantees that the return value will be >= 0 if and only if the key is found.

Throws
ClassCastException	if the list contains elements that are not mutually comparable using the specified comparator, or the search key is not mutually comparable with the elements of the list using this comparator.

binarySearch

Added in API level 1

public static int binarySearch (List<? extends Comparable<? super T>> list, 
                T key)

Searches the specified list for the specified object using the binary search algorithm. The list must be sorted into ascending order according to the natural ordering of its elements (as by the sort(java.util.List) method) prior to making this call. If it is not sorted, the results are undefined. If the list contains multiple elements equal to the specified object, there is no guarantee which one will be found.

This method runs in log(n) time for a "random access" list (which provides near-constant-time positional access). If the specified list does not implement the RandomAccess interface and is large, this method will do an iterator-based binary search that performs O(n) link traversals and O(log n) element comparisons.

Parameters
list	List: the list to be searched.
key	T: the key to be searched for.

Returns

int

the index of the search key, if it is contained in the list; otherwise, (-(insertion point) - 1). The insertion point is defined as the point at which the key would be inserted into the list: the index of the first element greater than the key, or list.size() if all elements in the list are less than the specified key. Note that this guarantees that the return value will be >= 0 if and only if the key is found.

Throws
ClassCastException	if the list contains elements that are not mutually comparable (for example, strings and integers), or the search key is not mutually comparable with the elements of the list.

checkedCollection

Added in API level 1

public static Collection<E> checkedCollection (Collection<E> c, 
                Class<E> type)

Returns a dynamically typesafe view of the specified collection. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a collection contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the collection takes place through the view, it is guaranteed that the collection cannot contain an incorrectly typed element.

The generics mechanism in the language provides compile-time (static) type checking, but it is possible to defeat this mechanism with unchecked casts. Usually this is not a problem, as the compiler issues warnings on all such unchecked operations. There are, however, times when static type checking alone is not sufficient. For example, suppose a collection is passed to a third-party library and it is imperative that the library code not corrupt the collection by inserting an element of the wrong type.

Another use of dynamically typesafe views is debugging. Suppose a program fails with a ClassCastException, indicating that an incorrectly typed element was put into a parameterized collection. Unfortunately, the exception can occur at any time after the erroneous element is inserted, so it typically provides little or no information as to the real source of the problem. If the problem is reproducible, one can quickly determine its source by temporarily modifying the program to wrap the collection with a dynamically typesafe view. For example, this declaration:

 Collection<String> c = new HashSet<>();
 

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.

Parameters
c	Collection: the collection for which a dynamically typesafe view is to be returned
type	Class: the type of element that c is permitted to hold

Returns
Collection<E>	a dynamically typesafe view of the specified collection

checkedList

Added in API level 1

public static List<E> checkedList (List<E> list, 
                Class<E> type)

Returns a dynamically typesafe view of the specified list. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a list contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the list takes place through the view, it is guaranteed that the list cannot contain an incorrectly typed element.

A discussion of the use of dynamically typesafe views may be found in the documentation for the checkedCollection method.

The returned list will be serializable if the specified list is serializable.

Since null is considered to be a value of any reference type, the returned list permits insertion of null elements whenever the backing list does.

Parameters
list	List: the list for which a dynamically typesafe view is to be returned
type	Class: the type of element that list is permitted to hold

Returns
List<E>	a dynamically typesafe view of the specified list

checkedMap

Added in API level 1

public static Map<K, V> checkedMap (Map<K, V> m, 
                Class<K> keyType, 
                Class<V> valueType)

Returns a dynamically typesafe view of the specified map. Any attempt to insert a mapping whose key or value have the wrong type will result in an immediate ClassCastException. Similarly, any attempt to modify the value currently associated with a key will result in an immediate ClassCastException, whether the modification is attempted directly through the map itself, or through a Map.Entry instance obtained from the map's entry set view.

Assuming a map contains no incorrectly typed keys or values prior to the time a dynamically typesafe view is generated, and that all subsequent access to the map takes place through the view (or one of its collection views), it is guaranteed that the map cannot contain an incorrectly typed key or value.

A discussion of the use of dynamically typesafe views may be found in the documentation for the checkedCollection method.

The returned map will be serializable if the specified map is serializable.

Since null is considered to be a value of any reference type, the returned map permits insertion of null keys or values whenever the backing map does.

Parameters
m	Map: the map for which a dynamically typesafe view is to be returned
keyType	Class: the type of key that m is permitted to hold
valueType	Class: the type of value that m is permitted to hold

Returns
Map<K, V>	a dynamically typesafe view of the specified map

checkedNavigableMap

Added in API level 26

public static NavigableMap<K, V> checkedNavigableMap (NavigableMap<K, V> m, 
                Class<K> keyType, 
                Class<V> valueType)

Returns a dynamically typesafe view of the specified navigable map. Any attempt to insert a mapping whose key or value have the wrong type will result in an immediate ClassCastException. Similarly, any attempt to modify the value currently associated with a key will result in an immediate ClassCastException, whether the modification is attempted directly through the map itself, or through a Map.Entry instance obtained from the map's entry set view.

Assuming a map contains no incorrectly typed keys or values prior to the time a dynamically typesafe view is generated, and that all subsequent access to the map takes place through the view (or one of its collection views), it is guaranteed that the map cannot contain an incorrectly typed key or value.

A discussion of the use of dynamically typesafe views may be found in the documentation for the checkedCollection method.

The returned map will be serializable if the specified map is serializable.

Since null is considered to be a value of any reference type, the returned map permits insertion of null keys or values whenever the backing map does.

Parameters
m	NavigableMap: the map for which a dynamically typesafe view is to be returned
keyType	Class: the type of key that m is permitted to hold
valueType	Class: the type of value that m is permitted to hold

Returns
NavigableMap<K, V>	a dynamically typesafe view of the specified map

checkedNavigableSet

Added in API level 26

public static NavigableSet<E> checkedNavigableSet (NavigableSet<E> s, 
                Class<E> type)

Returns a dynamically typesafe view of the specified navigable set. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a navigable set contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the navigable set takes place through the view, it is guaranteed that the navigable set cannot contain an incorrectly typed element.

A discussion of the use of dynamically typesafe views may be found in the documentation for the checkedCollection method.

The returned navigable set will be serializable if the specified navigable set is serializable.

Since null is considered to be a value of any reference type, the returned navigable set permits insertion of null elements whenever the backing sorted set does.

Parameters
s	NavigableSet: the navigable set for which a dynamically typesafe view is to be returned
type	Class: the type of element that s is permitted to hold

Returns
NavigableSet<E>	a dynamically typesafe view of the specified navigable set

checkedQueue

Added in API level 26

public static Queue<E> checkedQueue (Queue<E> queue, 
                Class<E> type)

Returns a dynamically typesafe view of the specified queue. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a queue contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the queue takes place through the view, it is guaranteed that the queue cannot contain an incorrectly typed element.

A discussion of the use of dynamically typesafe views may be found in the documentation for the checkedCollection method.

The returned queue will be serializable if the specified queue is serializable.

Since null is considered to be a value of any reference type, the returned queue permits insertion of null elements whenever the backing queue does.

Parameters
queue	Queue: the queue for which a dynamically typesafe view is to be returned
type	Class: the type of element that queue is permitted to hold

Returns
Queue<E>	a dynamically typesafe view of the specified queue

checkedSet

Added in API level 1

public static Set<E> checkedSet (Set<E> s, 
                Class<E> type)

Returns a dynamically typesafe view of the specified set. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a set contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the set takes place through the view, it is guaranteed that the set cannot contain an incorrectly typed element.

A discussion of the use of dynamically typesafe views may be found in the documentation for the checkedCollection method.

The returned set will be serializable if the specified set is serializable.

Since null is considered to be a value of any reference type, the returned set permits insertion of null elements whenever the backing set does.

Parameters
s	Set: the set for which a dynamically typesafe view is to be returned
type	Class: the type of element that s is permitted to hold

Returns
Set<E>	a dynamically typesafe view of the specified set

checkedSortedMap

Added in API level 1

public static SortedMap<K, V> checkedSortedMap (SortedMap<K, V> m, 
                Class<K> keyType, 
                Class<V> valueType)

Returns a dynamically typesafe view of the specified sorted map. Any attempt to insert a mapping whose key or value have the wrong type will result in an immediate ClassCastException. Similarly, any attempt to modify the value currently associated with a key will result in an immediate ClassCastException, whether the modification is attempted directly through the map itself, or through a Map.Entry instance obtained from the map's entry set view.

Assuming a map contains no incorrectly typed keys or values prior to the time a dynamically typesafe view is generated, and that all subsequent access to the map takes place through the view (or one of its collection views), it is guaranteed that the map cannot contain an incorrectly typed key or value.

A discussion of the use of dynamically typesafe views may be found in the documentation for the checkedCollection method.

The returned map will be serializable if the specified map is serializable.

Since null is considered to be a value of any reference type, the returned map permits insertion of null keys or values whenever the backing map does.

Parameters
m	SortedMap: the map for which a dynamically typesafe view is to be returned
keyType	Class: the type of key that m is permitted to hold
valueType	Class: the type of value that m is permitted to hold

Returns
SortedMap<K, V>	a dynamically typesafe view of the specified map

checkedSortedSet

Added in API level 1

public static SortedSet<E> checkedSortedSet (SortedSet<E> s, 
                Class<E> type)

Returns a dynamically typesafe view of the specified sorted set. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a sorted set contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the sorted set takes place through the view, it is guaranteed that the sorted set cannot contain an incorrectly typed element.

A discussion of the use of dynamically typesafe views may be found in the documentation for the checkedCollection method.

The returned sorted set will be serializable if the specified sorted set is serializable.

Since null is considered to be a value of any reference type, the returned sorted set permits insertion of null elements whenever the backing sorted set does.

Parameters
s	SortedSet: the sorted set for which a dynamically typesafe view is to be returned
type	Class: the type of element that s is permitted to hold

Returns
SortedSet<E>	a dynamically typesafe view of the specified sorted set

copy

Added in API level 1

public static void copy (List<? super T> dest, 
                List<? extends T> src)

Copies all of the elements from one list into another. After the operation, the index of each copied element in the destination list will be identical to its index in the source list. The destination list's size must be greater than or equal to the source list's size. If it is greater, the remaining elements in the destination list are unaffected.

This method runs in linear time.

Parameters
dest	List: The destination list.
src	List: The source list.

Throws
IndexOutOfBoundsException	if the destination list is too small to contain the entire source List.
UnsupportedOperationException	if the destination list's list-iterator does not support the set operation.

disjoint

Added in API level 1

public static boolean disjoint (Collection<?> c1, 
                Collection<?> c2)

Returns true if the two specified collections have no elements in common.

Care must be exercised if this method is used on collections that do not comply with the general contract for Collection. Implementations may elect to iterate over either collection and test for containment in the other collection (or to perform any equivalent computation). If either collection uses a nonstandard equality test (as does a SortedSet whose ordering is not compatible with equals, or the key set of an IdentityHashMap), both collections must use the same nonstandard equality test, or the result of this method is undefined.

Care must also be exercised when using collections that have restrictions on the elements that they may contain. Collection implementations are allowed to throw exceptions for any operation involving elements they deem ineligible. For absolute safety the specified collections should contain only elements which are eligible elements for both collections.

Note that it is permissible to pass the same collection in both parameters, in which case the method will return true if and only if the collection is empty.

Parameters
c1	Collection: a collection
c2	Collection: a collection

Returns
boolean	true if the two specified collections have no elements in common.

Throws
NullPointerException	if either collection is null.
NullPointerException	if one collection contains a null element and null is not an eligible element for the other collection. (optional)
ClassCastException	if one collection contains an element that is of a type which is ineligible for the other collection. (optional)

emptyEnumeration

Added in API level 19

public static Enumeration<T> emptyEnumeration ()

Returns an enumeration that has no elements. More precisely,

• 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.

Returns
Enumeration<T>	an empty enumeration

emptyIterator

Added in API level 19

public static Iterator<T> emptyIterator ()

Returns an iterator that has no elements. More precisely,

• 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.

Returns
Iterator<T>	an empty iterator

emptyList

Added in API level 1

public static final List<T> emptyList ()

Returns an empty list (immutable). This list is serializable.

This example illustrates the type-safe way to obtain an empty list:

     List<String> s = Collections.emptyList();
 

Implementation Note:

• Implementations of this method need not create a separate List object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.)

Returns
List<T>	an empty immutable list

See also:

• EMPTY_LIST

emptyListIterator

Added in API level 19

public static ListIterator<T> emptyListIterator ()

Returns a list iterator that has no elements. More precisely,

• 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.

Returns
ListIterator<T>	an empty list iterator

emptyMap

Added in API level 1

public static final Map<K, V> emptyMap ()

Returns an empty map (immutable). This map is serializable.

This example illustrates the type-safe way to obtain an empty map:

     Map<String, Date> s = Collections.emptyMap();
 

Implementation Note:

• Implementations of this method need not create a separate Map object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.)

Returns
Map<K, V>	an empty map

See also:

• EMPTY_MAP

emptyNavigableMap

Added in API level 26

public static final NavigableMap<K, V> emptyNavigableMap ()

Returns an empty navigable map (immutable). This map is serializable.

This example illustrates the type-safe way to obtain an empty map:

 NavigableMap<String, Date> s = Collections.emptyNavigableMap();
 

Implementation Note:

• Implementations of this method need not create a separate NavigableMap object for each call.

Returns
NavigableMap<K, V>	an empty navigable map

emptyNavigableSet

Added in API level 26

public static NavigableSet<E> emptyNavigableSet ()

Returns an empty navigable set (immutable). This set is serializable.

This example illustrates the type-safe way to obtain an empty navigable set:

 NavigableSet<String> s = Collections.emptyNavigableSet();
 

Implementation Note:

• Implementations of this method need not create a separate NavigableSet object for each call.

Returns
NavigableSet<E>	the empty navigable set

emptySet

Added in API level 1

public static final Set<T> emptySet ()

Returns an empty set (immutable). This set is serializable. Unlike the like-named field, this method is parameterized.

This example illustrates the type-safe way to obtain an empty set:

     Set<String> s = Collections.emptySet();
 

Implementation Note:

• Implementations of this method need not create a separate Set object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.)

Returns
Set<T>	the empty set

See also:

• EMPTY_SET

emptySortedMap

Added in API level 26

public static final SortedMap<K, V> emptySortedMap ()

Returns an empty sorted map (immutable). This map is serializable.

This example illustrates the type-safe way to obtain an empty map:

 SortedMap<String, Date> s = Collections.emptySortedMap();
 

Implementation Note:

• Implementations of this method need not create a separate SortedMap object for each call.

Returns
SortedMap<K, V>	an empty sorted map

emptySortedSet

Added in API level 26

public static SortedSet<E> emptySortedSet ()

Returns an empty sorted set (immutable). This set is serializable.

This example illustrates the type-safe way to obtain an empty sorted set:

 SortedSet<String> s = Collections.emptySortedSet();
 

Implementation Note:

• Implementations of this method need not create a separate SortedSet object for each call.

Returns
SortedSet<E>	the empty sorted set

enumeration

Added in API level 1

public static Enumeration<T> enumeration (Collection<T> c)

Returns an enumeration over the specified collection. This provides interoperability with legacy APIs that require an enumeration as input.

The iterator returned from a call to Enumeration#asIterator() does not support removal of elements from the specified collection. This is necessary to avoid unintentionally increasing the capabilities of the returned enumeration.

Parameters
c	Collection: the collection for which an enumeration is to be returned.

Returns
Enumeration<T>	an enumeration over the specified collection.

See also:

• Enumeration

fill

Added in API level 1

public static void fill (List<? super T> list, 
                T obj)

Replaces all of the elements of the specified list with the specified element.

This method runs in linear time.

Parameters
list	List: the list to be filled with the specified element.
obj	T: The element with which to fill the specified list.

Throws
UnsupportedOperationException	if the specified list or its list-iterator does not support the set operation.

frequency

Added in API level 1

public static int frequency (Collection<?> c, 
                Object o)

Returns the number of elements in the specified collection equal to the specified object. More formally, returns the number of elements e in the collection such that Objects.equals(o, e).

Parameters
c	Collection: the collection in which to determine the frequency of o
o	Object: the object whose frequency is to be determined

Returns
int	the number of elements in c equal to o

Throws
NullPointerException	if c is null

indexOfSubList

Added in API level 1

public static int indexOfSubList (List<?> source, 
                List<?> target)

Returns the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. More formally, returns the lowest index i such that source.subList(i, i+target.size()).equals(target), or -1 if there is no such index. (Returns -1 if target.size() > source.size())

This implementation uses the "brute force" technique of scanning over the source list, looking for a match with the target at each location in turn.

Parameters
source	List: the list in which to search for the first occurrence of target.
target	List: the list to search for as a subList of source.

Returns
int	the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.

lastIndexOfSubList

Added in API level 1

public static int lastIndexOfSubList (List<?> source, 
                List<?> target)

Returns the starting position of the last occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. More formally, returns the highest index i such that source.subList(i, i+target.size()).equals(target), or -1 if there is no such index. (Returns -1 if target.size() > source.size())

This implementation uses the "brute force" technique of iterating over the source list, looking for a match with the target at each location in turn.

Parameters
source	List: the list in which to search for the last occurrence of target.
target	List: the list to search for as a subList of source.

Returns
int	the starting position of the last occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.

list

Added in API level 1

public static ArrayList<T> list (Enumeration<T> e)

Returns an array list containing the elements returned by the specified enumeration in the order they are returned by the enumeration. This method provides interoperability between legacy APIs that return enumerations and new APIs that require collections.

Parameters
e	Enumeration: enumeration providing elements for the returned array list

Returns
ArrayList<T>	an array list containing the elements returned by the specified enumeration.

See also:

• Enumeration
• ArrayList

max

Added in API level 1

public static T max (Collection<? extends T> coll, 
                Comparator<? super T> comp)

Returns the maximum element of the given collection, according to the order induced by the specified comparator. All elements in the collection must be mutually comparable by the specified comparator (that is, comp.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the collection).

This method iterates over the entire collection, hence it requires time proportional to the size of the collection.

Parameters
coll	Collection: the collection whose maximum element is to be determined.
comp	Comparator: the comparator with which to determine the maximum element. A null value indicates that the elements' natural ordering should be used.

Returns
T	the maximum element of the given collection, according to the specified comparator.

Throws
ClassCastException	if the collection contains elements that are not mutually comparable using the specified comparator.
NoSuchElementException	if the collection is empty.

See also:

• Comparable

max

Added in API level 1

public static T max (Collection<? extends T> coll)

Returns the maximum element of the given collection, according to the natural ordering of its elements. All elements in the collection must implement the Comparable interface. Furthermore, all elements in the collection must be mutually comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the collection).

This method iterates over the entire collection, hence it requires time proportional to the size of the collection.

Parameters
coll	Collection: the collection whose maximum element is to be determined.

Returns
T	the maximum element of the given collection, according to the natural ordering of its elements.

Throws
ClassCastException	if the collection contains elements that are not mutually comparable (for example, strings and integers).
NoSuchElementException	if the collection is empty.

See also:

• Comparable

min

Added in API level 1

public static T min (Collection<? extends T> coll)

Returns the minimum element of the given collection, according to the natural ordering of its elements. All elements in the collection must implement the Comparable interface. Furthermore, all elements in the collection must be mutually comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the collection).

This method iterates over the entire collection, hence it requires time proportional to the size of the collection.

Parameters
coll	Collection: the collection whose minimum element is to be determined.

Returns
T	the minimum element of the given collection, according to the natural ordering of its elements.

Throws
ClassCastException	if the collection contains elements that are not mutually comparable (for example, strings and integers).
NoSuchElementException	if the collection is empty.

See also:

• Comparable

min

Added in API level 1

public static T min (Collection<? extends T> coll, 
                Comparator<? super T> comp)

Returns the minimum element of the given collection, according to the order induced by the specified comparator. All elements in the collection must be mutually comparable by the specified comparator (that is, comp.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the collection).

This method iterates over the entire collection, hence it requires time proportional to the size of the collection.

Parameters
coll	Collection: the collection whose minimum element is to be determined.
comp	Comparator: the comparator with which to determine the minimum element. A null value indicates that the elements' natural ordering should be used.

Returns
T	the minimum element of the given collection, according to the specified comparator.

Throws
ClassCastException	if the collection contains elements that are not mutually comparable using the specified comparator.
NoSuchElementException	if the collection is empty.

See also:

• Comparable

nCopies

Added in API level 1

public static List<T> nCopies (int n, 
                T o)

Returns an immutable list consisting of n copies of the specified object. The newly allocated data object is tiny (it contains a single reference to the data object). This method is useful in combination with the List.addAll method to grow lists. The returned list is serializable.

Parameters
n	int: the number of elements in the returned list.
o	T: the element to appear repeatedly in the returned list.

Returns
List<T>	an immutable list consisting of n copies of the specified object.

Throws
IllegalArgumentException	if n < 0

See also:

• List.addAll(Collection)
• List.addAll(int, Collection)

newSequencedSetFromMap

Added in API level 35

public static SequencedSet<E> newSequencedSetFromMap (SequencedMap<E, Boolean> map)

Returns a sequenced set backed by the specified map. The resulting set displays the same ordering, concurrency, and performance characteristics as the backing map. In essence, this factory method provides a SequencedSet implementation corresponding to any SequencedMap implementation.

Each method invocation on the set returned by this method results in exactly one method invocation on the backing map or its keySet view, with one exception. The addAll method is implemented as a sequence of put invocations on the backing map.

The specified map must be empty at the time this method is invoked, and should not be accessed directly after this method returns. These conditions are ensured if the map is created empty, passed directly to this method, and no reference to the map is retained.

API Note:

• The following example code creates a SequencedSet from 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; } });

Parameters
map	SequencedMap: the backing map

Returns
SequencedSet<E>	the set backed by the map

Throws
IllegalArgumentException	if map is not empty

newSetFromMap

Added in API level 9

public static Set<E> newSetFromMap (Map<E, Boolean> map)

Returns a set backed by the specified map. The resulting set displays the same ordering, concurrency, and performance characteristics as the backing map. In essence, this factory method provides a Set implementation corresponding to any Map implementation. There is no need to use this method on a Map implementation that already has a corresponding Set implementation (such as HashMap or TreeMap).

Each method invocation on the set returned by this method results in exactly one method invocation on the backing map or its keySet view, with one exception. The addAll method is implemented as a sequence of put invocations on the backing map.

The specified map must be empty at the time this method is invoked, and should not be accessed directly after this method returns. These conditions are ensured if the map is created empty, passed directly to this method, and no reference to the map is retained, as illustrated in the following code fragment:

    Set<Object> weakHashSet = Collections.newSetFromMap(
        new WeakHashMap<Object, Boolean>());
 

Parameters
map	Map: the backing map

Returns
Set<E>	the set backed by the map

Throws
IllegalArgumentException	if map is not empty

replaceAll

Added in API level 1

public static boolean replaceAll (List<T> list, 
                T oldVal, 
                T newVal)

Replaces all occurrences of one specified value in a list with another. More formally, replaces with newVal each element e in list such that (oldVal==null ? e==null : oldVal.equals(e)). (This method has no effect on the size of the list.)

Parameters
list	List: the list in which replacement is to occur.
oldVal	T: the old value to be replaced.
newVal	T: the new value with which oldVal is to be replaced.

Returns
boolean	true if list contained one or more elements e such that (oldVal==null ? e==null : oldVal.equals(e)).

Throws
UnsupportedOperationException	if the specified list or its list-iterator does not support the set operation.

reverse

Added in API level 1

public static void reverse (List<?> list)

Reverses the order of the elements in the specified list.

This method runs in linear time.

API Note:

• This method mutates the specified list in-place. To obtain a reverse-ordered view of a list without mutating it, use the List.reversed method.

Parameters
list	List: the list whose elements are to be reversed.

Throws
UnsupportedOperationException	if the specified list or its list-iterator does not support the set operation.

See also:

• List.reversed

reverseOrder

Added in API level 1

public static Comparator<T> reverseOrder ()

Returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface. (The natural ordering is the ordering imposed by the objects' own compareTo method.) This enables a simple idiom for sorting (or maintaining) collections (or arrays) of objects that implement the Comparable interface in reverse-natural-order. For example, suppose a is an array of strings. Then:

          Arrays.sort(a, Collections.reverseOrder());
 

sorts the array in reverse-lexicographic (alphabetical) order.

The returned comparator is serializable.

API Note:

• This method returns a Comparator that is suitable for sorting elements in reverse order. To obtain a reverse-ordered view of a sequenced collection, use the SequencedCollection.reversed method. Or, to obtain a reverse-ordered view of a sequenced map, use the SequencedMap.reversed method.

Returns
Comparator<T>	A comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface.

See also:

• Comparable

reverseOrder

Added in API level 1

public static Comparator<T> reverseOrder (Comparator<T> cmp)

Returns a comparator that imposes the reverse ordering of the specified comparator. If the specified comparator is null, this method is equivalent to reverseOrder() (in other words, it returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface).

The returned comparator is serializable (assuming the specified comparator is also serializable or null).

API Note:

• This method returns a Comparator that is suitable for sorting elements in reverse order. To obtain a reverse-ordered view of a sequenced collection, use the SequencedCollection.reversed method. Or, to obtain a reverse-ordered view of a sequenced map, use the SequencedMap.reversed method.

Parameters
cmp	Comparator: a comparator who's ordering is to be reversed by the returned comparator or null

Returns
Comparator<T>	A comparator that imposes the reverse ordering of the specified comparator.

rotate

Added in API level 1

public static void rotate (List<?> list, 
                int distance)

Rotates the elements in the specified list by the specified distance. After calling this method, the element at index i will be the element previously at index (i - distance) mod list.size(), for all values of i between 0 and list.size()-1, inclusive. (This method has no effect on the size of the list.)

For example, suppose list comprises[t, a, n, k, s]. After invoking Collections.rotate(list, 1) (or Collections.rotate(list, -4)), list will comprise [s, t, a, n, k].

Note that this method can usefully be applied to sublists to move one or more elements within a list while preserving the order of the remaining elements. For example, the following idiom moves the element at index j forward to position k (which must be greater than or equal to j):

     Collections.rotate(list.subList(j, k+1), -1);
 

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(java.util.List) method is invoked on each sublist view, and finally it is invoked on the entire list. For a more complete description of both algorithms, see Section 2.3 of Jon Bentley's Programming Pearls (Addison-Wesley, 1986).

Parameters
list	List: the list to be rotated.
distance	int: the distance to rotate the list. There are no constraints on this value; it may be zero, negative, or greater than list.size().

Throws
UnsupportedOperationException	if the specified list or its list-iterator does not support the set operation.

shuffle

Added in API level 1

public static void shuffle (List<?> list, 
                Random rnd)

Randomly permute the specified list using the specified source of randomness.

This method is equivalent to shuffle(java.util.List, java.util.random.RandomGenerator) and exists for backward compatibility. The shuffle(java.util.List, java.util.random.RandomGenerator) method is preferred, as it is not limited to random generators that extend the Random class.

Parameters
list	List: the list to be shuffled.
rnd	Random: the source of randomness to use to shuffle the list.

Throws
UnsupportedOperationException	if the specified list or its list-iterator does not support the set operation.

shuffle

Added in API level 1

public static void shuffle (List<?> list)

Randomly permutes the specified list using a default source of randomness. All permutations occur with approximately equal likelihood.

The hedge "approximately" is used in the foregoing description because default source of randomness is only approximately an unbiased source of independently chosen bits. If it were a perfect source of randomly chosen bits, then the algorithm would choose permutations with perfect uniformity.

This implementation traverses the list backwards, from the last element up to the second, repeatedly swapping a randomly selected element into the "current position". Elements are randomly selected from the portion of the list that runs from the first element to the current position, inclusive.

Implementation Requirements:

• This method runs in linear time. If the specified list does not implement the RandomAccess interface and is large, this implementation dumps the specified list into an array before shuffling it, and dumps the shuffled array back into the list. This avoids the quadratic behavior that would result from shuffling a "sequential access" list in place.

Parameters
list	List: the list to be shuffled.

Throws
UnsupportedOperationException	if the specified list or its list-iterator does not support the set operation.

shuffle

Added in API level 35

public static void shuffle (List<?> list, 
                RandomGenerator rnd)

Randomly permute the specified list using the specified source of randomness. All permutations occur with equal likelihood assuming that the source of randomness is fair.

This implementation traverses the list backwards, from the last element up to the second, repeatedly swapping a randomly selected element into the "current position". Elements are randomly selected from the portion of the list that runs from the first element to the current position, inclusive.

Implementation Requirements:

• This method runs in linear time. If the specified list does not implement the RandomAccess interface and is large, this implementation dumps the specified list into an array before shuffling it, and dumps the shuffled array back into the list. This avoids the quadratic behavior that would result from shuffling a "sequential access" list in place.

Parameters
list	List: the list to be shuffled.
rnd	RandomGenerator: the source of randomness to use to shuffle the list.

Throws
UnsupportedOperationException	if the specified list or its list-iterator does not support the set operation.

singleton

Added in API level 1

public static Set<T> singleton (T o)

Returns an immutable set containing only the specified object. The returned set is serializable.

Parameters
o	T: the sole object to be stored in the returned set.

Returns
Set<T>	an immutable set containing only the specified object.

singletonList

Added in API level 1

public static List<T> singletonList (T o)

Returns an immutable list containing only the specified object. The returned list is serializable.

Parameters
o	T: the sole object to be stored in the returned list.

Returns
List<T>	an immutable list containing only the specified object.

singletonMap

Added in API level 1

public static Map<K, V> singletonMap (K key, 
                V value)

Returns an immutable map, mapping only the specified key to the specified value. The returned map is serializable.

Parameters
key	K: the sole key to be stored in the returned map.
value	V: the value to which the returned map maps key.

Returns
Map<K, V>	an immutable map containing only the specified key-value mapping.

sort

Added in API level 1

public static void sort (List<T> list)

Sorts the specified list into ascending order, according to the natural ordering of its elements. All elements in the list must implement the Comparable interface. Furthermore, all elements in the list must be mutually comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the list).

This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort.

The specified list must be modifiable, but need not be resizable.

Implementation Note:

• This implementation defers to the List#sort(Comparator) method using the specified list and a null comparator. Do not call this method from List.sort() since that can lead to infinite recursion. Apps targeting APIs <= 25 observe backwards compatibility behavior where this method was implemented on top of List#toArray(), ListIterator#next() and ListIterator#set(Object).

Parameters
list	List: the list to be sorted.

Throws
ClassCastException	if the list contains elements that are not mutually comparable (for example, strings and integers).
UnsupportedOperationException	if the specified list's list-iterator does not support the set operation.
IllegalArgumentException	(optional) if the implementation detects that the natural ordering of the list elements is found to violate the Comparable contract

See also:

• List.sort(Comparator)

sort

Added in API level 1

public static void sort (List<T> list, 
                Comparator<? super T> c)

Sorts the specified list according to the order induced by the specified comparator. All elements in the list must be mutually comparable using the specified comparator (that is, c.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the list).

This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort.

The specified list must be modifiable, but need not be resizable.

Implementation Note:

• This implementation defers to the List#sort(Comparator) method using the specified list and comparator. Do not call this method from List.sort() since that can lead to infinite recursion. Apps targeting APIs <= 25 observe backwards compatibility behavior where this method was implemented on top of List#toArray(), ListIterator#next() and ListIterator#set(Object).

Parameters
list	List: the list to be sorted.
c	Comparator: the comparator to determine the order of the list. A null value indicates that the elements' natural ordering should be used.

Throws
ClassCastException	if the list contains elements that are not mutually comparable using the specified comparator.
UnsupportedOperationException	if the specified list's list-iterator does not support the set operation.
IllegalArgumentException	(optional) if the comparator is found to violate the Comparator contract

See also:

• List.sort(Comparator)

swap

Added in API level 1

public static void swap (List<?> list, 
                int i, 
                int j)

Swaps the elements at the specified positions in the specified list. (If the specified positions are equal, invoking this method leaves the list unchanged.)

Parameters
list	List: The list in which to swap elements.
i	int: the index of one element to be swapped.
j	int: the index of the other element to be swapped.

Throws
IndexOutOfBoundsException	if either i or j is out of range (i < 0 || i >= list.size() || j < 0 || j >= list.size()).

synchronizedCollection

Added in API level 1

public static Collection<T> synchronizedCollection (Collection<T> c)

Returns a synchronized (thread-safe) collection backed by the specified collection. In order to guarantee serial access, it is critical that all access to the backing collection is accomplished through the returned collection.

It is imperative that the user manually synchronize on the returned collection when traversing it via Iterator, Spliterator or Stream:

  Collection c = Collections.synchronizedCollection(myCollection);
     ...
  synchronized (c) {
      Iterator i = c.iterator(); // Must be in the synchronized block
      while (i.hasNext())
         foo(i.next());
  }
 

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.

Parameters
c	Collection: the collection to be "wrapped" in a synchronized collection.

Returns
Collection<T>	a synchronized view of the specified collection.

synchronizedList

Added in API level 1

public static List<T> synchronizedList (List<T> list)

Returns a synchronized (thread-safe) list backed by the specified list. In order to guarantee serial access, it is critical that all access to the backing list is accomplished through the returned list.

It is imperative that the user manually synchronize on the returned list when traversing it via Iterator, Spliterator or Stream:

  List list = Collections.synchronizedList(new ArrayList());
      ...
  synchronized (list) {
      Iterator i = list.iterator(); // Must be in synchronized block
      while (i.hasNext())
          foo(i.next());
  }
 

Failure to follow this advice may result in non-deterministic behavior.

The returned list will be serializable if the specified list is serializable.

Parameters
list	List: the list to be "wrapped" in a synchronized list.

Returns
List<T>	a synchronized view of the specified list.

synchronizedMap

Added in API level 1

public static Map<K, V> synchronizedMap (Map<K, V> m)

Returns a synchronized (thread-safe) map backed by the specified map. In order to guarantee serial access, it is critical that all access to the backing map is accomplished through the returned map.

It is imperative that the user manually synchronize on the returned map when traversing any of its collection views via Iterator, Spliterator or Stream:

  Map m = Collections.synchronizedMap(new HashMap());
      ...
  Set s = m.keySet();  // Needn't be in synchronized block
      ...
  synchronized (m) {  // Synchronizing on m, not s!
      Iterator i = s.iterator(); // Must be in synchronized block
      while (i.hasNext())
          foo(i.next());
  }
 

Failure to follow this advice may result in non-deterministic behavior.

The returned map will be serializable if the specified map is serializable.

Parameters
m	Map: the map to be "wrapped" in a synchronized map.

Returns
Map<K, V>	a synchronized view of the specified map.

synchronizedNavigableMap

Added in API level 26

public static NavigableMap<K, V> synchronizedNavigableMap (NavigableMap<K, V> m)

Returns a synchronized (thread-safe) navigable map backed by the specified navigable map. In order to guarantee serial access, it is critical that all access to the backing navigable map is accomplished through the returned navigable map (or its views).

It is imperative that the user manually synchronize on the returned navigable map when traversing any of its collection views, or the collections views of any of its subMap, headMap or tailMap views, via Iterator, Spliterator or Stream:

  NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
      ...
  Set s = m.keySet();  // Needn't be in synchronized block
      ...
  synchronized (m) {  // Synchronizing on m, not s!
      Iterator i = s.iterator(); // Must be in synchronized block
      while (i.hasNext())
          foo(i.next());
  }
 

or:

  NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
  NavigableMap m2 = m.subMap(foo, true, bar, false);
      ...
  Set s2 = m2.keySet();  // Needn't be in synchronized block
      ...
  synchronized (m) {  // Synchronizing on m, not m2 or s2!
      Iterator i = 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 navigable map will be serializable if the specified navigable map is serializable.

Parameters
m	NavigableMap: the navigable map to be "wrapped" in a synchronized navigable map

Returns
NavigableMap<K, V>	a synchronized view of the specified navigable map.

synchronizedNavigableSet

Added in API level 26

public static NavigableSet<T> synchronizedNavigableSet (NavigableSet<T> s)

Returns a synchronized (thread-safe) navigable set backed by the specified navigable set. In order to guarantee serial access, it is critical that all access to the backing navigable set is accomplished through the returned navigable set (or its views).

It is imperative that the user manually synchronize on the returned navigable set when traversing it, or any of its subSet, headSet, or tailSet views, via Iterator, Spliterator or Stream:

  NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
      ...
  synchronized (s) {
      Iterator i = s.iterator(); // Must be in the synchronized block
      while (i.hasNext())
          foo(i.next());
  }
 

or:

  NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
  NavigableSet s2 = s.headSet(foo, true);
      ...
  synchronized (s) {  // Note: s, not s2!!!
      Iterator i = s2.iterator(); // Must be in the synchronized block
      while (i.hasNext())
          foo(i.next());
  }
 

Failure to follow this advice may result in non-deterministic behavior.

The returned navigable set will be serializable if the specified navigable set is serializable.

Parameters
s	NavigableSet: the navigable set to be "wrapped" in a synchronized navigable set

Returns
NavigableSet<T>	a synchronized view of the specified navigable set

synchronizedSet

Added in API level 1

public static Set<T> synchronizedSet (Set<T> s)

Returns a synchronized (thread-safe) set backed by the specified set. In order to guarantee serial access, it is critical that all access to the backing set is accomplished through the returned set.

It is imperative that the user manually synchronize on the returned collection when traversing it via Iterator, Spliterator or Stream:

  Set s = Collections.synchronizedSet(new HashSet());
      ...
  synchronized (s) {
      Iterator i = s.iterator(); // Must be in the synchronized block
      while (i.hasNext())
          foo(i.next());
  }
 

Failure to follow this advice may result in non-deterministic behavior.

The returned set will be serializable if the specified set is serializable.

Parameters
s	Set: the set to be "wrapped" in a synchronized set.

Returns
Set<T>	a synchronized view of the specified set.

synchronizedSortedMap

Added in API level 1

public static SortedMap<K, V> synchronizedSortedMap (SortedMap<K, V> m)

Returns a synchronized (thread-safe) sorted map backed by the specified sorted map. In order to guarantee serial access, it is critical that all access to the backing sorted map is accomplished through the returned sorted map (or its views).

It is imperative that the user manually synchronize on the returned sorted map when traversing any of its collection views, or the collections views of any of its subMap, headMap or tailMap views, via Iterator, Spliterator or Stream:

  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
      ...
  Set s = m.keySet();  // Needn't be in synchronized block
      ...
  synchronized (m) {  // Synchronizing on m, not s!
      Iterator i = s.iterator(); // Must be in synchronized block
      while (i.hasNext())
          foo(i.next());
  }
 

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.

Parameters
m	SortedMap: the sorted map to be "wrapped" in a synchronized sorted map.

Returns
SortedMap<K, V>	a synchronized view of the specified sorted map.

synchronizedSortedSet

Added in API level 1

public static SortedSet<T> synchronizedSortedSet (SortedSet<T> s)

Returns a synchronized (thread-safe) sorted set backed by the specified sorted set. In order to guarantee serial access, it is critical that all access to the backing sorted set is accomplished through the returned sorted set (or its views).

It is imperative that the user manually synchronize on the returned sorted set when traversing it or any of its subSet, headSet, or tailSet views via Iterator, Spliterator or Stream:

  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
      ...
  synchronized (s) {
      Iterator i = s.iterator(); // Must be in the synchronized block
      while (i.hasNext())
          foo(i.next());
  }
 

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.

Parameters
s	SortedSet: the sorted set to be "wrapped" in a synchronized sorted set.

Returns
SortedSet<T>	a synchronized view of the specified sorted set.

unmodifiableCollection

Added in API level 1

public static Collection<T> unmodifiableCollection (Collection<? extends T> c)

Returns an unmodifiable view of the specified collection. Query operations on the returned collection "read through" to the specified collection, and attempts to modify the returned collection, whether direct or via its iterator, result in an UnsupportedOperationException.

The returned collection does not pass the hashCode and equals operations through to the backing collection, but relies on Object's equals and hashCode methods. This is necessary to preserve the contracts of these operations in the case that the backing collection is a set or a list.

The returned collection will be serializable if the specified collection is serializable.

Implementation Note:

• This method may return its argument if the argument is already unmodifiable.

Parameters
c	Collection: the collection for which an unmodifiable view is to be returned.

Returns
Collection<T>	an unmodifiable view of the specified collection.

unmodifiableList

Added in API level 1

public static List<T> unmodifiableList (List<? extends T> list)

Returns an unmodifiable view of the specified list. Query operations on the returned list "read through" to the specified list, and attempts to modify the returned list, whether direct or via its iterator, result in an UnsupportedOperationException.

The returned list will be serializable if the specified list is serializable. Similarly, the returned list will implement RandomAccess if the specified list does.

Implementation Note:

• This method may return its argument if the argument is already unmodifiable.

Parameters
list	List: the list for which an unmodifiable view is to be returned.

Returns
List<T>	an unmodifiable view of the specified list.

unmodifiableMap

Added in API level 1

public static Map<K, V> unmodifiableMap (Map<? extends K, ? extends V> m)

Returns an unmodifiable view of the specified map. Query operations on the returned map "read through" to the specified map, and attempts to modify the returned map, whether direct or via its collection views, result in an UnsupportedOperationException.

The returned map will be serializable if the specified map is serializable.

Implementation Note:

• This method may return its argument if the argument is already unmodifiable.

Parameters
m	Map: the map for which an unmodifiable view is to be returned.

Returns
Map<K, V>	an unmodifiable view of the specified map.

unmodifiableNavigableMap

Added in API level 26

public static NavigableMap<K, V> unmodifiableNavigableMap (NavigableMap<K, ? extends V> m)

Returns an unmodifiable view of the specified navigable map. Query operations on the returned navigable map "read through" to the specified navigable map. Attempts to modify the returned navigable map, whether direct, via its collection views, or via its subMap, headMap, or tailMap views, result in an UnsupportedOperationException.

The returned navigable map will be serializable if the specified navigable map is serializable.

Implementation Note:

• This method may return its argument if the argument is already unmodifiable.

Parameters
m	NavigableMap: the navigable map for which an unmodifiable view is to be returned

Returns
NavigableMap<K, V>	an unmodifiable view of the specified navigable map

unmodifiableNavigableSet

Added in API level 26

public static NavigableSet<T> unmodifiableNavigableSet (NavigableSet<T> s)

Returns an unmodifiable view of the specified navigable set. Query operations on the returned navigable set "read through" to the specified navigable set. Attempts to modify the returned navigable set, whether direct, via its iterator, or via its subSet, headSet, or tailSet views, result in an UnsupportedOperationException.

The returned navigable set will be serializable if the specified navigable set is serializable.

Implementation Note:

• This method may return its argument if the argument is already unmodifiable.

Parameters
s	NavigableSet: the navigable set for which an unmodifiable view is to be returned

Returns
NavigableSet<T>	an unmodifiable view of the specified navigable set

unmodifiableSequencedCollection

Added in API level 35

public static SequencedCollection<T> unmodifiableSequencedCollection (SequencedCollection<? extends T> c)

Returns an unmodifiable view of the specified SequencedCollection. Query operations on the returned collection "read through" to the specified collection, and attempts to modify the returned collection, whether direct or via its iterator, result in an UnsupportedOperationException.

The returned collection does not pass the hashCode and equals operations through to the backing collection, but relies on Object's equals and hashCode methods. This is necessary to preserve the contracts of these operations in the case that the backing collection is a set or a list.

The returned collection will be serializable if the specified collection is serializable.

Implementation Note:

• This method may return its argument if the argument is already unmodifiable.

Parameters
c	SequencedCollection: the collection for which an unmodifiable view is to be returned.

Returns
SequencedCollection<T>	an unmodifiable view of the specified collection.

unmodifiableSequencedMap

Added in API level 35

public static SequencedMap<K, V> unmodifiableSequencedMap (SequencedMap<? extends K, ? extends V> m)

Returns an unmodifiable view of the specified SequencedMap. Query operations on the returned map "read through" to the specified map, and attempts to modify the returned map, whether direct or via its collection views, result in an UnsupportedOperationException.

The returned map will be serializable if the specified map is serializable.

Implementation Note:

• This method may return its argument if the argument is already unmodifiable.

Parameters
m	SequencedMap: the map for which an unmodifiable view is to be returned.

Returns
SequencedMap<K, V>	an unmodifiable view of the specified map.

unmodifiableSequencedSet

Added in API level 35

public static SequencedSet<T> unmodifiableSequencedSet (SequencedSet<? extends T> s)

Returns an unmodifiable view of the specified SequencedSet. Query operations on the returned set "read through" to the specified set, and attempts to modify the returned set, whether direct or via its iterator, result in an UnsupportedOperationException.

The returned set will be serializable if the specified set is serializable.

Implementation Note:

• This method may return its argument if the argument is already unmodifiable.

Parameters
s	SequencedSet: the set for which an unmodifiable view is to be returned.

Returns
SequencedSet<T>	an unmodifiable view of the specified sequenced set.

unmodifiableSet

Added in API level 1

public static Set<T> unmodifiableSet (Set<? extends T> s)

Returns an unmodifiable view of the specified set. Query operations on the returned set "read through" to the specified set, and attempts to modify the returned set, whether direct or via its iterator, result in an UnsupportedOperationException.

The returned set will be serializable if the specified set is serializable.

Implementation Note:

• This method may return its argument if the argument is already unmodifiable.

Parameters
s	Set: the set for which an unmodifiable view is to be returned.

Returns
Set<T>	an unmodifiable view of the specified set.

unmodifiableSortedMap

Added in API level 1

public static SortedMap<K, V> unmodifiableSortedMap (SortedMap<K, ? extends V> m)

Returns an unmodifiable view of the specified sorted map. Query operations on the returned sorted map "read through" to the specified sorted map. Attempts to modify the returned sorted map, whether direct, via its collection views, or via its subMap, headMap, or tailMap views, result in an UnsupportedOperationException.

The returned sorted map will be serializable if the specified sorted map is serializable.

Implementation Note:

• This method may return its argument if the argument is already unmodifiable.

Parameters
m	SortedMap: the sorted map for which an unmodifiable view is to be returned.

Returns
SortedMap<K, V>	an unmodifiable view of the specified sorted map.

unmodifiableSortedSet

Added in API level 1

public static SortedSet<T> unmodifiableSortedSet (SortedSet<T> s)

Returns an unmodifiable view of the specified sorted set. Query operations on the returned sorted set "read through" to the specified sorted set. Attempts to modify the returned sorted set, whether direct, via its iterator, or via its subSet, headSet, or tailSet views, result in an UnsupportedOperationException.

The returned sorted set will be serializable if the specified sorted set is serializable.

Implementation Note:

• This method may return its argument if the argument is already unmodifiable.

Parameters
s	SortedSet: the sorted set for which an unmodifiable view is to be returned.

Returns
SortedSet<T>	an unmodifiable view of the specified sorted set.