java.util

Class Collections

java.lang.Object

java.util.Collections

public class Collections
extends Object

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.

Since:

1.2

Author:

Josh Bloch, Neal Gafter

See Also:

Collection, Set, List, Map

Field Summary

Fields

Modifier and Type	Field and Description
static List	EMPTY_LIST The empty list (immutable).
static Map	EMPTY_MAP The empty map (immutable).
static Set	EMPTY_SET The empty set (immutable).

Method Summary

Modifier and Type	Method and 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) Returns a view of a Deque as a Last-in-first-out (Lifo) Queue.
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 <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 <T> List<T>	emptyList() Returns the empty list (immutable).
static <T> ListIterator<T>	emptyListIterator() Returns a list iterator that has no elements.
static <K,V> Map<K,V>	emptyMap() Returns the empty map (immutable).
static <T> Set<T>	emptySet() Returns the empty 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>> T	max(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>> 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> 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) 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>> 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 <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 <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.

Methods inherited from class java.lang.Object

clone, equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

EMPTY_LIST

public static final List EMPTY_LIST

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

See Also:

emptyList()

EMPTY_MAP

public static final Map EMPTY_MAP

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

Since:

1.3

See Also:

emptyMap()

EMPTY_SET

public static final Set EMPTY_SET

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

See Also:

emptySet()

Method Detail

addAll

@SafeVarargs
public static <T> 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 behavior of this convenience method is identical to that of c.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");
 

Parameters:

c - the collection into which elements are to be inserted

elements - the elements to insert into c

Returns:

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

Since:

1.5

See Also:

Collection.addAll(Collection)

asLifoQueue

public static <T> 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.

Parameters:

deque - the deque

Returns:

the queue

Since:

1.6

binarySearch

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

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

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, 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.

binarySearch

public static <T> int binarySearch(List<? extends T> list,
                                   T key,
                                   @Nullable
                                   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 - the list to be searched.

key - the key to be searched for.

c - the comparator by which the list is ordered. A null value indicates that the elements' natural ordering should be used.

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, 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.

copy

public static <T> 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 must be at least as long as the source list. If it is longer, the remaining elements in the destination list are unaffected.

This method runs in linear time.

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 the set operation.

disjoint

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

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

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

c2 - a collection

Returns:

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)

Since:

1.5

emptyEnumeration

public static <T> 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:

an empty enumeration

Since:

1.7

emptyIterator

public static <T> 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:

an empty iterator

Since:

1.7

emptyList

public static final <T> List<T> emptyList()

Returns the 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.)

Since:

1.5

See Also:

EMPTY_LIST

emptyListIterator

public static <T> 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:

an empty list iterator

Since:

1.7

emptyMap

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

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

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

 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.)

Since:

1.5

See Also:

EMPTY_MAP

emptySet

public static final <T> Set<T> emptySet()

Returns the 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.)

Since:

1.5

See Also:

EMPTY_SET

enumeration

public static <T> 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.

Parameters:

c - the collection for which an enumeration is to be returned.

Returns:

an enumeration over the specified collection.

See Also:

Enumeration

fill

public static <T> 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 - 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 the set operation.

frequency

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

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

Parameters:

c - the collection in which to determine the frequency of o

o - the object whose frequency is to be determined

Throws:

NullPointerException - if c is null

Since:

1.5

indexOfSubList

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

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

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

Parameters:

source - the list in which to search for the first occurrence of target.

target - the list to search for as a subList of source.

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

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 - the list in which to search for the last occurrence of target.

target - the list to search for as a subList of source.

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

list

public 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. This method provides interoperability between legacy APIs that return enumerations and new APIs that require collections.

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

max

public static <T extends Object & 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. 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 - 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

public static <T> T max(Collection<? extends T> coll,
                        @Nullable
                        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 - the collection whose maximum element is to be determined.

comp - the comparator with which to determine the maximum element. A null value indicates that the elements' natural ordering should be used.

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

min

public static <T extends Object & 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. 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 - 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

public static <T> T min(Collection<? extends T> coll,
                        @Nullable
                        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 - the collection whose minimum element is to be determined.

comp - the comparator with which to determine the minimum element. A null value indicates that the elements' natural ordering should be used.

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

nCopies

public static <T> 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 - 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 - if n < 0

See Also:

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

newSetFromMap

public static <E> 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 - the backing map

Returns:

the set backed by the map

Throws:

IllegalArgumentException - if map is not empty

Since:

1.6

replaceAll

public static <T> 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 - the list in which replacement is to occur.

oldVal - the old value to be replaced.

newVal - the new value with which oldVal is to be replaced.

Returns:

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.

Since:

1.4

reverse

public static void reverse(List<?> list)

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 the set operation.

reverseOrder

public 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. (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.

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

public static <T> Comparator<T> reverseOrder(@Nullable
                                             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).

Parameters:

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

Returns:

A comparator that imposes the reverse ordering of the specified comparator.

Since:

1.5

rotate

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

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

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

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

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

To make this concrete, suppose list comprises [a, b, c, d, e]. To move the element at index 1 (b) forward two positions, perform the following invocation:

 Collections.rotate(l.subList(1, 4), -1);
 

The resulting list is [a, c, d, b, e].

To move more than one element forward, increase the absolute value of the rotation distance. To move elements backward, use a positive shift distance.

If the specified list is small or implements the RandomAccess interface, this implementation exchanges the first element into the location it should go, and then repeatedly exchanges the displaced element into the location it should go until a displaced element is swapped into the first element. If necessary, the process is repeated on the second and successive elements, until the rotation is complete. If the specified list is large and doesn't implement the RandomAccess interface, this implementation breaks the list into two sublist views around index -distance mod size. Then the reverse(List) method is invoked on each sublist view, and finally it is invoked on the entire list. For a more complete description of both algorithms, see Section 2.3 of Jon Bentley's Programming Pearls (Addison-Wesley, 1986).

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 than list.size().

Throws:

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

Since:

1.4

shuffle

public static void shuffle(List<?> list)

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

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

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

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 the set operation.

shuffle

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

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 the set operation.

singleton

public static <T> Set<T> singleton(T o)

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

Parameters:

o - the sole object to be stored in the returned set.

Returns:

an immutable set containing only the specified object.

singletonList

public static <T> List<T> singletonList(T o)

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

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

public static <K,V> 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 - the sole key to be stored in the returned map.

value - the value to which the returned map maps key.

Returns:

an immutable map containing only the specified key-value mapping.

Since:

1.3

sort

public 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. 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 is a stable, adaptive, iterative mergesort that requires far fewer than n lg(n) comparisons when the input array is partially sorted, while offering the performance of a traditional mergesort when the input array is randomly ordered. If the input array is nearly sorted, the implementation requires approximately n comparisons. Temporary storage requirements vary from a small constant for nearly sorted input arrays to n/2 object references for randomly ordered input arrays.

The implementation takes equal advantage of ascending and descending order in its input array, and can take advantage of ascending and descending order in different parts of the same input array. It is well-suited to merging two or more sorted arrays: simply concatenate the arrays and sort the resulting array.

The implementation was adapted from Tim Peters's list sort for Python ( TimSort). It uses techiques from Peter McIlroy's "Optimistic Sorting and Information Theoretic Complexity", in Proceedings of the Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474, January 1993.

This implementation dumps the specified list into an array, sorts the array, and iterates over the list resetting each element from the corresponding position in the array. This avoids the n² log(n) performance that would result from attempting to sort a linked list in place.

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 the set operation.

IllegalArgumentException - (optional) if the implementation detects that the natural ordering of the list elements is found to violate the Comparable contract

sort

public static <T> void sort(List<T> list,
                            @Nullable
                            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 is a stable, adaptive, iterative mergesort that requires far fewer than n lg(n) comparisons when the input array is partially sorted, while offering the performance of a traditional mergesort when the input array is randomly ordered. If the input array is nearly sorted, the implementation requires approximately n comparisons. Temporary storage requirements vary from a small constant for nearly sorted input arrays to n/2 object references for randomly ordered input arrays.

The implementation takes equal advantage of ascending and descending order in its input array, and can take advantage of ascending and descending order in different parts of the same input array. It is well-suited to merging two or more sorted arrays: simply concatenate the arrays and sort the resulting array.

The implementation was adapted from Tim Peters's list sort for Python ( TimSort). It uses techiques from Peter McIlroy's "Optimistic Sorting and Information Theoretic Complexity", in Proceedings of the Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474, January 1993.

This implementation dumps the specified list into an array, sorts the array, and iterates over the list resetting each element from the corresponding position in the array. This avoids the n² log(n) performance that would result from attempting to sort a linked list in place.

Parameters:

list - the list to be sorted.

c - the comparator to determine the order of the list. A null value indicates that the elements' natural ordering should be used.

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

swap

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

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

Parameters:

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

Since:

1.4

synchronizedCollection

public static <T> 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 iterating over it:

  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 - the collection to be "wrapped" in a synchronized collection.

Returns:

a synchronized view of the specified collection.

synchronizedList

public static <T> 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 iterating over it:

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

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

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

Parameters:

list - the list to be "wrapped" in a synchronized list.

Returns:

a synchronized view of the specified list.

synchronizedMap

public static <K,V> 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 iterating over any of its collection views:

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

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

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

Parameters:

m - the map to be "wrapped" in a synchronized map.

Returns:

a synchronized view of the specified map.

synchronizedSet

public static <T> 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 set when iterating over it:

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

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

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

Parameters:

s - the set to be "wrapped" in a synchronized set.

Returns:

a synchronized view of the specified set.

synchronizedSortedMap

public static <K,V> 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 iterating over any of its collection views, or the collections views of any of its subMap, headMap or tailMap views.

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

or:

  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
  SortedMap m2 = m.subMap(foo, bar);
      ...
  Set s2 = m2.keySet();  // Needn't be in synchronized block
      ...
  synchronized (m) {  // Synchronizing on m, not m2 or s2!
      Iterator i = s.iterator(); // Must be in synchronized block
      while (i.hasNext())
          foo(i.next());
  }
 

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

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

Parameters:

m - the sorted map to be "wrapped" in a synchronized sorted map.

Returns:

a synchronized view of the specified sorted map.

synchronizedSortedSet

public static <T> 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 iterating over it or any of its subSet, headSet, or tailSet views.

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

or:

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

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

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

Parameters:

s - the sorted set to be "wrapped" in a synchronized sorted set.

Returns:

a synchronized view of the specified sorted set.

unmodifiableCollection

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

Returns an unmodifiable view of the specified collection. This method allows modules to provide users with "read-only" access to internal collections. 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.

Parameters:

c - the collection for which an unmodifiable view is to be returned.

Returns:

an unmodifiable view of the specified collection.

unmodifiableList

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

Returns an unmodifiable view of the specified list. This method allows modules to provide users with "read-only" access to internal lists. 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.

Parameters:

list - the list for which an unmodifiable view is to be returned.

Returns:

an unmodifiable view of the specified list.

unmodifiableMap

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

Returns an unmodifiable view of the specified map. This method allows modules to provide users with "read-only" access to internal maps. 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.

Parameters:

m - the map for which an unmodifiable view is to be returned.

Returns:

an unmodifiable view of the specified map.

unmodifiableSet

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

Returns an unmodifiable view of the specified set. This method allows modules to provide users with "read-only" access to internal sets. 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.

Parameters:

s - the set for which an unmodifiable view is to be returned.

Returns:

an unmodifiable view of the specified set.

unmodifiableSortedMap

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

Returns an unmodifiable view of the specified sorted map. This method allows modules to provide users with "read-only" access to internal sorted maps. 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.

Parameters:

m - the sorted map for which an unmodifiable view is to be returned.

Returns:

an unmodifiable view of the specified sorted map.

unmodifiableSortedSet

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

Returns an unmodifiable view of the specified sorted set. This method allows modules to provide users with "read-only" access to internal sorted sets. 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.

Parameters:

s - the sorted set for which an unmodifiable view is to be returned.

Returns:

an unmodifiable view of the specified sorted set.