java.util
Class Collections

java.lang.Object
  java.util.Collections

public class Collections
extends Object

Collections contains static methods which operate on Collection classes.

Since:

1.2

Nested Class Summary

(package private) static class	Collections.SynchronizedCollection<E>
(package private) static class	Collections.SynchronizedList<E>
(package private) static class	Collections.SynchronizedMap<K,V>
(package private) static class	Collections.SynchronizedRandomAccessList<E>
(package private) static class	Collections.SynchronizedSet<E>
(package private) static class	Collections.SynchronizedSortedMap<K,V>
(package private) static class	Collections.SynchronizedSortedSet<E>

Field Summary

static List	EMPTY_LIST
static Map	EMPTY_MAP
static Set	EMPTY_SET

Method Summary

static <T> boolean	addAll(Collection<? super T> c, T... a) Adds all the specified elements to the specified collection
static <T> int	binarySearch(List<? extends Comparable<? super T>> list, T object) Performs a binary search for the specified element in the specified sorted List.
static <T> int	binarySearch(List<? extends T> list, T object, Comparator<? super T> comparator) Performs a binary search for the specified element in the specified sorted List using the specified Comparator.
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 <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.
(package private) static <E> E	checkType(E obj, Class<E> type) Checks if specified object is instance of specified class.
static <T> void	copy(List<? super T> destination, List<? extends T> source) Copies the elements from the source list to the destination list.
static boolean	disjoint(Collection<?> c1, Collection<?> c2) Returns true if the collections have no elements in common
static <T> List<T>	emptyList() Returns a type-safe empty, immutable List.
static <K,V> Map<K,V>	emptyMap() Returns a type-safe empty, immutable Map.
static <T> Set<T>	emptySet() Returns a type-safe empty, immutable Set.
static <T> Enumeration<T>	enumeration(Collection<T> collection) Returns an Enumeration on the specified Collection.
static <T> void	fill(List<? super T> list, T object) Fills the specified List with the specified element.
static int	frequency(Collection<?> c, Object o) Returns the number of elements in the Collection that match the Object passed.
static int	indexOfSubList(List<?> list, List<?> sublist) Searches the list for sublist and returns the beginning index of the first occurrence.
static int	lastIndexOfSubList(List<?> list, List<?> sublist) Searches the list for sublist and returns the beginning index of the last occurrence.
static <T> ArrayList<T>	list(Enumeration<T> enumeration) Returns an ArrayList with all the elements in the enumeration.
static <T extends Object & Comparable<? super T>> T	max(Collection<? extends T> collection) Searches the specified Collection for the maximum element.
static <T> T	max(Collection<? extends T> collection, Comparator<? super T> comparator) Searches the specified Collection for the maximum element using the specified Comparator.
static <T extends Object & Comparable<? super T>> T	min(Collection<? extends T> collection) Searches the specified Collection for the minimum element.
static <T> T	min(Collection<? extends T> collection, Comparator<? super T> comparator) Searches the specified Collection for the minimum element using the specified Comparator.
static <T> List<T>	nCopies(int length, T object) Returns a List containing the specified number of the specified element.
static <T> boolean	replaceAll(List<T> list, T obj, T obj2) Replaces all occurrences of Object obj in list with newObj.
static void	reverse(List<?> list) Returns the supplied List with the order of its contained elements reversed.
static <T> Comparator<T>	reverseOrder() A Comparator which reverses the natural order of the elements.
static <T> Comparator<T>	reverseOrder(Comparator<T> c) Returns a Comparator that reverses the order of the Comparator passed.
static void	rotate(List<?> lst, int dist) Rotates the elements in List list by the distance dist
static void	shuffle(List<?> list) Moves every element of the List to a random new position in the list.
static void	shuffle(List<?> list, Random random) Moves every element of the List to a random new position in the list using the specified random number generator.
static <E> Set<E>	singleton(E object) Returns a Set containing the specified element.
static <E> List<E>	singletonList(E object) Returns a List containing the specified element.
static <K,V> Map<K,V>	singletonMap(K key, V value) Returns a Map containing the specified key and value.
static <T extends Comparable<? super T>> void	sort(List<T> list) Sorts the specified List in ascending order.
static <T> void	sort(List<T> list, Comparator<? super T> comparator) Sorts the specified List using the specified Comparator.
static void	swap(List<?> list, int index1, int index2) Swaps the elements of List list at indices index1 and index2
static <T> Collection<T>	synchronizedCollection(Collection<T> collection) Returns a wrapper on the specified Collection which synchronizes all access to the Collection.
static <T> List<T>	synchronizedList(List<T> list) Returns a wrapper on the specified List which synchronizes all access to the List.
static <K,V> Map<K,V>	synchronizedMap(Map<K,V> map) Returns a wrapper on the specified Map which synchronizes all access to the Map.
static <E> Set<E>	synchronizedSet(Set<E> set) Returns a wrapper on the specified Set which synchronizes all access to the Set.
static <K,V> SortedMap<K,V>	synchronizedSortedMap(SortedMap<K,V> map) Returns a wrapper on the specified SortedMap which synchronizes all access to the SortedMap.
static <E> SortedSet<E>	synchronizedSortedSet(SortedSet<E> set) Returns a wrapper on the specified SortedSet which synchronizes all access to the SortedSet.
static <E> Collection<E>	unmodifiableCollection(Collection<? extends E> collection) Returns a wrapper on the specified Collection which throws an UnsupportedOperationException whenever an attempt is made to modify the Collection.
static <E> List<E>	unmodifiableList(List<? extends E> list) Returns a wrapper on the specified List which throws an UnsupportedOperationException whenever an attempt is made to modify the List.
static <K,V> Map<K,V>	unmodifiableMap(Map<? extends K,? extends V> map) Returns a wrapper on the specified Map which throws an UnsupportedOperationException whenever an attempt is made to modify the Map.
static <E> Set<E>	unmodifiableSet(Set<? extends E> set) Returns a wrapper on the specified Set which throws an UnsupportedOperationException whenever an attempt is made to modify the Set.
static <K,V> SortedMap<K,V>	unmodifiableSortedMap(SortedMap<K,? extends V> map) Returns a wrapper on the specified SortedMap which throws an UnsupportedOperationException whenever an attempt is made to modify the SortedMap.
static <E> SortedSet<E>	unmodifiableSortedSet(SortedSet<E> set) Returns a wrapper on the specified SortedSet which throws an UnsupportedOperationException whenever an attempt is made to modify the SortedSet.

Methods inherited from class java.lang.Object

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

Field Detail

EMPTY_LIST

public static final List EMPTY_LIST

EMPTY_SET

public static final Set EMPTY_SET

EMPTY_MAP

public static final Map EMPTY_MAP

Method Detail

binarySearch

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

Performs a binary search for the specified element in the specified sorted List.

Parameters:

list - the sorted List to search

object - the element to find

Returns:

the non-negative index of the element, or a negative index which is the -index - 1 where the element would be inserted

Throws:

ClassCastException - when an element in the List or the search element does not implement Comparable, or cannot be compared to each other

binarySearch

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

Performs a binary search for the specified element in the specified sorted List using the specified Comparator.

Parameters:

list - the sorted List to search

object - the element to find

comparator - the Comparator. If the comparator is null then the search uses the objects' natural ordering.

Returns:

the non-negative index of the element, or a negative index which is the -index - 1 where the element would be inserted

Throws:

ClassCastException - when an element in the list and the searched element cannot be compared to each other using the Comparator

copy

public static <T> void copy(List<? super T> destination,
                            List<? extends T> source)

Copies the elements from the source list to the destination list.

Parameters:

destination -

source -

Throws:

IndexOutOfBoundsException - when the destination List is smaller than the source List

UnsupportedOperationException - when replacing an element in the destination list is not supported

enumeration

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

Returns an Enumeration on the specified Collection.

Parameters:

collection - the Collection to enumerate

Returns:

an Enumeration

fill

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

Fills the specified List with the specified element.

Parameters:

list - the List to fill

object - the fill element

Throws:

UnsupportedOperationException - when replacing an element in the List is not supported

max

public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> collection)

Searches the specified Collection for the maximum element.

Parameters:

collection - the Collection to search

Returns:

the maximum element in the Collection

Throws:

ClassCastException - when an element in the Collection does not implement Comparable or elements cannot be compared to each other

max

public static <T> T max(Collection<? extends T> collection,
                        Comparator<? super T> comparator)

Searches the specified Collection for the maximum element using the specified Comparator.

Parameters:

collection - the Collection to search

comparator - the Comparator

Returns:

the maximum element in the Collection

Throws:

ClassCastException - when elements in the Collection cannot be compared to each other using the Comparator

min

public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> collection)

Searches the specified Collection for the minimum element.

Parameters:

collection - the Collection to search

Returns:

the minimum element in the Collection

Throws:

ClassCastException - when an element in the Collection does not implement Comparable or elements cannot be compared to each other

min

public static <T> T min(Collection<? extends T> collection,
                        Comparator<? super T> comparator)

Searches the specified Collection for the minimum element using the specified Comparator.

Parameters:

collection - the Collection to search

comparator - the Comparator

Returns:

the minimum element in the Collection

Throws:

ClassCastException - when elements in the Collection cannot be compared to each other using the Comparator

nCopies

public static <T> List<T> nCopies(int length,
                                  T object)

Returns a List containing the specified number of the specified element. The list cannot be modified.

Parameters:

length - the size of the returned List

object - the element

Returns:

a List containing length copies of the element

Throws:

IllegalArgumentException - when length < 0

reverse

public static void reverse(List<?> list)

Returns the supplied List with the order of its contained elements reversed.

Parameters:

list - the List to reverse

Throws:

UnsupportedOperationException - when replacing an element in the List is not supported

reverseOrder

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

A Comparator which reverses the natural order of the elements. The Comparator that's returned is Serializable.

Returns:

A Comparator instance.

See Also:

Comparator, Comparable, Serializable

reverseOrder

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

Returns a Comparator that reverses the order of the Comparator passed. If the Comparator passed is null, then this method is equivalent to reverseOrder().

The Comparator that's returned is Serializable if the Comparator passed is serializable or null.

Parameters:

c - The Comparator to reverse or null.

Returns:

A Comparator instance.

Since:

1.5

See Also:

Comparator

shuffle

public static void shuffle(List<?> list)

Moves every element of the List to a random new position in the list.

Parameters:

list - the List to shuffle

Throws:

UnsupportedOperationException - when replacing an element in the List is not supported

shuffle

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

Moves every element of the List to a random new position in the list using the specified random number generator.

Parameters:

list - the List to shuffle

random - the random number generator

Throws:

UnsupportedOperationException - when replacing an element in the List is not supported

singleton

public static <E> Set<E> singleton(E object)

Returns a Set containing the specified element. The set cannot be modified.

Parameters:

object - the element

Returns:

a Set containing the element

singletonList

public static <E> List<E> singletonList(E object)

Returns a List containing the specified element. The list cannot be modified.

Parameters:

object - the element

Returns:

a List containing the element

singletonMap

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

Returns a Map containing the specified key and value. The map cannot be modified.

Parameters:

key - the key

value - the value

Returns:

a Map containing the key and value

sort

public static <T extends Comparable<? super T>> void sort(List<T> list)

Sorts the specified List in ascending order.

Parameters:

list - the List to be sorted

Throws:

ClassCastException - when an element in the List does not implement Comparable or elements cannot be compared to each other

sort

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

Sorts the specified List using the specified Comparator.

Parameters:

list - the List to be sorted

comparator - the Comparator

Throws:

ClassCastException - when elements in the List cannot be compared to each other using the Comparator

swap

public static void swap(List<?> list,
                        int index1,
                        int index2)

Swaps the elements of List list at indices index1 and index2

Parameters:

list - the List to manipulate on

index1 - int position of the first element to swap with the element in index2

index2 - int position of the other element

Throws:

IndexOutOfBoundsException - if index1 or index2 is out of range of this list

replaceAll

public static <T> boolean replaceAll(List<T> list,
                                     T obj,
                                     T obj2)

Replaces all occurrences of Object obj in list with newObj. If the obj is null, then all occurrences of null is replaced with newObj.

Parameters:

list - the List to modify

obj - the Object to find and replace occurrences of.

obj2 - the Object to replace all occurrences of obj in list

Returns:

true, if at least one occurrence of obj has been found in list

Throws:

UnsupportedOperationException - if the list does not support setting elements

rotate

public static void rotate(List<?> lst,
                          int dist)

Rotates the elements in List list by the distance dist

e.g. for a given list with elements [1, 2, 3, 4, 5, 6, 7, 8, 9, 0], calling rotate(list, 3) or rotate(list, -7) would modify the list to look like this: [8, 9, 0, 1, 2, 3, 4, 5, 6, 7]

Parameters:

lst -

dist - It can be any integer: 0, positive, negative, larger than the list size

indexOfSubList

public static int indexOfSubList(List<?> list,
                                 List<?> sublist)

Searches the list for sublist and returns the beginning index of the first occurrence.

-1 is returned if the sublist does not exist in list

Parameters:

list - the List to search sublist in

sublist - the List to search in list

Returns:

the beginning index of the first occurrence of sublist in list, or -1

lastIndexOfSubList

public static int lastIndexOfSubList(List<?> list,
                                     List<?> sublist)

Searches the list for sublist and returns the beginning index of the last occurrence.

-1 is returned if the sublist does not exist in list

Parameters:

list - the List to search sublist in

sublist - the List to search in list

Returns:

the beginning index of the last occurrence of sublist in list, or -1

list

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

Returns an ArrayList with all the elements in the enumeration. The elements in the returned ArrayList are in the same order as in the enumeration.

Parameters:

enumeration - Enumeration

Returns:

and ArrayList

synchronizedCollection

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

Returns a wrapper on the specified Collection which synchronizes all access to the Collection.

Parameters:

collection - the Collection

Returns:

a synchronized Collection

synchronizedList

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

Returns a wrapper on the specified List which synchronizes all access to the List.

Parameters:

list - the List

Returns:

a synchronized List

synchronizedMap

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

Returns a wrapper on the specified Map which synchronizes all access to the Map.

Parameters:

map - the Map

Returns:

a synchronized Map

synchronizedSet

public static <E> Set<E> synchronizedSet(Set<E> set)

Returns a wrapper on the specified Set which synchronizes all access to the Set.

Parameters:

set - the Set

Returns:

a synchronized Set

synchronizedSortedMap

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

Returns a wrapper on the specified SortedMap which synchronizes all access to the SortedMap.

Parameters:

map - the SortedMap

Returns:

a synchronized SortedMap

synchronizedSortedSet

public static <E> SortedSet<E> synchronizedSortedSet(SortedSet<E> set)

Returns a wrapper on the specified SortedSet which synchronizes all access to the SortedSet.

Parameters:

set - the SortedSet

Returns:

a synchronized SortedSet

unmodifiableCollection

public static <E> Collection<E> unmodifiableCollection(Collection<? extends E> collection)

Returns a wrapper on the specified Collection which throws an UnsupportedOperationException whenever an attempt is made to modify the Collection.

Parameters:

collection - the Collection

Returns:

an unmodifiable Collection

unmodifiableList

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

Returns a wrapper on the specified List which throws an UnsupportedOperationException whenever an attempt is made to modify the List.

Parameters:

list - the List

Returns:

an unmodifiable List

unmodifiableMap

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

Returns a wrapper on the specified Map which throws an UnsupportedOperationException whenever an attempt is made to modify the Map.

Parameters:

map - the Map

Returns:

a unmodifiable Map

unmodifiableSet

public static <E> Set<E> unmodifiableSet(Set<? extends E> set)

Returns a wrapper on the specified Set which throws an UnsupportedOperationException whenever an attempt is made to modify the Set.

Parameters:

set - the Set

Returns:

a unmodifiable Set

unmodifiableSortedMap

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

Returns a wrapper on the specified SortedMap which throws an UnsupportedOperationException whenever an attempt is made to modify the SortedMap.

Parameters:

map - the SortedMap

Returns:

a unmodifiable SortedMap

unmodifiableSortedSet

public static <E> SortedSet<E> unmodifiableSortedSet(SortedSet<E> set)

Returns a wrapper on the specified SortedSet which throws an UnsupportedOperationException whenever an attempt is made to modify the SortedSet.

Parameters:

set - the SortedSet

Returns:

a unmodifiable SortedSet

frequency

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

Returns the number of elements in the Collection that match the Object passed. If the Object is null, then the number of null elements is returned.

Parameters:

c - The Collection to search.

o - The Object to search for.

Returns:

The number of matching elements.

Throws:

NullPointerException - if the Collection parameter is null.

Since:

1.5

emptyList

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

Returns a type-safe empty, immutable List.

Returns:

An empty List.

Since:

1.5

See Also:

EMPTY_LIST

emptySet

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

Returns a type-safe empty, immutable Set.

Returns:

An empty Set.

Since:

1.5

See Also:

EMPTY_SET

emptyMap

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

Returns a type-safe empty, immutable Map.

Returns:

An empty Map.

Since:

1.5

See Also:

EMPTY_MAP

checkedCollection

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

Returns a dynamically typesafe view of the specified collection.

Parameters:

c - the collection

type - the type of the elements permitted to insert

Returns:

a typesafe collection

checkedMap

public 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.

Parameters:

m - the map

keyType - the type of the keys permitted to insert

valueType - the type of the values permitted to insert

Returns:

a typesafe map

checkedList

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

Returns a dynamically typesafe view of the specified list.

Parameters:

list - the list

type - the type of the elements permitted to insert

Returns:

a typesafe list

checkedSet

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

Returns a dynamically typesafe view of the specified set.

Parameters:

s - the set

type - the type of the elements permitted to insert

Returns:

a typesafe set

checkedSortedMap

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

Returns a dynamically typesafe view of the specified sorted map.

Parameters:

m - the sorted map

keyType - the type of the keys permitted to insert

valueType - the type of the values permitted to insert

Returns:

a typesafe sorted map

checkedSortedSet

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

Returns a dynamically typesafe view of the specified sorted set.

Parameters:

s - the sorted set

type - the type of the elements permitted to insert

Returns:

a typesafe sorted set

addAll

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

Adds all the specified elements to the specified collection

Parameters:

c - the collection the elements are to be inserted into

a - the elements to insert

Returns:

true if the collection changed during insertion

Throws:

UnsupportedOperationException - when the method is not supported

NullPointerException - when c or elements is null, or elements contains one or more null elements and c doesn't support null elements

disjoint

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

Returns true if the collections have no elements in common

Parameters:

c1 - the first collection

c2 - the second collection

Returns:

true if the collections have no elements in common

Throws:

NullPointerException - if one of the collections is null

checkType

static <E> E checkType(E obj,
                       Class<E> type)

Checks if specified object is instance of specified class. Used for a dynamically typesafe view of the collections.

Parameters:

obj - - object is to be checked

type - - class of object that should be

Returns:

specified object