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public class

Collections

extends Object
java.lang.Object
   ↳ java.util.Collections

Class Overview

Collections contains static methods which operate on Collection classes.

Summary

Constants
List EMPTY_LIST
Map EMPTY_MAP
Set EMPTY_SET
Public Methods
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.
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
final static <T> List<T> emptyList()
Returns a type-safe empty, immutable List.
final static <K, V> Map<K, V> emptyMap()
Returns a type-safe empty, immutable Map.
final 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(Comparator<T> c)

Returns a Comparator that reverses the order of the Comparator passed.

static <T> Comparator<T> reverseOrder()

A Comparator which reverses the natural order of the elements.

static void rotate(List<?> lst, int dist)
Rotates the elements in List list by the distance dist

e.g.

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> void sort(List<T> list, Comparator<? super T> comparator)
Sorts the specified List using the specified Comparator.
static <T extends Comparable<? super T>> void sort(List<T> list)
Sorts the specified List in ascending order.
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.
[Expand]
Inherited Methods
From class java.lang.Object

Constants

public static final List EMPTY_LIST

public static final Map EMPTY_MAP

public static final Set EMPTY_SET

Public Methods

public static 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

public static 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

public static 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

public static 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

public static 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

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.

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

public static 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

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.

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

public static 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

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

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

Throws
IndexOutOfBoundsException when the destination List is smaller than the source List
UnsupportedOperationException when replacing an element in the destination list is not supported

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

public static final List<T> emptyList ()

Returns a type-safe empty, immutable List.

Returns
See Also

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

Returns a type-safe empty, immutable Map.

Returns
See Also

public static final Set<T> emptySet ()

Returns a type-safe empty, immutable Set.

Returns
See Also

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

Returns an Enumeration on the specified Collection.

Parameters
collection the Collection to enumerate
Returns
  • an Enumeration

public static 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

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.

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

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

public static 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

public static 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

public static 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

public static 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

public static 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

public static 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

public static 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

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

public static 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.
See Also

public static 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

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

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

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

public static 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

public static 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

public static 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

public static 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

public static 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

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

public static 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

public static 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

public static 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

public static 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

public static 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

public static 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

public static 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

public static 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

public static 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

public static 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

public static 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

public static 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