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 An empty immutable instance of List.
Map EMPTY_MAP An empty immutable instance of Map.
Set EMPTY_SET An empty immutable instance of 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 whether the specified 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)
Modifies the specified List by reversing the order of the elements.
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 natural 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

An empty immutable instance of List.

public static final Map EMPTY_MAP

An empty immutable instance of Map.

public static final Set EMPTY_SET

An empty immutable instance of 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 a is null, or a contains one or more null elements and c doesn't support null elements.
IllegalArgumentException if at least one of the elements can't be inserted into the collection.

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. The List needs to be already sorted in natural sorting order. Searching in an unsorted array has an undefined result. It's also undefined which element is found if there are multiple occurrences of the same element.

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 if 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. The List needs to be already sorted according to the comparator passed. Searching in an unsorted array has an undefined result. It's also undefined which element is found if there are multiple occurrences of the same element.

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. Trying to insert an element of the wrong type into this collection throws a ClassCastException. At creation time the types in c are not checked for correct type.

Parameters
c the collection to be wrapped in a typesafe 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. Trying to insert an element of the wrong type into this list throws a ClassCastException. At creation time the types in list are not checked for correct type.

Parameters
list the list to be wrapped in a typesafe 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. Trying to insert an element of the wrong type into this map throws a ClassCastException. At creation time the types in m are not checked for correct type.

Parameters
m the map to be wrapped in a typesafe 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. Trying to insert an element of the wrong type into this set throws a ClassCastException. At creation time the types in s are not checked for correct type.

Parameters
s the set to be wrapped in a typesafe 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. Trying to insert an element of the wrong type into this sorted map throws a ClassCastException. At creation time the types in m are not checked for correct type.

Parameters
m the sorted map to be wrapped in a typesafe 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. Trying to insert an element of the wrong type into this sorted set throws a ClassCastException. At creation time the types in s are not checked for correct type.

Parameters
s the sorted set to be wrapped in a typesafe 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. At the end both lists will have the same objects at the same index. If the destination array is larger than the source list, the elements in the destination list with index >= source.size() will be unchanged.

Parameters
destination the list whose elements are set from the source list.
source the list with the elements to be copied into the destination.
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 whether the specified 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, false otherwise.
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 element to fill the list with.
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 the source Enumeration.
Returns
  • an ArrayList from enumeration.

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. The list is serializable.

Parameters
length the size of the returned list.
object the element to be added length times to a list.
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 are 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)

Modifies the specified List by reversing the order of the elements.

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
lst the list whose elements are to be rotated.
dist is the distance the list is rotated. This can be any valid integer. Negative values rotate the list backwards.

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. The set is serializable.

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. The list is serializable.

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. The map is serializable.

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. The algorithm is stable which means equal elements don't get reordered.

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 natural order. The algorithm is stable which means equal elements don't get reordered.

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.
index1 position of the first element to swap with the element in index2.
index2 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 to wrap in a synchronized 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 to wrap in a synchronized 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 to wrap in a synchronized 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 to wrap in a synchronized 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 to wrap in a synchronized sorted map.
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 to wrap in a synchronized sorted set.
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 to wrap in an unmodifiable 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 to wrap in an unmodifiable 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 to wrap in an unmodifiable 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 to wrap in an unmodifiable 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 to wrap in an unmodifiable sorted map.
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 to wrap in an unmodifiable sorted set.
Returns
  • a unmodifiable SortedSet.