Java.util.Collections Class


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Introduction

The java.util.Collections class consists exclusively of static methods that operate on or return collections.Following are the important points about Collections:

  • It contains polymorphic algorithms that operate on collections, "wrappers", which return a new collection backed by a specified collection.

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

Class declaration

Following is the declaration for java.util.Collections class:

public class Collections
   extends Object

Field

Following are the fields for java.util.Collections class:

  • static List EMPTY_LIST -- This is the empty list (immutable).

  • static Map EMPTY_MAP -- This is the empty map (immutable).

  • static Set EMPTY_SET -- This is the empty set (immutable).

Class methods

S.N. Method & Description
1

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

This method adds all of the specified elements to the specified collection.

2

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

This method returns a view of a Deque as a Last-in-first-out (Lifo) Queue.

3

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

This method searches the specified list for the specified object using the binary search algorithm.

4

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

This method searches the specified list for the specified object using the binary search algorithm.

5

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

This method returns a dynamically typesafe view of the specified collection.

6

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

This method returns a dynamically typesafe view of the specified list.

7

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

This method returns a dynamically typesafe view of the specified map.

8

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

This method returns a dynamically typesafe view of the specified set.

9

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

This method returns a dynamically typesafe view of the specified sorted map.

10

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

This method returns a dynamically typesafe view of the specified sorted set.

11

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

This method copies all of the elements from one list into another.

12

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

This method returns true if the two specified collections have no elements in common.

13

static <T> List<T> emptyList()

This method returns the empty list (immutable).

14

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

This method returns the empty map (immutable).

15

static <T> Set<T> emptySet()

This method returns the empty set (immutable).

16

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

This method returns an enumeration over the specified collection.

17

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

This method replaces all of the elements of the specified list with the specified element.

18

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

This method returns the number of elements in the specified collection equal to the specified object.

19

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

This method 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.

20

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

This method 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.

21

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

This method returns an array list containing the elements returned by the specified enumeration in the order they are returned by the enumeration.

22

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

This method returns the maximum element of the given collection, according to the natural ordering of its elements.

23

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

This method returns the maximum element of the given collection, according to the order induced by the specified comparator.

24

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

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

25

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

This method returns the minimum element of the given collection, according to the order induced by the specified comparator.

26

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

This method returns an immutable list consisting of n copies of the specified object.

27

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

This method returns a set backed by the specified map.

28

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

This method replaces all occurrences of one specified value in a list with another.

29

static void reverse(List<?> list)

This method reverses the order of the elements in the specified list.

30

static <T> Comparator<T> reverseOrder()

This method returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface.

31

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

This method returns a comparator that imposes the reverse ordering of the specified comparator.

32

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

This method rotates the elements in the specified list by the specified distance.

33

static void shuffle(List<?> list)

This method randomly permutes the specified list using a default source of randomness.

34

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

This method randomly permute the specified list using the specified source of randomness.

35

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

This method returns an immutable set containing only the specified object.

36

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

This method returns an immutable list containing only the specified object.

37

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

This method returns an immutable map, mapping only the specified key to the specified value.

38

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

This method sorts the specified list into ascending order, according to the natural ordering of its elements.

39

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

This method sorts the specified list according to the order induced by the specified comparator.

40

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

This method swaps the elements at the specified positions in the specified list.

41

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

This method returns a synchronized (thread-safe) collection backed by the specified collection.

42

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

This method returns a synchronized (thread-safe) list backed by the specified list.

43

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

This method returns a synchronized (thread-safe) map backed by the specified map.

44

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

This method returns a synchronized (thread-safe) set backed by the specified set.

45

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

This method returns a synchronized (thread-safe) sorted map backed by the specified sorted map.

46

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

This method returns a synchronized (thread-safe) sorted set backed by the specified sorted set.

47

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

This method returns an unmodifiable view of the specified collection.

48

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

This method returns an unmodifiable view of the specified list.

49

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

This method returns an unmodifiable view of the specified map.

50

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

This method returns an unmodifiable view of the specified set.

51

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

This method returns an unmodifiable view of the specified sorted map.

52

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

This method returns an unmodifiable view of the specified sorted set.

Methods inherited

This class inherits methods from the following classes:

  • java.util.Object


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