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

Sr.No. 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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