Table of Contents

Java Collections Class

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Introduction

The Java 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 <E> Collection<E> checkedCollection(Collection<E> c, Class<E> type) This method returns a dynamically typesafe view of the specified collection.
5	static <E> List<E> checkedList(List<E> list, Class<E> type) This method returns a dynamically typesafe view of the specified list.
6	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.
7	static <K,V> Navigable<K,V> checkedNavigableMap(Navigable<K,V> m, Class<K> keyType, Class<V> valueType) This method returns a dynamically typesafe view of the specified navigable map.
8	static <E> NavigableSet<E> checkedNavigableSet(NavigableSet<E> s, Class<E> type) This method returns a dynamically typesafe view of the specified navigable set.
9	static <E> Queue<E> checkedQueue(Queue<E> s, Class<E> type) This method returns a dynamically typesafe view of the specified queue.
10	static <E> Set<E> checkedSet(Set<E> s, Class<E> type) This method returns a dynamically typesafe view of the specified set.
11	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.
12	static <E> SortedSet<E>checkedSortedSet(SortedSet<E> s, Class<E> type) This method returns a dynamically typesafe view of the specified sorted set.
13	static <T> void copy(List<? super T> dest, List<? extends T> src) This method copies all of the elements from one list into another.
14	static boolean disjoint(Collection<?> c1, Collection<?> c2) This method returns true if the two specified collections have no elements in common.
15	static <T> Enumeration <T> emptyEnumeration() This method returns the empty enumeration.
16	static <T> Iterator <T> emptyIterator() This method returns the empty iterator.
17	static <T> List<T> emptyList() This method returns the empty list (immutable).
18	static <T> ListIterator <T> emptyListIterator() This method returns the empty list iterator.
19	static <K,V> Map<K,V> emptyMap() This method returns the empty map (immutable).
20	static <K,V> NavigableMap<K,V> emptyNavigableMap() This method returns the empty navigable map (immutable).
21	static <T> NavigableSet<T> emptyNavigableSet() This method returns the empty navigable set (immutable).
22	static <T> Set<T> emptySet() This method returns the empty set (immutable).
23	static <K,V> SortedMap<K,V> emptySortedMap() This method returns the empty sorted map (immutable).
24	static <T> SortedSet<T> emptySortedSet() This method returns the empty sorted set (immutable).
25	static <T> Enumeration<T> enumeration(Collection<T> c) This method returns an enumeration over the specified collection.
26	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.
27	static int frequency(Collection<?> c, Object o) This method returns the number of elements in the specified collection equal to the specified object.
28	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.
29	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.
30	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.
31	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.
32	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.
33	static <T> List<T> nCopies(int n, T o) This method returns an immutable list consisting of n copies of the specified object.
34	static <E> Set<E> newSetFromMap(Map<E,Boolean> map) This method returns a set backed by the specified map.
35	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.
36	static void reverse(List<?> list) This method reverses the order of the elements in the specified list.
37	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.
38	static void rotate(List<?> list, int distance) This method rotates the elements in the specified list by the specified distance.
39	static void shuffle(List<?> list) This method randomly permutes the specified list using a default source of randomness.
40	static <T> Set<T> singleton(T o) This method returns an immutable set containing only the specified object.
41	static <T> List<T> singletonList(T o) This method returns an immutable list containing only the specified object.
42	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.
43	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.
44	static void swap(List<?> list, int i, int j) This method swaps the elements at the specified positions in the specified list.
45	static <T> Collection<T> synchronizedCollection(Collection<T> c) This method returns a synchronized (thread-safe) collection backed by the specified collection.
46	static <T> List<T> synchronizedList(List<T> list) This method returns a synchronized (thread-safe) list backed by the specified list.
47	static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) This method returns a synchronized (thread-safe) map backed by the specified map.
48	static <K,V> NavigableMap<K,V> synchronizedNavigableMap() This method returns the synchronized navigable map (immutable).
49	static <T> NavigableSet<T> synchronizedNavigableSet() This method returns the synchronized navigable set (immutable).
50	static <T> Set<T> synchronizedSet(Set<T> s) This method returns a synchronized (thread-safe) set backed by the specified set.
51	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.
52	static <T> SortedSet<T> synchronizedSortedSet(SortedSet<T> s) This method returns a synchronized (thread-safe) sorted set backed by the specified sorted set.
53	static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c) This method returns an unmodifiable view of the specified collection.
54	static <T> List<T> unmodifiableList(List<? extends T> list) This method returns an unmodifiable view of the specified list.
55	static <K,V> Map<K,V> unmodifiableMap(Map<? extends K,? extends V> m) This method returns an unmodifiable view of the specified map.
56	static <K,V> NavigableMap<K,V> unmodifiableNavigableMap() This method returns the unmodifiable navigable map (immutable).
57	static <T> NavigableSet<T> unmodifiableNavigableSet() This method returns the unmodifiable navigable set (immutable).
58	static <T> Set<T> unmodifiableSet(Set<? extends T> s) This method returns an unmodifiable view of the specified set.
59	static <K,V> SortedMap<K,V> unmodifiableSortedMap(SortedMap<K,? extends V> m) This method returns an unmodifiable view of the specified sorted map.
60	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

Adding Multiple Elements to the Collection of Integers Example

The following example shows the usage of Java Collection addAll(Collection,T… ) method to add a collection of Integers. We”ve created a List object with some integers, printed the original list. Using addAll(collection, T…) method, we”ve added few more elements to the list and then printed the updated list.

package com.tutorialspoint;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class CollectionsDemo {

   public static void main(String[] args) {
      List<Integer> list = new ArrayList<>(Arrays.asList(1,2,3,4,5));

      System.out.println("Initial collection value: " + list);
      // add values to this collection
      Collections.addAll(list, 6, 7, 8);
      System.out.println("Final collection value: "+list);
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Initial collection value: [1, 2, 3, 4, 5]
Final collection value: [1, 2, 3, 4, 5, 6, 7, 8]

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Java – Collection