The Java Collections Framework includes the Collections class in the Java.util.Collections package.  The static methods that work with collections or return collections are essentially utilized with the collections class. If the collection or object supplied to the methods is null, all the methods of this class raise the NullPointerException.

Syntax: 

public class Collections
extends Object

Collections class fields:

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:

The following are the methods that have been listed below in a tabular format:

Sr.No.	Method & Description
1	static <T> boolean addAll(Collection<? super T> c, T… elements)The provided collection receives all of the elements added by this method.
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 technique use the binary search algorithm to search the supplied list for the desired object.
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)The result of this method is a dynamically typesafe view of the given list.
7	static <K,V> Map<K,V> checkedMap(Map<K,V> m, Class<K> keyType, Class<V> valueType)A dynamically typesafe view of the supplied map is returned by this method.
8	static <E> Set<E> checkedSet(Set<E> s, Class<E> type)A dynamically typesafe view of the given set is the result of this method.
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 the enumeration returns them.
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)According to the order created by the designated comparator, this function returns the minimal member of the supplied collection.
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 technique swaps out every instance of one specified value in a list for another.
29	static void reverse(List<?> list)TThe elements in the given list are rearranged by this method.
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 turns 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 permutes 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 set 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.

Example: Sorting a Collection

java.util.Collections.sort() is used to sort the elements present in the specified list of Collections in ascending order. Sorting in reverse order with java.util.Collections.reverseOrder() produces the desired results.

// Java program to demonstrate sorting
// a Collections using sort() method

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

// Main Class
// SortingCollectionExample
class Coderz{

	// Main driver method
	public static void main(String[] args)
	{
		// Creating a list
		// Declaring object of string type
		List<String> items = new ArrayList<>();

		// Adding elements to the list
		// using add() method
		items.add("Cake");
		items.add("Stick");

		// Adding one or more elements using addAll()
		Collections.addAll(items, "Dog", "Cat", "Mouse");

		// Sorting according to default ordering
		// using sort() method
		Collections.sort(items);

		// Printing the elements
		for (int i = 0; i < items.size(); i++) {
			System.out.print(items.get(i) + " ");
		}

		System.out.println();

		// Sorting according to reverse ordering
		Collections.sort(items, Collections.reverseOrder());

		// Printing the reverse order
		for (int i = 0; i < items.size(); i++) {
			System.out.print(items.get(i) + " ");
		}
	}
}

Output:

Cake Cat Dog Mouse Stick 
Stick Mouse Dog Cat Cake 

Note: also read about the Interfaces of Java Collection framework

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