Java: Collections Class Methods and Examples


java.util.Collections class extends Object class. Collections also called as utility class is used exclusively with static methods that operate on or return collections.

Points to remember

  • Collection class supports the polymorphic to operate on collections.
  • Collection class throws a NullPointerException if passing objects are null.

Collections Declaration

public class Collections extends Object 

Collections Class Methods

Methods Descriptions
static boolean addAll() Adds all of the specified elements to the given collection.
static Queue asLifoQueue() Returns a view of a Deque as a LIFO(Last in first out) Queue.
static int binarySearch() Searches the list for the specified object and returns their index position in a sorted list.
static Collection checkedCollection() Returns a dynamically type safe view of the given collection.
static List checkedList() Returns a dynamically type safe view of the given list.
static Map checkedMap() Returns a dynamically type safe view of the given map.
static NavigableMap checkedNavigableMap() Returns a dynamically type safe view of the given navigable map.
static NavigableSet checkedNavigableSet() Returns a dynamically type safe view of the given navigable set.
static Queue checkedQueue() Returns a dynamically type safe view of the given queue.
static Set checkedSet() Returns a dynamically type safe view of the given set.
static SortedMap checkedSortedMap() Returns a dynamically type safe view of the given sorted map.
static SortedSet checkedSortedSet() Returns a dynamically type safe view of the given sorted set.
static void copy() Copy all the elements from one list into a another list.
static boolean disjoint() Returns true if the two specified collections have no common elements.
static Enumeration emptyEnumeration() Get an enumeration that has no elements.
static Iterator emptyIterator() Get an Iterator that has no elements.
static List emptyList() Get a List that has no elements.
static ListIterator emptyListIterator() Get a List Iterator that has no elements.
static Map emptyMap() Returns an empty map which is immutable.
static NavigableMap emptyNavigableMap() Returns an empty navigable map which is immutable.
static NavigableSet emptyNavigableSet() Get an empty navigable set which is immutable in nature.
static Set emptySet() Get the set that has no elements.
static SortedMap emptySortedMap() Returns an empty sorted map which is immutable.
static SortedSet emptySortedSet() Get the sorted set that has no elements.
static Enumeration enumeration() Get the enumeration over the specified collection.
static void fill() Replace all of the elements of the specified list with the specified elements.
static int frequency() Get the number of elements in the specified collection equal to the given object.
static int indexOfSubList() Get the starting index position of the first occurrence of the specified target list within the specified source list. It returns -1 if there is no elements found in the specified list.
static int lastIndexOfSubList() Get the starting index position of the last occurrence of the specified target list within the specified source list. It returns -1 if there is no elements found in the specified list.
static ArrayList list() Get an array list containing the elements returned by the specified enumeration in the order in which they are returned by the enumeration.
static > T max() Get the maximum value of the given collection, according to the natural ordering of its elements.
static > T min() Get the minimum value of the given collection, according to the natural ordering of its elements.
static List nCopies() Get an immutable list consisting of n copies of the specified object.
static Set newSetFromMap() Return a set backed by the specified map.
static boolean replaceAll() Replace all occurrences of one specified value in a list with the other specified value.
static void reverse() Reverse the order of the elements in the given list.
static Comparator reverseOrder() Get the comparator that imposes the reverse of the natural ordering of elements on a collection of objects which implement the Comparable interface.
static void rotate() Rotate the elements in the specified list by a given distance.
static void shuffle() Randomly reorders the specified list elements using a default randomness.
static Set singleton() Get an immutable set which contains only the specified object.
static List singletonList() Get an immutable list which contains only the specified object.
static Map singletonMap() Get an immutable map, mapping only the specified key to the specified value.
static >void sort() Sort the elements presents in the specified list of collection in ascending order.
static void swap() Swap the elements at the specified positions in the given list.
static Collection synchronizedCollection() Get a synchronized (thread-safe) collection backed by the given collection.
static List synchronizedList() Get a synchronized (thread-safe) collection backed by the given list.
static Map synchronizedMap() Get a synchronized (thread-safe) map backed by the given map.
static NavigableMap synchronizedNavigableMap() Get a synchronized (thread-safe) navigable map backed by the given navigable map.
static NavigableSet synchronizedNavigableSet() Get a synchronized (thread-safe) navigable set backed by the given navigable set.
static Set synchronizedSet() Get a synchronized (thread-safe) set backed by the given set.
static SortedMap synchronizedSortedMap() Get a synchronized (thread-safe) sorted map backed by the given sorted map.
static SortedSet synchronizedSortedSet() Get a synchronized (thread-safe) sorted set backed by the given sorted set.
static Collection unmodifiableCollection() Get an unmodifiable view of the given collection.
static List unmodifiableList() Get an unmodifiable view of the given list.
static Map unmodifiableMap() Get an unmodifiable view of the given map.
static NavigableMap unmodifiableNavigableMap() Get an unmodifiable view of the given navigable map.
static NavigableSet unmodifiableNavigableSet() Get an unmodifiable view of the given navigable set.
static Set unmodifiableSet() Get an unmodifiable view of the given set.
static SortedMap unmodifiableSortedMap() Get an unmodifiable view of the given sorted map.
static SortedSet unmodifiableSortedSet() Get an unmodifiable view of the given sorted set.

Collections Example : Add elements in list

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

public class CollectionsExample1 {

	public static void main(String[] args) {
		 List<String> list = new ArrayList<String>();
	        list.add("C");
	        list.add("C++");
	        list.add("Python");
	        list.add("Lisp");
	        list.add("Java Script");
	        System.out.println("Initial collection value:"+list);
	        //add some more element in collection
	        Collections.addAll(list, "Servlet","JSP");
	        System.out.println("After adding elements collection value:"+list);
	        String[] strArr = {"C#", "Closure",".Net"};
	        //add some more elements
	        Collections.addAll(list, strArr);
	        System.out.println("After adding array collection value:"+list);
	}

}

Output :


Initial collection value:[C, C++, Python, Lisp, Java Script]
After adding elements collection value:[C, C++, Python, Lisp, Java Script, Servlet, JSP]
After adding array collection value:[C, C++, Python, Lisp, Java Script, Servlet, JSP, C#, Closure, .Net]

Collections Example :max()

import java.util.*;

public class CollectionsExample2 {
	public static void main(String a[]) {
		List<Integer> list = new ArrayList<Integer>();
		list.add(15);
		list.add(50);
		list.add(3);
		list.add(90);
		list.add(2);
		list.add(16);
		System.out.println("Max element from the collection: " + Collections.max(list));
	}
}

Output :


Max element from the collection: 90

Output:

Collections Example :min()

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

public class CollectionsExample3 {
	public static void main(String a[]) {
		List<Integer>list = new ArrayList<Integer>();
		list.add(15);
		list.add(50);
		list.add(3);
		list.add(90);
		list.add(2);
		list.add(16);
		System.out.println("Min element from the collection: " + Collections.min(list));
	}
}

Output :


Min element from the collection: 2
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