java.util.Collections class inherits Object class. The Collections utility class is used exclusively with static methods that operate on or return collections.
Points to remember
- Collections class supports the polymorphic to operate on collections.
- Collections class throws a NullPointerException if passing objects are null.
See Also:
- Collection Framework Hierarchy
- Java : Sort Date Time Object and Text Format
- Java : Sort Date Object and Text Format
- Java :Sort object by Comparator interface in ascending and descending order
- Java: Sort ArrayList in Ascending or Descending Order or Natural or Chronological Order
- Java: Sort By Comparable Interface in Ascending and Descending Order
- Java : Comparable Vs Comparator
Collections Declaration
public class Collections extends Object
Collections Utility Class Methods
| Method | Descriptions |
| static boolean addAll() | It is used to adds all of the specified element to the specified collection. |
| static Queue asLifoQueue() | It returns a view of a Deque as a Last-in-first-out (LIFO) Queue. |
| static int binarySearch() | It searches the list for the specified object and returns its position in a sorted list. |
| static Collection checkedCollection() | It is used to returns a dynamically typesafe view of the specified collection. |
| static List checkedList() | It is used to returns a dynamically typesafe view of the specified list. |
| static Map checkedMap() | It is used to returns a dynamically typesafe view of the specified map. |
| static NavigableMap checkedNavigableMap() | It is used to returns a dynamically typesafe view of the specified navigable map. |
| static NavigableSet checkedNavigableSet() | It is used to returns a dynamically typesafe view of the specified navigable set. |
| static Queue checkedQueue() | It is used to returns a dynamically typesafe view of the specified queue. |
| static Set checkedSet() | It is used to returns a dynamically typesafe view of the specified set. |
| static SortedMap checkedSortedMap() | It is used to returns a dynamically typesafe view of the specified sorted map. |
| static SortedSet checkedSortedSet() | It is used to returns a dynamically typesafe view of the specified sorted set. |
| static void copy() | It is used to copy all the elements from one list into another list. |
| static boolean disjoint() | It returns true if the two specified collections have no elements in common. |
| static Enumeration emptyEnumeration() | It is used to get an enumeration that has no elements. |
| static Iterator emptyIterator() | It is used to get an Iterator that has no elements. |
| static List emptyList() It is used to get a List that has no elements. | |
| static ListIterator emptyListIterator() | It is used to get a List Iterator that has no elements. |
| static Map emptyMap() | It returns an empty map that is immutable. |
| static NavigableMap emptyNavigableMap() | It returns an empty navigable map that is immutable. |
| static NavigableSet emptyNavigableSet() | It is used to get an empty navigable set which is immutable in nature. |
| static Set emptySet() | It is used to get the set that has no elements. |
| static SortedMap emptySortedMap() | It returns an empty sorted map which is immutable. |
| static SortedSet emptySortedSet() | It is used to get the sorted set that has no elements. |
| static Enumeration enumeration() | It is used to get the enumeration over the specified collection. |
| static void fill() | It is used to replace all of the elements of the specified list with the specified elements. |
| static int frequency() | It is used to get the number of elements in the specified collection equal to the specified object. |
| static int indexOfSubList() | It is used to get the starting position of the first occurrence of the specified target list within the specified source list. It returns -1 if there is no such occurrence in the specified list. |
| static int lastIndexOfSubList() | It is used to get the starting position of the last occurrence of the specified target list within the specified source list. It returns -1 if there is no such occurrence in the specified list. |
| static ArrayList list() | It is used to 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() | It is used to get the maximum value of the given collection, according to the natural ordering of its elements. |
| static > T min() | It is used to get the minimum value of the given collection, according to the natural ordering of its elements. |
| static List nCopies() | It is used to get an immutable list consisting of n copies of the specified object. |
| static Set newSetFromMap() | It is used to return a set backed by the specified map. |
| static boolean replaceAll() | It is used to replace all occurrences of one specified value in a list with the other specified value. |
| static void reverse() | It is used to reverse the order of the elements in the specified list. |
| static Comparator reverseOrder() | It is used to get the comparator that imposes the reverse of the natural ordering on a collection of objects which internally implement the Comparable interface. |
| static void rotate() | It is used to rotate the elements in the specified list by a given distance. |
| static void shuffle() | It is used to randomly reorders the specified list elements using default randomness. |
| static Set singleton() | It is used to get an immutable set that contains only the specified object. |
| static List singletonList() | It is used to get an immutable list that contains only the specified object. |
| static Map singletonMap() | Use to get an immutable map, mapping only the specified key to the specified value. |
| static >void sort() | It is used to sort the elements present in the specified list of the collection in ascending order. |
| static void swap() | It is used to swap the elements at the specified positions in the specified list. |
| static Collection synchronizedCollection() | It is used to get a synchronized (thread-safe) collection backed by the specified collection. |
| static List synchronizedList() | It is used to get a synchronized (thread-safe) collection backed by the specified list. |
| static Map synchronizedMap() | It is used to get a synchronized (thread-safe) map backed by the specified map. |
| static NavigableMap synchronizedNavigableMap() | It is used to get a synchronized (thread-safe) navigable map backed by the specified navigable map. |
| static NavigableSet synchronizedNavigableSet() | It is used to get a synchronized (thread-safe) navigable set backed by the specified navigable set. |
| static Set synchronizedSet() | It is used to get a synchronized (thread-safe) set backed by the specified set. |
| static SortedMap synchronizedSortedMap() | It is used to get a synchronized (thread-safe) sorted map backed by the specified sorted map. |
| static SortedSet synchronizedSortedSet() | It is used to get a synchronized (thread-safe) sorted set backed by the specified sorted set. |
| static Collection unmodifiableCollection() | It is used to get an unmodifiable view of the specified collection. |
| static List unmodifiableList() | It is used to get an unmodifiable view of the specified list. |
| static Map unmodifiableMap() | It is used to get an unmodifiable view of the specified map. |
| static NavigableMap unmodifiableNavigableMap() | It is used to get an unmodifiable view of the specified navigable map. |
| static NavigableSet unmodifiableNavigableSet() | It is used to get an unmodifiable view of the specified navigable set. |
| static Set unmodifiableSet() | It is used to get an unmodifiable view of the specified set. |
| static SortedMap unmodifiableSortedMap() | It is used to get an unmodifiable view of the specified sorted map. |
| static SortedSet unmodifiableSortedSet() | It is used to get an unmodifiable view of the specified sorted set. |
See Also:
- Java: Unmodifiable Collection Methods and Example
- Java: Synchronized Collection Methods and Examples
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 list = new ArrayList();
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 list = new ArrayList();
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 list = new ArrayList();
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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