Collections.sort() in Java with Examples

java.util.Collections.sort() method is present in java.util.Collections class. It is used to sort the elements present in the specified list of Collection in ascending order. It works similar to java.util.Arrays.sort() method but it is better than as it can sort the elements of Array as well as linked list, queue and many more present in it.

public static void sort(List myList)

myList : A List type object we want to sort.

This method doesn't return anything

Example:

Let us suppose that our list contains
{"Geeks For Geeks", "Friends", "Dear", "Is", "Superb"}

After using Collection.sort(), we obtain a sorted list as
{"Dear", "Friends", "Geeks For Geeks", "Is", "Superb"}

Sorting an ArrayList in ascending order

JAVA

// Java program to demonstrate working of Collections.sort()
import java.util.*;

public class Collectionsorting
{
    public static void main(String[] args)
    {
        // Create a list of strings
        ArrayList<String> al = new ArrayList<String>();
        al.add("Geeks For Geeks");
        al.add("Friends");
        al.add("Dear");
        al.add("Is");
        al.add("Superb");

        /* Collections.sort method is sorting the
        elements of ArrayList in ascending order. */
        Collections.sort(al);

        // Let us print the sorted list
        System.out.println("List after the use of" +
                        " Collection.sort() :\n" + al);
    }
}

Output

List after the use of Collection.sort() :
[Dear, Friends, Geeks For Geeks, Is, Superb]

Time Complexity: O(N log N) as time complexity of Collections.sort() is O(nlog(n)).
Auxiliary Space: O(1)  

Sorting an ArrayList in descending order

JAVA

// Java program to demonstrate working of Collections.sort()
// to descending order.
import java.util.*;

public class Collectionsorting
{
    public static void main(String[] args)
    {
        // Create a list of strings
        ArrayList<String> al = new ArrayList<String>();
        al.add("Geeks For Geeks");
        al.add("Friends");
        al.add("Dear");
        al.add("Is");
        al.add("Superb");

        /* Collections.sort method is sorting the
        elements of ArrayList in ascending order. */
        Collections.sort(al, Collections.reverseOrder());

        // Let us print the sorted list
        System.out.println("List after the use of" +
                        " Collection.sort() :\n" + al);
    }
}

Output

List after the use of Collection.sort() :
[Superb, Is, Geeks For Geeks, Friends, Dear]

Time Complexity: O(N log N) as time complexity of Collections.sort() is O(nlog(n)).
Auxiliary Space: O(1)  

Sorting an ArrayList according to user defined criteria. We can use Comparator Interface for this purpose. 

Java

// Java program to demonstrate working of Comparator
// interface and Collections.sort() to sort according
// to user defined criteria.
import java.util.*;
import java.lang.*;
import java.io.*;

// A class to represent a student.
class Student
{
    int rollno;
    String name, address;

    // Constructor
    public Student(int rollno, String name,
                            String address)
    {
        this.rollno = rollno;
        this.name = name;
        this.address = address;
    }

    // Used to print student details in main()
    public String toString()
    {
        return this.rollno + " " + this.name +
                        " " + this.address;
    }
}

class Sortbyroll implements Comparator<Student>
{
    // Used for sorting in ascending order of
    // roll number
    public int compare(Student a, Student b)
    {
        return a.rollno - b.rollno;
    }
}

// Driver class
class Main
{
    public static void main (String[] args)
    {
        ArrayList<Student> ar = new ArrayList<Student>();
        ar.add(new Student(111, "bbbb", "london"));
        ar.add(new Student(131, "aaaa", "nyc"));
        ar.add(new Student(121, "cccc", "jaipur"));

        System.out.println("Unsorted");
        for (int i=0; i<ar.size(); i++)
            System.out.println(ar.get(i));

        Collections.sort(ar, new Sortbyroll());

        System.out.println("\nSorted by rollno");
        for (int i=0; i<ar.size(); i++)
            System.out.println(ar.get(i));
    }
}

Output

Unsorted
111 bbbb london
131 aaaa nyc
121 cccc jaipur

Sorted by rollno
111 bbbb london
121 cccc jaipur
131 aaaa nyc

Arrays.sort() vs Collections.sort() Arrays.sort works for arrays which can be of primitive data type also. Collections.sort() works for objects Collections like ArrayList, LinkedList, etc. We can use Collections.sort() to sort an array after creating an ArrayList of given array items.

JAVA

// Using Collections.sort() to sort an array
import java.util.*;
public class Collectionsort
{
    public static void main(String[] args)
    {
        // create an array of string objs
        String domains[] = {"Practice", "Geeks",
                            "Code", "Quiz"};

        // Here we are making a list named as Collist
        List colList =
            new ArrayList(Arrays.asList(domains));

        // Collection.sort() method is used here
        // to sort the list elements.
        Collections.sort(colList);

        // Let us print the sorted list
        System.out.println("List after the use of" +
                        " Collection.sort() :\n" +
                        colList);
    }
}

Output

List after the use of Collection.sort() :
[Code, Geeks, Practice, Quiz]

Arrays.sort() vs Collections.sort() time complexity :

Arrays.sort() uses a Dual-Pivot Quicksort algorithm which gives a time complexity of O(N.log N) which is typically faster than traditional Quicksort algorithms. On the other hand, Collections.sort() creates an array of list elements, sorts them using an adaptive Mergesort algorithm, and iterates over the list to position each element at its correct location. Thus for primitive datatypes like int, char, double, etc. Arrays.sort() proves to be way more time efficient than Collections.sort(). Problems involving primitive datatypes should be tried to solve using Arrays.sort() for better optimisation.

Below is the code to demonstrate the difference:

Java

/*package whatever //do not write package name here */

import java.io.*;
import java.util.*;

class GFG {
    public static void main (String[] args) {
        int len = 5000000;

          // creating a large test array
        int[] arr = new int[len];
        for (int i = len; i > 0; i--)
            arr[len - i] = i;

          // creating a large test arraylist
        ArrayList<Integer> list = new ArrayList<>();
        for (int i = len; i > 0; i--)
            list.add(i);

          // calculating time used by arrays.sort()
        long startA = System.currentTimeMillis();
        Arrays.sort(arr);
        long stopA = System.currentTimeMillis();

          // calculating time used by collections.sort()
          long startAL = System.currentTimeMillis();
          Collections.sort(list);
        long stopAL = System.currentTimeMillis();

          System.out.println("Time taken by Arrays.sort(): " + (stopA - startA));
        System.out.println("Time taken by Collections.sort(): " + (stopAL - startAL));
    }
}

// This code is contributed by godcoder28

Output

Time taken by Arrays.sort(): 29
Time taken by Collections.sort(): 42

This article is contributed by Mohit Gupta. The article is wished to be useful to the esteemed Geeks. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. .