Java ArrayList sort()

import java.util.ArrayList;
import java.util.Comparator;

class Main {
  public static void main(String[] args) {

    // create an ArrayList
    ArrayList<Integer> numbers = new ArrayList<>();
    numbers.add(7);
    numbers.add(3);
    numbers.add(9);
    numbers.add(-33);
    System.out.println("Unsorted ArrayList: " + numbers);

    // sort the ArrayList in ascending order
    numbers.sort(Comparator.naturalOrder());

    System.out.println("Sorted ArrayList: " + numbers);
  }
}

// Output: Unsorted ArrayList: [7, 3, 9, -33]
//         Sorted ArrayList: [-33, 3, 7, 9]

Syntax of ArrayList sort()

The syntax of the sort() method is:

arraylist.sort(Comparator c)

Here, arraylist is an object of the ArrayList class.

sort() Parameters

The sort() method takes a single parameter.

• comparator - specifies the sort order of the arraylist

sort() Return Values

The sort() method does not return any value. Rather it only changes the order of elements in an arraylist.

Example 1: Sort the ArrayList in Natural Order

import java.util.ArrayList;
import java.util.Comparator;

class Main {
  public static void main(String[] args) {

    // create an ArrayList
    ArrayList<String> languages = new ArrayList<>();

    // add elements to ArrayList
    languages.add("Python");
    languages.add("Swift");
    languages.add("C");
    languages.add("JavaScript");
    System.out.println("Unsorted ArrayList: " + languages);

    // sort the ArrayList in ascending order
    languages.sort(Comparator.naturalOrder());

    System.out.println("Sorted ArrayList: " + languages);
  }
}

Output

Unsorted ArrayList: [Python, Swift, C, JavaScript]
Sorted ArrayList: [C, JavaScript, Python, Swift]

In the above example, we have used the sort() method to sort the arraylist named languages. Notice the line,

languages.sort(Comparator.naturalOrder());

Here, the naturalOrder() method of the Java Comparator Interface specifies that elements are sorted in natural order (i.e. ascending order).

The Comparator interface also provides a method to sort elements in descending order. For example,

Example 2: Sort the ArrayList in Reverse Order

import java.util.ArrayList;
import java.util.Comparator;

class Main {
  public static void main(String[] args) {

    // create an ArrayList
    ArrayList<String> languages = new ArrayList<>();

    // add elements to ArrayList
    languages.add("Python");
    languages.add("Swift");
    languages.add("C");
    languages.add("JavaScript");
    System.out.println("Unsorted ArrayList: " + languages);

    // sort the ArrayList in ascending order
    languages.sort(Comparator.reverseOrder());

    System.out.println("Sorted ArrayList: " + languages);
  }
}

Output

Unsorted ArrayList: [Python, Swift, C, JavaScript]
Sorted ArrayList: [Swift, Python, JavaScript, C]

Here, the reverseOrder() method of the Comparator interface specifies that elements are sorted in reverse order (i.e. descending order).

Note: The Collections.sort() method is the more convenient method for sorting an arraylist.