What is the Comparator.nullsFirst() method in Java?

The nullsFirst() method is used to return a Comparator, which considers the null elements to be lesser than the non-null elements. It takes a Comparator as a parameter, which is then used for the comparison of the non-null elements.

Given two inputs and a non-null comparator as the parameter, the following scenarios are possible:

First input Second input Behaviour
null null The inputs are considered equal.
non-null non-null The supplied comparator is used for comparison.
null non-null First input is considered to be less than the second input.
non-null null Second input is considered to be less than the first input.

The Comparator interface is defined in the java.util package. To import the Comparator interface, we check the following import statement:

import java.util.Comparator;

Syntax

public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator)

Parameters

Comparator<? super T> comparator: This is the Comparator that is used for comparing the non-null values.

Return value

This method returns a Comparator which considers the null elements to be lesser than the non-null elements.

import java.util.Arrays;
import java.util.Comparator;
public class Main{
    public static void main(String[] args) {
        // Collection of strings
        String[] strings = {null, "hello", "educative", null, "edpresso"};
        // print the string collection
        System.out.println("Before sorting: " + Arrays.toString(strings));
        // Using the nullsFirst method to sort the array
        Arrays.sort(strings, Comparator.nullsFirst(Comparator.naturalOrder()));
        // print the sorted array
        System.out.println("After sorting: " + Arrays.toString(strings));
    }
}

Code explanation

  • Lines 1-2: We import the relevant packages.
  • Line 8: We define a collection of strings containing null and non-null values.
  • Line 11: We print the string collection that we defined in line 8.
  • Line 14: We sort the collection by using the Arrays.sort() method and passing the Comparator.nullsFirst() method as a parameter.
  • Line 17: We print the sorted collection.
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