Comparator nullsFirst() method in Java with examples

Last Updated : 29 Apr, 2019

The nullsFirst(java.util.Comparator) method returns comparator that is a null-friendly comparator and considers null values to be less than non-null. The null first operates by the following logic:

The null element is considered to be less than non-null.
When both elements are null, then they are considered equal.
When both elements are non-null, the specified Comparator determines the order.
If the specified comparator is null, then the returned comparator considers all non-null elements equal.
The returned comparator is serializable if the specified comparator is serializable.

Syntax:

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

Parameters: This method accepts a single parameter comparator which is a Comparator for comparing non-null values Return value: This method returns a comparator that considers null to be less than non-null and compares non-null objects with the supplied Comparator. Below programs illustrate nullsFirst(java.util.Comparator) method: Program 1:

Java

// Java program to demonstrate
// Comparator.nullsFirst(java.util.Comparator) method

import java.util.Arrays;
import java.util.Comparator;

public class GFG {

    public static void main(String[] args)
    {

        // Create a collection of an array
        // of names also contain nulls
        String[] strings = { "aman", "suvam",
                             null, "sahil",
                             null };

        // print the array
        System.out.println("Before sorting: "
                           + Arrays.toString(strings));

        // apply nullsFirst method
        // and sort the array
        Arrays.sort(strings,
                    Comparator.nullsFirst(
                        Comparator.naturalOrder()));

        // print the array
        System.out.println("After sorting: "
                           + Arrays.toString(strings));
    }
}

The output printed on console of IDE is shown below. Output:

Program 2:

Java

// Java program to demonstrate
// Comparator.nullsFirst(java.util.Comparator) method

import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class GFG {
    public static void main(String[] args)
    {

        // Create some user objects
        User u1 = new User("Aaman", 25);
        User u2 = new User("Joyita", 22);
        User u3 = new User("Suvam", 28);
        User u4 = new User("mahafuj", 25);

        System.out.println("One null Objects");
        List<User> list
            = Arrays.asList(u1, u2, u3, null, u4);
        Collections.sort(list,
                         Comparator.nullsFirst(
                             Comparator.comparing(
                                 User::getName)));
        list.forEach(user -> System.out.println(user));

        System.out.println("\nMore than One null Objects");
        list = Arrays.asList(u1, u4, null, u2, u3, null, null);
        Collections.sort(list,
                         Comparator.nullsFirst(
                             Comparator.comparing(
                                 User::getName)));
        list.forEach(user -> System.out.println(user));
    }
}

class User implements Comparable<User> {
    public String name;
    public int age;

    public User(String name, int age)
    {
        this.name = name;
        this.age = age;
    }

    public int compareTo(User u1)
    {
        return name.compareTo(u1.name);
    }

    public String getName()
    {
        return name;
    }

    public void setName(String name)
    {
        this.name = name;
    }

    public int getAge()
    {
        return age;
    }

    public void setAge(int age)
    {
        this.age = age;
    }

    @Override
    public String toString()
    {
        return "User [name=" + name + ", age=" + age + "]";
    }
}