Comparator thenComparingLong() method in Java with examples

The thenComparingLong(java.util.function.ToLongFunction) method of Comparator Interface in Java returns a lexicographic-order comparator with a function that extracts a Long sort key.

Syntax:

default Comparator <T> thenComparingLong(
    ToLongFunction <T> keyExtractor)

Parameters: This method accepts keyExtractor which is the function used to extract the Long sort key.

Return value: This method returns a lexicographic-order comparator composed of this and then the Long sort key.

Exception: This method throws NullPointerException if the argument is null.

Below programs illustrate thenComparingLong(java.util.function.ToLongFunction) method:
Program 1:

// Java program to demonstrate Comparator
// thenComparingLong(ToLongFunction) 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)
    {
        List<Users> list = createUsers();
        System.out.printf("before sort: %s%n", list);
        Collections.sort(list,
                         Comparator
                             .comparing(Users::getDepartment)
                             .thenComparingLong(Users::getId));
        System.out.printf("after sort: %s%n", list);
    }

    private static List<Users> createUsers()
    {
        return Arrays.asList(
            new Users(12311, "ME"),
            new Users(10211, "CSE"),
            new Users(33111, "CSE"),
            new Users(21000, "IT"),
            new Users(12133, "IT"),
            new Users(21445, "CSE"));
    }

    private static class Users {
        private long id;
        private String department;

        public Users(long id, String department)
        {
            this.id = id;
            this.department = department;
        }

        public long getId()
        {
            return id;
        }

        public void setId(long id)
        {
            this.id = id;
        }

        public String getDepartment()
        {
            return department;
        }

        public void setDepartment(String department)
        {
            this.department = department;
        }

        @Override
        public String toString()
        {
            return department + " - " + id;
        }
    }
}

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

You can see in example first sorting is done on department wise and if the department is same then ID wise.

Program 2:

// Java program to demonstrate Comparator
// thenComparingLong(ToLongFunction) method

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

public class GFG {
    public static void main(String... args)
    {

        List<String> list
            = Arrays.asList("KKR", "CSK",
                            "MI", "KXIP", "RCB",
                            "SRH", "DC", "RR");

        try {

            // apply thenComparingLong
            Comparator.comparing(list::get)
                .thenComparingLong(null);
        }
        catch (Exception e) {

            System.out.printf("Exception:" + e);
        }
    }
}

The output printed on console is shown below.
Output:

References: https://docs.oracle.com/javase/10/docs/api/java/util/Comparator.html#thenComparingLong(java.util.function.ToLongFunction)()