Comparator thenComparingLong() method in Java with examples

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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); 
        } 
    } 
} 