Comparator thenComparingDouble() method in Java with examples

The thenComparingDouble(java.util.function.ToDoubleFunction) method of Comparator Interface in Java returns a lexicographic-order comparator with a function that extracts a double sort key. This method is applied after comparing method if you want to apply another comparing for those values which are equal in the comparing method.

Syntax:

default Comparator <T> thenComparingDouble(
    ToDoubleFunction <T> keyExtractor)

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

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

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

Below programs illustrate thenComparingDouble(java.util.function.ToDoubleFunction) method:
Program 1:

// Java program to demonstrate Comparator
// thenComparingDouble(ToDoubleFunction) 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<Student> list = getStudentList();

        Comparator<Student> comparator
            = Comparator
                  .comparing(Student::getSchool)
                  .thenComparingDouble(Student::getpercentageMarks);

        Collections.sort(list, comparator);
        System.out.println("After sort");
        list.forEach(s -> System.out.println(s));
    }

    public static List<Student> getStudentList()
    {

        Student s1 = new Student("Ram", 85.5, "SJV");
        Student s2 = new Student("Shyam", 83.25, "MSH");
        Student s3 = new Student("Mohan", 86.55, "SJV");
        Student s4 = new Student("Sohan", 81.00, "MSH");
        Student s5 = new Student("Rabi", 55.6, "SJV");
        List<Student> list = Arrays.asList(s1, s2, s3, s4, s5);

        return list;
    }
}

class Student {

    private String name;
    private double percentageMarks;
    private String school;

    public Student(String name,
                   double percentageMarks,
                   String school)
    {
        this.name = name;
        this.percentageMarks = percentageMarks;
        this.school = school;
    }

    public String getName()
    {
        return name;
    }

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

    public double getpercentageMarks()
    {
        return percentageMarks;
    }

    public void setpercentageMarks(int percentageMarks)
    {
        this.percentageMarks = percentageMarks;
    }

    public String getSchool()
    {
        return school;
    }

    public void setSchool(String school)
    {
        this.school = school;
    }

    @Override
    public String toString()
    {
        return "Student [name=" + name + ",
            percentageMarks
            = " + percentageMarks
              + ", school=" + school + "]";
    }
}

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

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

Program 2:

// Java program to demonstrate Comparator
// thenComparingDouble(ToDoubleFunction)  method

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

public class GFG {

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

        List<Integer> list
            = Arrays.asList(1, 2, 3, 4, 5, 6);

        try {

            // apply thenComparingDouble
            Comparator.comparing(list::get)
                .thenComparingDouble(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#thenComparingDouble(java.util.function.ToDoubleFunction)()