Comparator comparingDouble() method in Java with examples

The comparingDouble(java.util.function.ToDoubleFunction) method of Comparator Interface in Java accepts a function as parameter that extracts an double sort key from a type T, and returns a Comparator that compares by that sort key.The returned comparator is serializable if the specified function is also serializable.

Syntax:

static <T> Comparator<T> comparingDouble(
    ToDoubleFunction <T> keyExtractor)

Parameters: This method accepts a single parameter keyExtractor which is the function used to extract the double sort key.

Return value: This method returns a comparator that compares by an extracted key

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

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

// Java program to demonstrate Comparator
// comparingDouble(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)
    {

        // create some user objects
        User u1 = new User("Aaman", 3500.56);
        User u2 = new User("Joyita", 4222.99);
        User u3 = new User("Suvam", 2832.34);
        User u4 = new User("mahafuj", 2522.76);

        // before sort
        List<User> list = Arrays.asList(u2, u1, u4, u3);
        System.out.println("Before Sort:");
        list.forEach(user -> System.out.println(user));

        Collections.sort(list,
                         Comparator.comparingDouble(User::getSalary));
        System.out.println("\nAfterSort:");
        list.forEach(user -> System.out.println(user));
    }
}

class User implements Comparable<User> {
    public String name;
    public double salary;

    public User(String name, double d)
    {
        this.name = name;
        this.salary = d;
    }

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

    public String getName()
    {
        return name;
    }

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

    public double getSalary()
    {
        return salary;
    }

    public void setSalary(double salary)
    {
        this.salary = salary;
    }

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

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

Program 2:

// Java program to demonstrate Comparator
// comparingDouble(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)
    {

        // before sort
        List<Order> list = Arrays.asList(
            new Order("A382y482y48", 32684.82),
            new Order("Vvekhfbkje2", 28348.47),
            new Order("efkhfbekjfbe", 47674.49),
            new Order("bhdhdfaddvad", 78449.43),
            new Order("abkasbcjabjc", 10104.43));

        System.out.println("Before Sort:");
        list.forEach(order -> System.out.println(order));

        Collections.sort(list,
                         Comparator.comparingDouble(Order::getValue));
        System.out.println("\nAfter Sort:");
        list.forEach(order -> System.out.println(order));
    }
}

class Order implements Comparable<Order> {
    public String orderNo;
    public double value;

    public int compareTo(Order o1)
    {
        return orderNo.compareTo(o1.orderNo);
    }

    public Order(String orderNo, double value)
    {
        super();
        this.orderNo = orderNo;
        this.value = value;
    }

    @Override
    public String toString()
    {
        return "Order [orderNo=" + orderNo
            + ", value=" + value + "]";
    }

    public String getOrderNo()
    {
        return orderNo;
    }

    public void setOrderNo(String orderNo)
    {
        this.orderNo = orderNo;
    }

    public double getValue()
    {
        return value;
    }

    public void setValue(double value)
    {
        this.value = value;
    }
}

The output printed on console is shown below.
Output:

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