Comparator comparingLong() method in Java with examples

The comparingLong(java.util.function.ToLongFunction) method of Comparator Interface in Java accepts a function as parameter that extracts an long 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> 
    comparingLong(ToLongFunction <T> keyExtractor)

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

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

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

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

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

        // create some user objects
        User u1 = new User("Aaman", 25724474);
        User u2 = new User("Joyita", 2262848);
        User u3 = new User("Suvam", 2824444);
        User u4 = new User("mahafuj", 2544244);

        // 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.comparingLong(
                             User::getregistrationId));
        System.out.println("\nAfterSort:");
        list.forEach(user -> System.out.println(user));
    }
}

class User implements Comparable<User> {
    public String name;
    public long registrationId;

    public User(String name, long registrationId)
    {
        this.name = name;
        this.registrationId = registrationId;
    }

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

    public String getName()
    {
        return name;
    }

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

    public long getregistrationId()
    {
        return registrationId;
    }

    public void setregistrationId(long registrationId)
    {
        this.registrationId = registrationId;
    }

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

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

Program 2:

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

        // before sort
        List<Order> list = Arrays.asList(
            new Order("A382y482y48", 3204355),
            new Order("Vvekhfbkje2", 2425355),
            new Order("efkhfbekjfbe", 1345435),
            new Order("bhdhdfaddvad", 2305535),
            new Order("abkasbcjabjc", 1005454));
        System.out.println("Before Sort:");
        list.forEach(order -> System.out.println(order));

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

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

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

    public Order(String orderNo, long 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 long getValue()
    {
        return value;
    }

    public void setValue(long 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#comparingLong(java.util.function.ToLongFunction)