JAVA 8 Sum function with custom logic

Question

Class A{
   Double x;
   Double y
}

List aList = new ArrayList<>();//It contains list of A class object.

Now I want to get sum(x-y) from above list using Java 8.

Thanks in advance

Answer 1

By using Stream API:

import java.util.*;
import java.util.stream.Collectors;

public class TestSum {

    static class A{
        private Double x;
        private Double y;

        public A(Double x, Double y) {
            this.x = x;
            this.y = y;
        }
    }

    public static void main(String[] args) {

        List<A> aList = new ArrayList<>();

        aList.add(new A(2.2, 1.1));
        aList.add(new A(3.3, 2.2));

        //using converting to DoubleStream
        double d1 = aList.stream()
                         .mapToDouble(a -> a.x-a.y)
                         .sum();

        System.out.println(d1);

        //using Collector and getting DoubleSummaryStatistics
        double d2 = aList.stream()
                         .collect(Collectors.summarizingDouble(a-> a.x-a.y))
                         .getSum();

        System.out.println(d2);
    }
}

Answer 2

You can try this:

List<A> aList = new ArrayList<>();
A a1 = new A();
a1.x = 5D;
a1.y = 2D;
aList.add(a1);

A a2 = new A();
a2.x = 9D;
a2.y = 2D;
aList.add(a2);

Double sum = aList.stream()
        .mapToDouble(a -> a.x - a.y)
        .sum();
System.out.println("Sum of x - y of each A of the list: " + sum);

Output is:

Sum of x - y of each A of the list: 10.0

You can use this as well:

Double sumUsingStatistics = aList.stream()
            .mapToDouble(a -> a.x - a.y)
            .summaryStatistics()
            .getSum();

System.out.println("Sum of all x - y of each A of the list: " + sumUsingStatistics);

You will get same output.