We can use functional Programming in Java to calculate sum of an int array.

You can watch my live coding video to see how it is done.

Traditional Way :-

int sum = 0;
for(int intValue : ints) sum += intValue;

Functional Programming Way 1 :-

Arrays.stream(ints).sum();

Functional Programming Way 2 :-

Arrays.stream(ints).reduce(0, Integer::sum);

Functional Programming Way 3:-

final AtomicInteger sumInt = new AtomicInteger(0);
Arrays.stream(ints).forEach( intValue -> sumInt.set( intValue + sumInt.get()));

Source Code:-

import java.util.Arrays;
import java.util.concurrent.atomic.AtomicInteger;

public class SumOfIntArray{
    public static void main(String ...args){
        final int[] ints = {1, 2, 3, 4, 5, 6};

    int sum = 0;
    for(int intValue : ints) sum += intValue;
    System.out.println("Tradiiontal Sum " + sum);
    
    sum = 0;

    sum = Arrays.stream(ints).sum();
    System.out.println("Functional Stream 'sum' function Sum " + sum);

    sum = 0;

    sum = Arrays.stream(ints).reduce(0, Integer::sum);
    System.out.println("Functional Stream 'reduce' function Sum " + sum);

    final AtomicInteger sumInt = new AtomicInteger(0);
    Arrays.stream(ints).forEach( intValue -> sumInt.set( intValue + sumInt.get()));
    System.out.println("Functional Stream should not be done this way " + sumInt.get()); 

    }
}