Sum of elements of an Integer ArrayList without looping

Question

I have gone through a few web resources and links, however, I am unable to find the sum of elements of an Integer ArrayList without looping. I am looking for a function which will allow me to do this in a single line.

I was able to find the same for a normal array as follows

import java.util.ArrayList;
import java.util.stream.IntStream;

public class sumArray
{
    public static void main(String[] args)
    {
        int[] a = {10,20,30,40,50};
        int sum = IntStream.of(a).sum();
        System.out.println("The sum is " + sum);
    }
}

I can write a user-defined function as follows

import java.util.ArrayList;

public class sumAL
{
    public static void main(String[] args)
    {
        ArrayList<Integer> al = new ArrayList<Integer>();
        al.add(1);
        al.add(3);
        al.add(5);

        System.out.println(sum(al));
    }

    static int sum(ArrayList<Integer> al)
    {
        int value = 0;
        for(int i : al)
        {
            value += i;
        }
        return value;
    }
}

However, I'm looking for something in-built. Please advise.

EDIT : I have tried the following but I get build errors

import java.util.ArrayList;

public class sumAL
{
    public static void main(String[] args)
    {
        System.out.println(getVersion());

        ArrayList<Integer> al = new ArrayList<Integer>();
        al.add(1);
        al.add(3);
        al.add(5);
        System.out.println(al.stream().mapToInt(Integer::intValue).sum());
    }

    static double getVersion () {
        String version = System.getProperty("java.version");
        int pos = version.indexOf('.');
        pos = version.indexOf('.', pos+1);
        return Double.parseDouble (version.substring (0, pos));
    }
}

Errors

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
    Integer cannot be resolved to a variable
    Syntax error on tokens, delete these tokens

one way or another some looping will be used, you can use a divide-and-conquer recursive (or iterative) approach since sums satisfy the partial sums property, i.e a **sum is equal to the sum of partial sums** — Nikos M., Aug 19 '15 at 16:07
possible duplicate of [Is there possibility of sum of ArrayList without looping](http://stackoverflow.com/questions/5963847/is-there-possibility-of-sum-of-arraylist-without-looping) — Swapnil, Aug 19 '15 at 16:07
Are you sure you are using java 8? Run `java -version` from command line or check java instalation folder for version. Streams (you are using those) are only avaliable in java 8. — kajacx, Aug 19 '15 at 17:13
java -version gives me 1.8. I also tested using getVersion() code above and it gives me 1.8 — saltandwater, Aug 19 '15 at 17:17

score 1 · Answer 1 · answered Aug 19 '15 at 16:07

1

You can easily map a Stream<Integer> to an IntStream and then calculate the sum :

a1.stream().mapToInt(Integer::intValue).sum();

answered Aug 19 '15 at 16:07

Eran

387,369
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`map` uses loops implicitly! – Nikos M. Aug 19 '15 at 16:08
@NikosM. I don't think the OP minds that. He's just looking for a built-in method. – Swapnil Aug 19 '15 at 16:08
3

@NikosM. You'll always have some loop behind the scenes, since you must iterate over all the elements in order to calculate the sum. – Eran Aug 19 '15 at 16:09
@Eran, thanks for the response. I have tried the above, but I get some build errors, please see EDIT above. Is it something related to java versioning? – saltandwater Aug 19 '15 at 16:27
@saltandwater You must have java 8. – takendarkk Aug 19 '15 at 16:29
@Takendarkk, I do have java version 1.8. Please see edit above. – saltandwater Aug 19 '15 at 16:57
@Eran, sorry didn't get you. I said, I do have java version 1.8 – saltandwater Aug 19 '15 at 17:04
@saltandwater sorry about that, not enough sleep – Eran Aug 19 '15 at 17:05
@Eran, :) guessed it. Are you able to compile my code from the edit above? – saltandwater Aug 19 '15 at 17:09
@saltandwater I tested the code on ideone.com and it worked - http://ideone.com/e.js/76vf9v – Eran Aug 19 '15 at 17:11

score 1 · Answer 2 · answered Aug 19 '15 at 16:32

1

al.stream().reduce(0, (x,y) -> x+y)

answered Aug 19 '15 at 16:32

FoxyBOA

5,788
8
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2

While this code may answer the question, providing additional context regarding _how_ and/or _why_ it solves the problem would improve the answer's long-term value. – spongebob Aug 19 '15 at 16:55

Sum of elements of an Integer ArrayList without looping

2 Answers2