Java 8 stream - sum of objects

Question

Let's say I have a list of objects implementing below interface:

public interface Summable<T> {
    T add(T o1);
}

Let's say I have also some class which is able to sum these objects:

public class Calculator<T extends Summable<T>> {
    public T sum(final List<T> objects) {
        if (null == objects) {
            throw new IllegalArgumentException("Ups, list of objects cannot be null!");
        }
        T resultObject = null;
        for (T object : objects) {
            resultObject = object.add(resultObject);
        }
        return resultObject;
   }
}

How can I achieve the same using Java 8 streams?

I'm playing around a custom Collector, but couldn't figure out some neat solution.

It looks like it should probably be `public interface Summable>` — Paul Boddington, May 03 '15 at 21:04
Not necessarily, Calculator has it and it is enough I suppose. — Piotr Kozlowski, May 03 '15 at 21:19
Not directly related to the question but in your example,`resultObject` will always be null. — Chetan Kinger, May 04 '15 at 04:02
Only in first iteration, but what's the point? Have you some better idea how can I write it? — Piotr Kozlowski, May 04 '15 at 09:17

score 25 · Accepted Answer · answered May 03 '15 at 20:59

25

What you have is a reduction:

return objects.stream().reduce(T::add).orElse(null);

answered May 03 '15 at 20:59

Radiodef

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score 2 · Answer 2 · answered May 03 '15 at 21:09

2

list.stream().reduce(Summable::add);

interface Summable {
    Summable add(Summable o1);
}

answered May 03 '15 at 21:09

user2418306

2,352
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Java 8 stream - sum of objects

2 Answers2

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