Multiply & Sum two ArrayLists in Java8

Question

I have two ArrayLists, named A and B, of equal size containing some numbers. Now I want to calculate something like this:

int sum = 0;
for(int i=0; i<A.size() && i<B.size(); i++) {
  sum += A.get(i)*B.get(i);
}

How can I achieve what I am doing above, calculating the sum, by using Java 8 features (streams, lambda expressions, etc) without using any extra user-defined methods?

You might get a better answer if you explain why the loop is not acceptable to you, but you can use the rather literal stream translation of that loop with IntStream.range(0, A.size()).map(i -> A.get(i) * B.get(i)).sum(). — Jeffrey Bosboom
– Jeffrey Bosboom, Commented Jun 28, 2015 at 2:38
Just for good programming measure: I don't see any proof that A and B are the same size. — Mike 'Pomax' Kamermans
– Mike 'Pomax' Kamermans, Commented Jun 28, 2015 at 2:39
@JeffreyBosboom I have no problems with using loops. I asked it because I am learning Java 8 features, should have included that in the post. — Mubin
– Mubin, Commented Jun 28, 2015 at 2:55

Jeffrey Bosboom · Accepted Answer · 2015-06-28 02:45:42Z

15

int sum = 
    IntStream.range(0, min(a.size(), b.size())
             .map(i -> a.get(i) * b.get(i))
             .sum();

edited Jun 28, 2015 at 2:45

Jeffrey Bosboom

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answered Jun 28, 2015 at 2:37

Brian Goetz

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9 Comments

almightyGOSU Over a year ago

I think it's multiplication, not addition. a.get(i) + b.get(i) should be a.get(i) * b.get(i)?

Alain O'Dea Over a year ago

That's a neat solution. I hadn't considered emitting the range of indices as a Stream.

PM 77-1 Over a year ago

@Mubin - It will work "as is" for any collections that support positional access.

Alain O'Dea Over a year ago

@Mubin the IntStream.range generates a Stream of indices, not a Stream of values to be multiplied. All Lists are indexable by integer indexes regardless of contained types.

Mubin Over a year ago

For using ArrayLists with Double values, I had to use mapToDouble instead of map, otherwise it was giving Type mismatch: cannot convert from double to int

|

Daenerys · Accepted Answer · 2019-09-16 21:29:09Z

0

int sum = IntStream
            .range(0, min(a.size(), b.size())
            .map(i -> 
              (a.get(i).getValue() == null || b.get(i).getValue() == null) ? 0 :
                       (a.get(i).getValue() * b.get(i).getValue()))
            .sum();

Neat Solution!
I had a similar requirement, and I added ternary operator null check. (In my scenario, my lists where of generic type and calculation was done on values within the object which could be null)

answered Sep 16, 2019 at 21:29

Daenerys

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