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stream sum on list of objects

1 points · submitted Jun 2014 by u/Master_Rux · archived

In C# I had this

List<Learner> Learners;
double learnerSum = this.Learners.Sum(learner => learner.Predicted * learner.Alpha);

I want to do the same in java 8. I get as far as

double learnerSum = this.Learners.stream()...

It seems i need a reduce but all the examples are on simple arrays of integers. what am i missing?

2 comments · 67% upvoted · other discussions

comments

Master_Rux [OP] 2 points · Jun 2014

This seems to work unless someone has a better solution.

Learners.stream().collect(Collectors.summingDouble(learner -> learner.Predicted * learner.Alpha ));
Master_Rux [OP] 2 points · Jun 2014

Better answer

double learnerSum = Learners.stream()
.mapToDouble(l -> l.Predicted * l.Alpha).sum();