Is there a way to use streams to return value from doing operations on elements

Question

I have a Train class, with a List<TrainCar> containing objects from a TrainCar class. I have a function in Train called getTotalWeight(). Each TrainCar has a function called getWeight() which returns a value. Can I use streams in some way to make the body of the getTotalWeight() function looking like this?

return cars.stream().//some function//;

Answer 1

You could use mapToDouble to get a stream of the weights and then sum them:

return cars.stream().mapToDouble(TrainCar::getWeight).sum();