Enthuware

Why is this question in the category "09 - Parallel Streams"?

Question and answer options don't talk about parallel streams at all!

Yes, it should be in "Builtin Functional interface"
thank you for your feedback!

Using Stream<Double> and BinaryOperator<Double>:
BinaryOperator<Double> bo =  (a, b)->a+b;
double total = bkStrm.map(b->b.getPrice()).reduce(dbo).get(); //reduce returns Optional<Double>, so, need to call get()
OR
double total = bkStrm.map(b->b.getPrice()).reduce(0.0, dbo).get(); //reduce returns Double

It should be:
double total = bkStrm.map(b->b.getPrice()).reduce(bo).get(); //reduce returns Optional<Double>, so, need to call get()
OR
double total = bkStrm.map(b->b.getPrice()).reduce(0.0, bo).get(); //without get()

RIght. Fixed.
thank you for your feedback!

on second option (and explanation):
double total = bkStrm.map(b->b.getPrice()).reduce((a, b)->{ return a+b;}).ifPresent(p->p.doubleValue());
The single parameter reduce method returns an OptionalDouble (if it is a DoubleStream) or Optional<Double> if it is Stream<Double>.
OptionalDouble has getAsDouble() that returns a Double and Optional<Double> has get() that returns a Double.

Shouldn't is say that the problem here is that ifPresent returns void?? is ok the explanation about the OptionalDouble but the real issue is you cant assign void to a variable. Looks to me that the explanation doesn't have anything about the issue or the real problem of this answer option.

Yes, the explanation should explain that. Updated.
thank you for your feedback!

regarding the comment in the Explanation:

DoubleBinaryOperator dbo =  (a, b)->a+b;
double total = bkStrm.mapToDouble(b->b.getPrice()).reduce(dbo).getAsDouble(); //reduce returns OptionalDouble, so, need to call getAsDouble()
OR
double total = bkStrm.mapToDouble(b->b.getPrice()).reduce(0.0, dbo);//reduce returns Double

the mapToDouble returns a DoubleStream and the reduce method from that type of Stream returns a double instead of Double.

You are right, it should say Double.

did you mean double?

No, the code given there is bkStrm.map(b->b.getPrice()).reduce(0.0, bo)
Here, reduce returns Double.

I mean this one:
bkStrm.mapToDouble(b->b.getPrice()).reduce(0.0, dbo);

does it return a Double?

Oh right. I see what you mean. Yes, it should be double. Fixed.
thank you for your feedback!

Not fixed yet

Could you please tell me where are you seeing it not fixed? I checked that is it fixed in 819 and 829 question banks.