Question on concurrency (Scott & Jeanne's OCP 8 book)

Hi all,

I am currently preparing the Java OCP 8 exam, with Jeanne and Scott's book. In the chapter on Concurrency, review question 16 (page 400) confuses me. I am not questioning the correctness of the book. I just have troubles to wrap my head around it. The question goes as follows: What is the result of executing the following code:


The possible answers are:
A) It compiles and outputs the two numbers, followed by Printed
B) The code will not compile because of line b1
C) The code will not compile because of line b2
D) The code will not compile because of line b3
E) The code will not compile because of line b_4
F) It compiles but the output cannot be determined ahead of time
G) It compiles but throws an exception at runtime
H) It compiles but waits forever at runtime

The correct answers are G, H.

My way of thinking: The forEach() on line 4 takes a DoubleConsumer as argument. A DoubleConsumer takes one argument and returns no result (just like a regular Consumer). The argument is represented by c and that means that whatever comes after the arrow token should return void. But it was my understanding that the service() method always returns a Future object, regardless of whether it takes a Callable or a Runnable. So why doesn't this result in a compilation error? Could someone help me understand? That would be greatly appreciated.

I tried it on Java9 and got unpredictable output:-

java Question16Demo
27.18281828459045
31.41592653589793
Printed
^[[A

^C[campbell@... ~]$ java Question16Demo
31.41592653589793
Printed
27.18281828459045
^[[A
^C[campbell@... ~]$ java Question16Demo
31.41592653589793
27.18281828459045
Printed

The control characters are caused by my trying to run the program again; it seemed to print output but not in any particular order, and then keep running doing nothing.

When a function that returns void is expected, you can pass it a function that returns anything. The return value is simply ignored.

It seems to me that the correct answers should be F and H: you can't determine what the output will be because the tasks may be performed in any order and the program will keep running because the executor is never shutdown.

@Stephan: The correct answers are indeed F and H. That was a typo from my part.

Thanks Campbell and Stephan for answering.

Stephan van Hulst wrote:When a function that returns void is expected, you can pass it a function that returns anything. The return value is simply ignored.

Stephan, could you eleborate on what you mean by this? I am somewhat confused now.

The reason the outputs are not in any particular order is that the three Threads complete their “calculations” much faster than printing to the terminal, so they access System.out at any old time. This is a common finding when multi‑threading; the individual Threads run at different speeds and you cannot predict which will complete first.

I assumed that you thought that the application wouldn't compile because you pass forEach() a function that returns a Future, while forEach() expects a function that returns void.

To that I answered that it doesn't matter, forEach() just ignores the return type of your lambda expression.

That is indeed what is confusing me.

If I try to implement:



It should compile - but it doesn't. What am I not getting?

Right. When the return type is void, the body of the lambda expression must be a statement expression. Statement expressions are expressions that you can use as a statement, because they DO something. The following are statement expressions:

Variable assignment

Variable increment/decrement

Method call

Object creation

An expression like a+2 or just i is not a statement and therefore can't be used as the body of a function that returns void.

Ok, that is something that I did not know at all, even though it seems like it is a crucial concept to grasp. I learned something new.  

Thanks Stephan and Campbell for your insights! They are much appreciated.

Brecht Geeraerts wrote:. . . Thanks Stephan and Campbell . . .

That's a pleasure