Scala for-comprehension with concurrently running futures

Can you tell what’s the difference between the following two? If yes, then you’re great and you don’t need to read further.

 
 
 
 
 
 
 
 

Version 1:

val milkFuture = future { getMilk() }
val flourFuture = future { getFlour() }

for {
  milk <- milkFuture
  flour <- flourFuture
} yield (milk + flour)

Version 2:

for {
  milk <- future { getMilk() }
  flour <- future { getFlour() }
} yield (milk + flour)

You are at least curious if you got here. The difference is that the two futures in version 1 (can possibly) run in parallel, but in version 2 they can not. Function getFlour() is executed only after getMilk() is completed.

In the first version both futures are created before they are used in the for-comprehension. Once they exists it’s only up to execution context when they run, but nothing prevents them to be executed. I am trying not to say that they for sure run in parallel becuase that depends on many factors like thread pool size, execution time, etc. But the point is that they can run in parallel.

The second version looks very similar, but the problem is that the “getFlour()” future is created only once the “getMilk()” future is already completed. Therefore the two futures can never run concurrently no matter what. Don’t forget that the for-comprehension is just a syntactic sugar for methods “map”, “flatMap” and “withFilter”. There’s no magic behind.

That’s all folks. Happy futures to you.

Related Whitepaper:

Java Essential Training

Author David Gassner explores Java SE (Standard Edition), the language used to build mobile apps for Android devices, enterprise server applications, and more!

The course demonstrates how to install both Java and the Eclipse IDE and dives into the particulars of programming. The course also explains the fundamentals of Java, from creating simple variables, assigning values, and declaring methods to working with strings, arrays, and subclasses; reading and writing to text files; and implementing object oriented programming concepts. Exercise files are included with the course.

Get it Now!  

Leave a Reply


two + 7 =



Java Code Geeks and all content copyright © 2010-2014, Exelixis Media Ltd | Terms of Use | Privacy Policy
All trademarks and registered trademarks appearing on Java Code Geeks are the property of their respective owners.
Java is a trademark or registered trademark of Oracle Corporation in the United States and other countries.
Java Code Geeks is not connected to Oracle Corporation and is not sponsored by Oracle Corporation.
Do you want to know how to develop your skillset and become a ...
Java Rockstar?

Subscribe to our newsletter to start Rocking right now!

To get you started we give you two of our best selling eBooks for FREE!

Get ready to Rock!
You can download the complementary eBooks using the links below:
Close