Simple concurrency with Scala Futures (Futures tutorial)
Table of Contents
- Problem
- Solution
- Run one task, but block
- Run one thing, but don’t block, use callback
- The onSuccess and onFailure callback methods
- Creating a method to return a Future[T]
- How to use multiple Futures in a for loop
- Discussion
- A future and ExecutionContext
- Callback methods
- For-comprehensions (combinators: map, flatMap, filter, foreach, recoverWith, fallbackTo, andThen)
- See Also
This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 13.9, “Simple concurrency with Scala Futures.”
Back to topProblem
You want a simple way to run one or more tasks concurrently in a Scala application, including a way to handle their results when the tasks finish. For instance, you may want to make several web service calls in parallel, and then work with their results after they all return.
Back to topSolution
A Future gives you a simple way to run an algorithm concurrently. A future starts running concurrently when you create it and returns a result at some point, well, in the future. In Scala, it’s said that a future returns “eventually.”
Scala Cookbook errata page
Wow, many years ago I thought there should be an errata page for the Scala Cookbook, but to the best of my knowledge we never had one. But now, to my surprise, there is one.
How to disassemble and decompile Scala code (javap, scalac, jad)
This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 14.6, “How to disassemble and decompile Scala code.”
Back to topProblem
In the process of learning Scala, or trying to understand a particular problem, you want to examine the source code and bytecode the Scala compiler generates from your original source code.
Back to topSolution
You can use several different approaches to see how your Scala source code is translated:
How to run shell commands from the Scala REPL
This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 14.4, “How to run a shell command from the Scala REPL.”
Problem
You want to be able to run a shell command from within the Scala REPL, such as listing the files in the current directory.
Solution
Run the command using the :sh REPL command, then print the output. The following example shows how to run the Unix ls -al command from within the REPL, and then show the results of the command:
I can’t write a book
Over time I’ve discovered a number of things that I have no memory of from the years 2014 to 2016, when I was at my sickest with the mast cell disease. Apparently I created this image and wrote these words on August 24, 2015. (Here’s a link to the original post, if you’re into that sort of thing.)
How to define an `equals` method in a Scala class (object equality)
Scala problem: You want to define an equals method for your class so you can compare object instances to each other.
Solution
If you’re new to Scala, a first thing to know is that object instances are compared with ==:
"foo" == "foo" // true
"foo" == "bar" // false
"foo" == null // false
null == "foo" // false
1 == 1 // true
1 == 2 // false
1d == 1.0d // true
case class Person(name: String)
Person("Jess") == Person("Jessie") // false
This is different than Java, which uses == for primitive values and equals for object comparisons.
Writing a book is an adventure. To begin with it is a toy and an amusement.
“Writing a book is an adventure. To begin with it is a toy and an amusement. Then it becomes a mistress, then it becomes a master, then it becomes a tyrant. The last phase is that just as you are about to be reconciled to your servitude, you kill the monster and fling him to the public.”
~ Winston Churchill
I never got a final PDF version of the Scala Cookbook
A funny thing about writing books, or at least writing books with O’Reilly in 2013, is that I never received a final copy of the Scala Cookbook in PDF format. Fortunately I have the original Word docs, which is what they used at the time, so I carry those around on my laptop.
Scala Cookbook in Library of Congress
I’ll guess that nobody in the U.S. Congress has read it, but the Scala Cookbook is in the Library of Congress, which is kinda cool.
Authors love to hear “thanks”
I doubt that most people know it, but authors always enjoy receiving letters of “thanks” like this one. Writing is often a lonely, solitary business, and to hear that your work has helped other people is always satisfying. If there’s a book you’ve read that you really enjoyed, I encourage you to write the author a brief letter of thanks. They’ll appreciate it more than you’ll know.