scala interview questions
Top scala frequently asked interview questions
Scala doesn't have type-safe enums like Java has. Given a set of related constants, what would be the best way in Scala to represent those constants?
Source: (StackOverflow)
I'm using Java's java.util.date class in Scala and want to compare a date object and the current time. I know I can calculate the delta by using getTime():
(new java.util.Date()).getTime() - oldDate.getTime()
However, this just leaves me with a Long representing milliseconds. Is there any simpler, nicer way to get a time delta?
Source: (StackOverflow)
All I know about TypeTags is that they somehow replaced Manifests. Information on the Internet is scarce and doesn't provide me with a good sense of the subject.
So I'd be happy if someone shared a link to some useful materials on TypeTags including examples and popular use-cases. Detailed answers and explanations are also welcome.
Source: (StackOverflow)
In a simple way, what are context and view bounds and what is the difference between them?
Some easy-to-follow examples would be great too!
Source: (StackOverflow)
A self-type for a trait A:
trait B
trait A { this: B => }
says that "A cannot be mixed into a concrete class that does not also extend B".
On the other hand, the following:
trait B
trait A extends B
says that "any (concrete or abstract) class mixing in A will also be mixing in B".
Don't these two statements mean the same thing? The self-type seems to serve only to create the possibility of a simple compile-time error.
What am I missing?
Source: (StackOverflow)
I have followed the tutorial at Scala and Android with Scala 2.7.3 final. The resulting Android App works but even the most basic application takes several minutes (!) to compile and needs 900 kb compressed, which is a show stopper for mobile applications. Additionally, the IDE runs out of memory every now and then. I assume dex is not made for big libraries like the scala-library.
- So my question is: Has anyone actually done this and is there any cure for this?
Source: (StackOverflow)
Can someone please explain the major differences between Scala, Groovy and Clojure. I know each of these compiles to run on the JVM but I'd like a simple comparison between them.
Source: (StackOverflow)
Sealed classes are described in 'Programming in Scala', but sealed traits are not.
Where can I find more information about a sealed trait?
I would like to know, if a sealed trait is the same as a sealed class?
Or, if not, what are the differences?
When is it a good idea to use a sealed trait (and when not)?
Source: (StackOverflow)
In Scala 2.8, there is an object in scala.collection.package.scala:
def breakOut[From, T, To](implicit b : CanBuildFrom[Nothing, T, To]) =
new CanBuildFrom[From, T, To] {
def apply(from: From) = b.apply() ; def apply() = b.apply()
}
I have been told that this results in:
> import scala.collection.breakOut
> val map : Map[Int,String] = List("London", "Paris").map(x => (x.length, x))(breakOut)
map: Map[Int,String] = Map(6 -> London, 5 -> Paris)
What is going on here? Why is breakOut being called as an argument to my List?
Source: (StackOverflow)
It's a sad fact of life on Scala that if you instantiate a List[Int], you can verify that your instance is a List, and you can verify that any individual element of it is an Int, but not that it is a List[Int], as can be easily verified:
scala> List(1,2,3) match {
| case l : List[String] => println("A list of strings?!")
| case _ => println("Ok")
| }
warning: there were unchecked warnings; re-run with -unchecked for details
A list of strings?!
The -unchecked option puts the blame squarely on type erasure:
scala> List(1,2,3) match {
| case l : List[String] => println("A list of strings?!")
| case _ => println("Ok")
| }
<console>:6: warning: non variable type-argument String in type pattern is unchecked since it is eliminated by erasure
case l : List[String] => println("A list of strings?!")
^
A list of strings?!
Why is that, and how do I get around it?
Source: (StackOverflow)
I am trying to use a .format method of a string. But if I place %1, %2, etc. in the string, java.util.UnknownFormatConversionException is thrown pointing to a confusing Java source code piece:
private void checkText(String s) {
int idx;
// If there are any '%' in the given string, we got a bad format
// specifier.
if ((idx = s.indexOf('%')) != -1) {
char c = (idx > s.length() - 2 ? '%' : s.charAt(idx + 1));
throw new UnknownFormatConversionException(String.valueOf(c));
}
}
From this I understand that % char is forbidden. If so, then what should I use for argument placeholders?
I use Scala 2.8.
Source: (StackOverflow)
I have heard the term "coalgebras" several times in functional programming and PLT circles, especially when the discussion is about objects, comonads, lenses, and such. Googling this term gives pages that give mathematical description of these structures which is pretty much incomprehensible to me. Can anyone please explain what coalgebras mean in the context of programming, what is their significance, and how they relate to objects and comonads?
Source: (StackOverflow)
I'm just going over some Scala tutorials on the Internet and have noticed in some examples an object is declared at the start of the example.
What is the difference between class and object in Scala?
Source: (StackOverflow)
I have heard lots of raving about Akka framework (Java/Scala service platform), but so far have not seen many actual examples of use cases it would be good for. So I would be interested in hearing about things developers have used it succesfully.
Only one limitation: please do not include case of writing a chat server. :-)
(why? since this has been overused as an example for lots of similar things)
EDIT: Since there are many good answers, my choice of accepted answer is bit arbitrary; I thought highest voted one seems like a reasonable choice. Thank you everyone for good answers!
Source: (StackOverflow)