Accessing scala.None from Java

Question

How can you access scala.None from Java?

The last line causes the compiler to die with "type scala.None does not take parameters".

import scala.Option;
import scala.Some;
import scala.None;
final Option<String> object1 = new Some<String>("Hi there");
final Option<String> object2 = new None<String>();

This fails with "cannot find symbol constructor None()":

final Option<String> object2 = new None();

This fails with "cannot find symbol variable None":

final Option<String> object2 = None;

In 2007 this used to work, but then Scala changed. The Java compiler gives error: incompatible types:

final Option<String> object2 = scala.None$.MODULE$;

By the way... scala.None$.MODULE$ should still work. What problem did you have with it? — Daniel C. Sobral
– Daniel C. Sobral, Commented Feb 2, 2012 at 14:20

Peter Mortensen · Accepted Answer · 2013-06-11 14:02:53Z

36

This might work:

final scala.Option<String> x = scala.Option.apply(null);

def apply [A] (x: A): Option[A]
An Option factory which creates Some(x) if the argument is not null, and None if it is null.

edited Jun 11, 2013 at 14:02

Peter Mortensen

31.4k22 gold badges110 silver badges134 bronze badges

answered Jan 30, 2012 at 5:21

om-nom-nom

62.9k13 gold badges186 silver badges231 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Guillaume Massé Over a year ago

just like java: ugly but effective

Alvaro Agea Over a year ago

What is your Scala version? In Scala 2.11, it does not work.

om-nom-nom Over a year ago

@AlvaroAgea checked on scala v2.11.7, works like a charm. Can you elaborate how exactly it doesn't work?

Dave Moten · Accepted Answer · 2013-11-14 02:20:55Z

18

Using Scala 2.10.2 (not sure at what version when this came in):

final Option<String> none = Option.empty();

answered Nov 14, 2013 at 2:20

Dave Moten

12.1k2 gold badges45 silver badges48 bronze badges

Comments

Daniel C. Sobral · Accepted Answer · 2012-02-01 14:14:51Z

Not to put too fine a point on it, but one shouldn't rely on static forwarders, as they might not get generated under certain circumstances. For instance, Some doesn't have static forwarders for its object companion.

The current way of accessing methods on an object companion is this:

final scala.Option<String> x = scala.Option$.MODULE$.apply(null);

Option$ is the class for the object companion of Option. MODULE$ is a final public static field that contains the sole instance of Option$ (in other words, the object companion).

Christian Vielma · Accepted Answer · 2017-05-08 16:05:45Z

1

You can import

import scala.compat.java8.OptionConverters;

And then:

OptionConverters.<YourType>toScala(Optional.empty())

answered May 8, 2017 at 16:05

Christian Vielma

16.2k12 gold badges55 silver badges64 bronze badges

Comments

vladmore · Accepted Answer · 2012-01-30 05:29:44Z

0

You can do the following (I tested it out and it works fine for me with Scala 2.9.1 and Java 1.6):

 final Option<String> obj1 = Some.apply(null) // This will give you a None;
 System.out.println(obj1.getClass()); // will print "class scala.None$

 final Option<String> obj2 = Some.apply("Something");
 System.out.println(obj2.getClass()); //will print "class scala.Some

answered Jan 30, 2012 at 5:29

vladmore

1043 bronze badges

6 Comments

vladmore Over a year ago

I didn't see the answer @om-nom-nom posted when I wrote this but I think that should work as well.

Daniel C. Sobral Over a year ago

Some(null) is Some(null), not None.

vladmore Over a year ago

That's what I expected, because it would make sense and that's what happens when I try that through the Scala interpreter. But when I try the code I posted above in a java file and run that I get obj2.getClass() giving me None$. I'm not sure why though.

vladmore Over a year ago

When I do this through Java it looks like because Some extends Option it calls Option's factory apply method which in turn returns None if the argument to apply is null.

Daniel C. Sobral Over a year ago

Oh, wow, that is something I had never imagined! Ok, here's the deal: Java has static inheritance, Scala doesn't. The object Some doesn't, and can't, extend the object Option, but Scala creates static forwarders from the class to the object unless some conflict shows up. The correct way to invoke apply' is to do it on scala.Some$.MODULE$` (or Option$, etc). It just happens that Option has static forwarders but Some doesn't, so Java's static inheritance comes into play. So weird.

|

simou · Accepted Answer · 2016-09-08 11:58:19Z

0

Using Scala version 2.11.4 the only thing that worked out in my case, was

Option<String> maybeAmount = (amount != MISSING_STOCKAMOUNT) 
? Some.apply(amount+"") : Option.<String>apply(null); // can you feel the pain??

Update: a day later...

Option.apply(null);

without the type parameter does work too, as it should. For some reason I couldn't get Intellij to compile the code on my first attempt. I had to pass the secondary type parameter. Now it compiles, just don't ask

edited Sep 8, 2016 at 11:58

answered Sep 8, 2016 at 1:51

simou

2,5064 gold badges32 silver badges41 bronze badges

2 Comments

Mike Slinn Over a year ago

Yes, that seems painful. What version of Java are you using?

simou Over a year ago

Oracle JDK 1.8.0_65

Collectives™ on Stack Overflow

Accessing scala.None from Java

6 Answers 6

3 Comments

Comments

Comments

Comments

6 Comments

2 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

3 Comments

Comments

Comments

Comments

6 Comments

2 Comments

Linked

Related