Over a million developers have joined DZone.

Scala: Replacing a trait with a fake one for testing

· Java Zone

Navigate the Maze of the End-User Experience and pick up this APM Essential guide, brought to you in partnership with CA Technologies

We recently wanted to replace a trait mixed into one of our classes with a fake version to make it easier to test but forgot how exactly to do that!

The class is roughly like this:

trait Foo { def foo : String = "real foo" } 
class Mark extends Foo {}

We originally tried to replace it like this:

trait BrokenFakeFoo { def foo : String = "broken fake foo" }
val m = new Mark with BrokenFakeFoo
error: overriding method foo in trait Foo of type => String;
 method foo in trait BrokenFakeFoo of type => String needs `override' modifier
       val m = new Mark with BrokenFakeFoo

If m compiled it would have two versions of foo but it wouldn’t know which one to use, hence the error message.

Attempt two was this:

trait BrokenFakeFoo { override def foo : String = "broken fake foo" }
error: method foo overrides nothing
       trait BrokenFakeFoo { override def foo : String = "broken fake foo" }

As Uday pointed out, what we actually need to do is make our fake trait extend the original one and then override the method.

trait FakeFoo extends Foo { override def foo : String = "fake foo" }
val m = new Mark with FakeFoo
> res5: String = fake foo

Since FakeFoo is the right most of the traits mixed into Mark its foo method will be used over the Foo one mixed into Mark on its class definition.

From http://www.markhneedham.com/blog/2011/09/25/scala-replacing-a-trait-with-a-fake-one-for-testing/

Thrive in the application economy with an APM model that is strategic. Be E.P.I.C. with CA APM.  Brought to you in partnership with CA Technologies.


Opinions expressed by DZone contributors are their own.

The best of DZone straight to your inbox.

Please provide a valid email address.

Thanks for subscribing!

Awesome! Check your inbox to verify your email so you can start receiving the latest in tech news and resources.

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}