Question

Why do I get the error below? How to workaround it?

I assumed that since A and B compile to (interface,class) pairs, it s a matter of choosing the right static method call to implement when compiling C. I would expect the priority to be according to order.

scala> trait A { def hi = println("A") }
defined trait A

scala> trait B { def hi = println("B") }
defined trait B

scala> class C extends B with A
<console>:6: error: error overriding method hi in trait B of type => Unit;
 method hi in trait A of type => Unit needs `override  modifier
       class C extends B with A

scala> trait A { override def hi = println("A") }
<console>:4: error: method hi overrides nothing
       trait A {override def hi = println("A")}

Note that in Ruby this works well:

>> module B; def hi; puts  B ; end; end
=> nil
>> module A; def hi; puts  A ; end; end
=> nil
>> class C; include A; include B; end
=> C
>> c = C.new
=> #<C:0xb7c51068>
>> c.hi
B
=> nil

Answer 1

This works for me in 2.8 and 2.11, and would allow you to be non-intrusive in traits A or B:

trait A { def hi = println("A") }
trait B { def hi = println("B") }

class C extends A with B {
  override def hi = super[B].hi
  def howdy = super[A].hi // if you still want A#hi available
}

object App extends Application {
  (new C).hi // prints "B"
}

Answer 2

You could use a common base trait, say Base, as follows:

trait Base {def hi: Unit}
trait A extends Base {override def hi = println("A")}
trait B extends Base {override def hi = println("B")}
class C extends A with B

With the type hierarchy the result of calling hi s is as follows (note the use of {} to instantiate the traits):

scala> (new A {}).hi
A

scala> (new B {}).hi
B

scala> (new C).hi
B

Answer 3

A trait adds methods to the class that mixes it in. If two traits adds the same method, the class would end up with two identical methods, which, of course, can t happen.

If the method is private in the trait, however, it won t cause problem. And if you want the methods to stack over each other, you may define a base trait and then abstract override on the inheriting traits. It requires a class to define the method, however. Here is an example of this:

scala> trait Hi { def hi: Unit }
defined trait Hi

scala> trait A extends Hi { abstract override def hi = { println("A"); super.hi } }
defined trait A

scala> trait B extends Hi { abstract override def hi = { println("B"); super.hi } }
defined trait B

scala> class NoHi extends Hi { def hi = () }
defined class NoHi

scala> class C extends NoHi with B with A
defined class C

scala> new C().hi
A
B

If, however, you truly want two separate methods from each trait, then you ll need to compose instead of inherit.

Answer 4

This is the diamond problem. Which method hi should be inherited, the one from A, or the one from B? You can get around this as Don suggested, by using a common base trait.

Answer 5

I had the same problem and I didn t like to have to create an intermediary trait because I can have 4,5 or even 6 traits with the same methods, because it is traits containing CRUD operations (find, create...). Furthermore I needed to use those trait together only for test purpose and I always try to avoid as much as possible to modify the structure of my project only to make my test easier. So I simply implemented those traits in different objects :

class somethingToTest {
  object AImpl extends ATrait 
  object BImpl extends BTrait

  val a = AImpl.methodDuplicated()
  val b = BImpl.methodDuplicated()
}

It is probably not the most clever way to use traits but it doesn t require any change in the project s code, it only implies to have a bit more code in the tests.

友情链接